Module 8.1. Rhythms of Waking and Sleeping
Introduction to Sleep
Imagine astronauts discovering Earth and being surprised by sleep, which would seem mysterious to them.
The chapter adopts the astronauts' perspective, questioning why we spend a third of our lives sleeping.
Rhythms of Waking and Sleeping
Learning Objectives:
Define and describe endogenous rhythms.
Explain the mechanisms that set and reset the biological clock.
Describe the role of the suprachiasmatic nucleus in controlling sleep and wakefulness.
Outline the biochemical basis of the circadian rhythm.
Earlier psychologists believed wakefulness and sleep depended on external stimuli.
Curt Richter's research (1922) suggested the body generates its own cycles.
The idea of self-generated rhythms views animals as active producers of behaviors.
Endogenous Rhythms
Animals need to anticipate environmental changes rather than merely respond to stimuli.
Migratory birds fly south before it gets too cold.
Squirrels store nuts and gain fat before winter.
Endogenous Circannual Rhythm: Brain keeps track of the date to generate a rhythm lasting about a year.
Circannual: From Latin circum (about) and annum (year).
Endogenous Circadian Rhythms: Rhythms that last about a day.
Circadian: From Latin circum (about) and dies (day).
Even after sleep deprivation, people feel more alert in the morning due to the circadian rhythm.
Activity in the posterior areas of the cerebral cortex correlates with the circadian rhythm.
Flying squirrels in total darkness maintain a consistent activity and sleep rhythm.
The self-generated cycle may be slightly shorter or longer than 24 hours.
Adjusting to Different Schedules
Humans generate 24-hour wake-sleep rhythms, modifiable only a little.
Astronauts on Mars (24 hours and 39 minutes) need to adjust. Engineers monitoring the Phoenix mission lived on a Martian schedule.
Most adapted, but they slept less and some experienced alertness loss.
Circadian rhythm cannot easily adjust to departures from a 24-hour schedule.
Submariners face months without sunlight, living under artificial light.
Their bodies generate rhythms averaging over 24 hours, and alertness suffers.
Circadian Rhythms Affect Many Bodily Functions
Circadian Rhythms include hunger, thirst, liver activity, insulin secretion, DNA repair, and intestinal activity.
Body temperature fluctuates, reaching a low at night and a peak in the late afternoon.
Mood also follows a circadian rhythm.
Positive mood increases from waking until late afternoon, then declines until bedtime.
In an unchanging environment, people report the most pleasant mood around 5 p.m. and the least around 5 a.m., regardless of when they started a 30-hour period of wakefulness.
Setting and Resetting the Biological Clock
Circadian rhythm is close to 24 hours but not perfect, so it requires resetting.
Zeitgeber: German term for “time-giver,” the stimulus that resets the circadian rhythm.
Light is the dominant zeitgeber for land animals; tides are important for marine animals.
Other Zeitgebers include exercise, arousal, meals, and environmental temperature.
Social stimuli alone are ineffective as zeitgebers unless they induce physical activity.
In Antarctica's constant darkness, people maintain a 24-hour rhythm but with variations, causing difficulties in working together.
Astronauts in orbit, with alternating 45-minute periods of daylight and darkness, experience constant dim light.
They are less alert during wakeful periods and don't sleep deeply during rest, leading to depression and impaired performance.
Setting wake-sleep cycles by the clock is influenced by sunlight.
Daylight Saving Time causes people to be ill-rested for days after the shift, especially those already sleep-deprived.
Staying up late on weekends leads to a biological clock that is misaligned on Monday mornings.
Importance of Sunlight
Sunrise occurs earlier in eastern Germany than in western Germany.
People at the eastern edge have a sleep, midpoint about 30 minutes earlier than those at the west.
Blind people, need to set rhythms by non-light zeitgebers, but their rhythms are a little longer than 24 hours if they are not sensitive to the secondary zeitgebers.
This can cause insomnia at night and sleepiness during the day.
Most blind people report frequent sleep problems.
Jet Lag
A disruption of circadian rhythms after crossing time zones.
Travelers experience sleepiness during the day, sleeplessness at night, and impaired concentration.
Easier to adjust to westward travel (phase-delaying) rather than eastward (phase-advancing).
Adjusting to jet lag is stressful, raising cortisol levels.
Flight attendants on frequent long-distance flights have smaller hippocampus volumes and memory impairments.
Night Work
Night-shift workers sleep during the day but often remain groggy and have poor sleep.
Body temperature peaks during daytime sleep instead of nighttime work.
Night-shift workers have more accidents than day-shift workers.
Artificial lighting (150 to 180 lux) is not as effective in resetting the rhythm.
Adjustment is better with very dark rooms during the day and bright lights at night.
Short-wavelength (bluish) light resets the circadian rhythm more strongly.
Morning People and Evening People
Morning people (larks) awaken early and peak early.
Evening people (owls) warm up slowly, peaking late.
Morning people are most impaired on the night shift, and evening people are most impaired on the morning shift.
People differ by age; adolescents tend to stay up later and wake up later.
This trend occurs in every culture and in other species, triggered by sex hormones.
Speculation exists as to why staying up late and waking up late might be advantageous for adolescents.
Factors Affecting Circadian Rhythms
Being a “morning person” or an “evening person” depends partly on age, genetics, and environmental factors.
In low-tech societies, people sleep about three hours after sunset and wake at sunrise.
Electric lights cause people to stay awake later and get less sleep.
City dwellers are more likely to stay up late than rural people.
High school classes start early, causing drowsy teenagers to have lower test scores.
Morning types perform better in morning classes.
Morning people report being happier, as their biological rhythms align with school or work schedule.
Mechanisms of the Biological Clock
Curt Richter demonstrated that the brain generates its own rhythms.
Biological clock is insensitive to interference (blindness, deafness, food/water deprivation, X-rays, tranquilizers, alcohol, anesthesia, lack of oxygen, most brain damage or removal of endocrine organs).
Biological clock is a hardy mechanism.
Suprachiasmatic Nucleus (SCN): Main driver of sleep and body temperature rhythms, located in the hypothalamus.
Damage to the SCN causes erratic body rhythms.
The Suprachiasmatic Nucleus (SCN)
SCN generates circadian rhythms automatically.
SCN neurons produce a circadian rhythm of action potentials even when disconnected or in tissue culture.
Interactions among neurons and astrocytes sharpen the rhythm's accuracy.
SCN transplants from hamsters with a 20-hour rhythm into adults with SCN lesions resulted in the recipients producing a 20-hour rhythm.
SCN rhythms come from the SCN itself.
How Light Resets the SCN
A retinohypothalamic path from the retina to the SCN alters the SCN’s settings.
Input does not come from normal retinal receptors.
Blind mole rats respond to light by resetting their biological clocks.
Light resets their circadian rhythms, enabling them to be awake only at night.
Retinohypothalamic path comes from retinal ganglion cells with melanopsin.
These cells receive input from rods and cones but respond directly to light.
They respond slowly to the overall average amount of light, which the SCN needs to gauge the time of day.
These ganglion cells respond mainly to short-wavelength (blue) light.
Evening exposure to television, video games, computers, and eReaders phase-delays the circadian rhythm.
Wearing glasses that filter out blue light might help.
Biochemistry of the Circadian Rhythm
Research on circadian rhythm began in Drosophila (fruit flies).
Genes period (PER) and timeless (TIM) produce proteins PER and TIM, which promote sleep.
Concentration oscillates over a day based on feedback interactions.
Messenger RNA levels for PER and TIM start low, increase during the day, and increase protein synthesis.
PER and TIM proteins inhibit the genes producing the RNA molecules.
During the night, PER and TIM concentrations are high, promoting sleep, but messenger RNA concentrations are declining.
By next morning, PER and TIM protein levels have declined, the fly awakens, and the cycle starts again.
Light activates a chemical that breaks down the TIM protein about as fast as it forms during the day, keeping the fly awake and synchronizing the internal clock to the external world.
Relevance to Mammals
Analyzing the mechanism in flies told researchers what to look for in humans and other mammals.
Mammals have variants of the PER and other proteins found in flies.
People with PER mutations have circadian rhythms shorter than 24 hours, consistently getting sleepy early and awakening early, while other genetic mutations can cause the reverse condition.
Melatonin and the Pineal Gland
The suprachiasmatic nucleus controls activity levels in other brain areas, including the pineal gland.
The pineal gland releases melatonin.
Melatonin is released mostly at night in all animals.
In diurnal animals like humans, it increases sleepiness.
Melatonin also helps control the onset of puberty and adjustments to changes of season.
Melatonin secretion starts to increase about 2 or 3 hours before bedtime.
Taking a melatonin pill earlier can hasten the onset of sleepiness and shift the circadian rhythm.
Melatonin pills are helpful when people travel across time zones if they need to sleep at an unaccustomed time.
Sleep–Wake Cycles Summary
Animals have circadian rhythms, internally generated rhythms of activity and sleep lasting about 24 hours.
Timing of sunrise influences circadian rhythm.
It is easier for most people to follow a cycle slightly longer than 24 hours (westward travel).
The shifting of the circadian rhythm can be achieved with bright lights at night and darkness during the day.
There are differences in alertness at different times of the day. The greatest preference for staying awake late and sleeping late the next morning is most apparent in young adults.
The Suprachiasmatic Nucleus generates the body’s circadian rhythms for sleep and temperature.
Light resets the biological clock.
Genes controlling the circadian rhythm are similar in mammals as in insects, with circadian rhythms resulting from genes producing proteins that feed back to inhibit the genes that produce them.
Some people have genetic variations causing circadian rhythms to run faster or slower than average, affecting their schedule
The Suprachiasmatic Nucleus controls the body’s rhythm partly by directing the release of melatonin by the pineal gland, with the hormone increasing sleepiness and resetting the circadian rhythm.
Key Terms
Endogenous Circadian Rhythms
Endogenous Circannual Rhythm
Jet Lag
Melatonin
Pineal Gland
Suprachiasmatic Nucleus
Zeitgeber
Thought Question
Why would evolution have enabled blind mole rats to synchronize their SCN activity to light if they cannot see well enough to make any use of the light?
End-of-Module Quiz
Migratory birds know when to fly north for spring because of endogenous circannual rhythm.
Submariners on a 6-hour work/relax/sleep schedule continue producing the usual 24-hour rhythm.
People in Antarctica find it difficult to work together because their circadian rhythms drift out of phase.
For most young adults, mood tends to be most pleasant in the late afternoon or early evening.
Evidence of sunlight resetting humans’ circadian rhythm: People awaken earlier in eastern Germany than in western Germany.
High school students perform worse in the morning because teenagers tend to stay up late and awaken late.
Blind people experience something similar to jet lag because their circadian rhythms drift out of phase.
SCN produces the Circadian Rhythm itself: SCN cells isolated from the body continue to produce a circadian rhythm.
Light Resets the SCN even after damage: SCN receives input from ganglion cells that respond to light.
To get to sleep on time, avoid short-wavelength light late in the evening
After high levels of TIM and PER during day, their level decreases at night: High levels of the proteins inhibit the genes that produce these proteins.
Melatonin increases sleepiness.
Introduction to Sleep
Imagine astronauts discovering Earth and being surprised by sleep, which would seem mysterious to them.
The chapter adopts the astronauts' perspective, questioning why we spend a third of our lives sleeping.
Rhythms of Waking and Sleeping
Learning Objectives:- Define and describe endogenous rhythms.
Explain the mechanisms that set and reset the biological clock.
Describe the role of the suprachiasmatic nucleus in controlling sleep and wakefulness.
Outline the biochemical basis of the circadian rhythm.
Earlier psychologists believed wakefulness and sleep depended on external stimuli.
Curt Richter's research (1922) suggested the body generates its own cycles.
The idea of self-generated rhythms views animals as active producers of behaviors.
Endogenous Rhythms
Animals need to anticipate environmental changes rather than merely respond to stimuli.- Migratory birds fly south before it gets too cold.
Squirrels store nuts and gain fat before winter.
Endogenous Circannual Rhythm: Brain keeps track of the date to generate a rhythm lasting about a year.- Circannual: From Latin circum (about) and annum (year).
Endogenous Circadian Rhythms: Rhythms that last about a day.- Circadian: From Latin circum (about) and dies (day).
Even after sleep deprivation, people feel more alert in the morning due to the circadian rhythm.
Activity in the posterior areas of the cerebral cortex correlates with the circadian rhythm.
Flying squirrels in total darkness maintain a consistent activity and sleep rhythm.
The self-generated cycle may be slightly shorter or longer than 24 hours.
Adjusting to Different Schedules
Humans generate 24-hour wake-sleep rhythms, modifiable only a little.
Astronauts on Mars (24 hours and 39 minutes) need to adjust. Engineers monitoring the Phoenix mission lived on a Martian schedule.
Most adapted, but they slept less and some experienced alertness loss.
Circadian rhythm cannot easily adjust to departures from a 24-hour schedule.
Submariners face months without sunlight, living under artificial light.
Their bodies generate rhythms averaging over 24 hours, and alertness suffers.
Circadian Rhythms Affect Many Bodily Functions
Circadian Rhythms include hunger, thirst, liver activity, insulin secretion, DNA repair, and intestinal activity.
Body temperature fluctuates, reaching a low at night and a peak in the late afternoon.
Mood also follows a circadian rhythm.
Positive mood increases from waking until late afternoon, then declines until bedtime.
In an unchanging environment, people report the most pleasant mood around 5 p.m. and the least around 5 a.m., regardless of when they started a 30-hour period of wakefulness.
Setting and Resetting the Biological Clock
Circadian rhythm is close to 24 hours but not perfect, so it requires resetting.
Zeitgeber: German term for “time-giver,” the stimulus that resets the circadian rhythm.
Light is the dominant zeitgeber for land animals; tides are important for marine animals.
Other Zeitgebers include exercise, arousal, meals, and environmental temperature.
Social stimuli alone are ineffective as zeitgebers unless they induce physical activity.
In Antarctica's constant darkness, people maintain a 24-hour rhythm but with variations, causing difficulties in working together.
Astronauts in orbit, with alternating 45-minute periods of daylight and darkness, experience constant dim light.
They are less alert during wakeful periods and don't sleep deeply during rest, leading to depression and impaired performance.
Setting wake-sleep cycles by the clock is influenced by sunlight.
Daylight Saving Time causes people to be ill-rested for days after the shift, especially those already sleep-deprived.
Staying up late on weekends leads to a biological clock that is misaligned on Monday mornings.
Importance of Sunlight
Sunrise occurs earlier in eastern Germany than in western Germany.
People at the eastern edge have a sleep, midpoint about 30 minutes earlier than those at the west.
Blind people, need to set rhythms by non-light zeitgebers, but their rhythms are a little longer than 24 hours if they are not sensitive to the secondary zeitgebers.
This can cause insomnia at night and sleepiness during the day.
Most blind people report frequent sleep problems.
Jet Lag
A disruption of circadian rhythms after crossing time zones.
Travelers experience sleepiness during the day, sleeplessness at night, and impaired concentration.
Easier to adjust to westward travel (phase-delaying) rather than eastward (phase-advancing).
Adjusting to jet lag is stressful, raising cortisol levels.
Flight attendants on frequent long-distance flights have smaller hippocampus volumes and memory impairments.
Night Work
Night-shift workers sleep during the day but often remain groggy and have poor sleep.
Body temperature peaks during daytime sleep instead of nighttime work.
Night-shift workers have more accidents than day-shift workers.
Artificial lighting (150 to 180 lux) is not as effective in resetting the rhythm.
Adjustment is better with very dark rooms during the day and bright lights at night.
Short-wavelength (bluish) light resets the circadian rhythm more strongly.
Morning People and Evening People
Morning people (larks) awaken early and peak early.
Evening people (owls) warm up slowly, peaking late.
Morning people are most impaired on the night shift, and evening people are most impaired on the morning shift.
People differ by age; adolescents tend to stay up later and wake up later.
This trend occurs in every culture and in other species, triggered by sex hormones.
Speculation exists as to why staying up late and waking up late might be advantageous for adolescents.
Factors Affecting Circadian Rhythms
Being a “morning person” or an “evening person” depends partly on age, genetics, and environmental factors.
In low-tech societies, people sleep about three hours after sunset and wake at sunrise.
Electric lights cause people to stay awake later and get less sleep.
City dwellers are more likely to stay up late than rural people.
High school classes start early, causing drowsy teenagers to have lower test scores.
Morning types perform better in morning classes.
Morning people report being happier, as their biological rhythms align with school or work schedule.
Mechanisms of the Biological Clock
Curt Richter demonstrated that the brain generates its own rhythms.
Biological clock is insensitive to interference (blindness, deafness, food/water deprivation, X-rays, tranquilizers, alcohol, anesthesia, lack of oxygen, most brain damage or removal of endocrine organs).
Biological clock is a hardy mechanism.
Suprachiasmatic Nucleus (SCN): Main driver of sleep and body temperature rhythms, located in the hypothalamus.
Damage to the SCN causes erratic body rhythms.
The Suprachiasmatic Nucleus (SCN)
SCN generates circadian rhythms automatically.
SCN neurons produce a circadian rhythm of action potentials even when disconnected or in tissue culture.
Interactions among neurons and astrocytes sharpen the rhythm's accuracy.
SCN transplants from hamsters with a 20-hour rhythm into adults with SCN lesions resulted in the recipients producing a 20-hour rhythm.
SCN rhythms come from the SCN itself.
How Light Resets the SCN
A retinohypothalamic path from the retina to the SCN alters the SCN’s settings.
Input does not come from normal retinal receptors.
Blind mole rats respond to light by resetting their biological clocks.
Light resets their circadian rhythms, enabling them to be awake only at night.
Retinohypothalamic path comes from retinal ganglion cells with melanopsin.
These cells receive input from rods and cones but respond directly to light.
They respond slowly to the overall average amount of light, which the SCN needs to gauge the time of day.
These ganglion cells respond mainly to short-wavelength (blue) light.
Evening exposure to television, video games, computers, and eReaders phase-delays the circadian rhythm.
Wearing glasses that filter out blue light might help.
Biochemistry of the Circadian Rhythm
Research on circadian rhythm began in Drosophila (fruit flies).
Genes period (PER) and timeless (TIM) produce proteins PER and TIM, which promote sleep.
Concentration oscillates over a day based on feedback interactions.
Messenger RNA levels for PER and TIM start low, increase during the day, and increase protein synthesis.
PER and TIM proteins inhibit the genes producing the RNA molecules.
During the night, PER and TIM concentrations are high, promoting sleep, but messenger RNA concentrations are declining.
By next morning, PER and TIM protein levels have declined, the fly awakens, and the cycle starts again.
Light activates a chemical that breaks down the TIM protein about as fast as it forms during the day, keeping the fly awake and synchronizing the internal clock to the external world.
Relevance to Mammals
Analyzing the mechanism in flies told researchers what to look for in humans and other mammals.
Mammals have variants of the PER and other proteins found in flies.
People with PER mutations have circadian rhythms shorter than 24 hours, consistently getting sleepy early and awakening early, while other genetic mutations can cause the reverse condition.
Melatonin and the Pineal Gland
The suprachiasmatic nucleus controls activity levels in other brain areas, including the pineal gland.
The pineal gland releases melatonin.
Melatonin is released mostly at night in all animals.
In diurnal animals like humans, it increases sleepiness.
Melatonin also helps control the onset of puberty and adjustments to changes of season.
Melatonin secretion starts to increase about 2 or 3 hours before bedtime.
Taking a melatonin pill earlier can hasten the onset of sleepiness and shift the circadian rhythm.
Melatonin pills are helpful when people travel across time zones if they need to sleep at an unaccustomed time.
Sleep–Wake Cycles Summary
Animals have circadian rhythms, internally generated rhythms of activity and sleep lasting about 24 hours.
Timing of sunrise influences circadian rhythm.
It is easier for most people to follow a cycle slightly longer than 24 hours (westward travel).
The shifting of the circadian rhythm can be achieved with bright lights at night and darkness during the day.
There are differences in alertness at different times of the day. The greatest preference for staying awake late and sleeping late the next morning is most apparent in young adults.
The Suprachiasmatic Nucleus generates the body’s circadian rhythms for sleep and temperature.
Light resets the biological clock.
Genes controlling the circadian rhythm are similar in mammals as in insects, with circadian rhythms resulting from genes producing proteins that feed back to inhibit the genes that produce them.
Some people have genetic variations causing circadian rhythms to run faster or slower than average, affecting their schedule
The Suprachiasmatic Nucleus controls the body’s rhythm partly by directing the release of melatonin by the pineal gland, with the hormone increasing sleepiness and resetting the circadian rhythm.
Key Terms
Endogenous Circadian Rhythms
Endogenous Circannual Rhythm
Jet Lag
Melatonin
Pineal Gland
Suprachiasmatic Nucleus
Zeitgeber
Thought Question
Why would evolution have enabled blind mole rats to synchronize their SCN activity to light if they cannot see well enough to make any use of the light?
End-of-Module Quiz
Migratory birds know when to fly north for spring because of endogenous circannual rhythm.
Submariners on a 6-hour work/relax/sleep schedule continue producing the usual 24-hour rhythm.
People in Antarctica find it difficult to work together because their circadian rhythms drift out of phase.
For most young adults, mood tends to be most pleasant in the late afternoon or early evening.
Evidence of sunlight resetting humans’ circadian rhythm: People awaken earlier in eastern Germany than in western Germany.
High school students perform worse in the morning because teenagers tend to stay up late and awaken late.
Blind people experience something similar to jet lag because their circadian rhythms drift out of phase.
SCN produces the Circadian Rhythm itself: SCN cells isolated from the body continue to produce a circadian rhythm.
Light Resets the SCN even after damage: SCN receives input from ganglion cells that respond to light.
To get to sleep on time, avoid short-wavelength light late in the evening.
After high levels of TIM and PER during day, their level decreases at night: High levels of the proteins inhibit the genes that produce these proteins.
Melatonin increases sleepiness.
Introduction to Sleep
Imagine astronauts discovering Earth and being surprised by sleep, which would seem mysterious to them.
The chapter adopts the astronauts' perspective, questioning why we spend a third of our lives sleeping.
Rhythms of Waking and Sleeping
Learning Objectives:- Define and describe endogenous rhythms.
Explain the mechanisms that set and reset the biological clock.
Describe the role of the suprachiasmatic nucleus in controlling sleep and wakefulness.
Outline the biochemical basis of the circadian rhythm.
Earlier psychologists believed wakefulness and sleep depended on external stimuli.
Curt Richter's research (1922) suggested the body generates its own cycles.
The idea of self-generated rhythms views animals as active producers of behaviors.
Endogenous Rhythms
Animals need to anticipate environmental changes rather than merely respond to stimuli.- Migratory birds fly south before it gets too cold.
Squirrels store nuts and gain fat before winter.
Endogenous Circannual Rhythm: Brain keeps track of the date to generate a rhythm lasting about a year.- Circannual: From Latin circum (about) and annum (year).
Endogenous Circadian Rhythms: Rhythms that last about a day.- Circadian: From Latin circum (about) and dies (day).
Even after sleep deprivation, people feel more alert in the morning due to the circadian rhythm.
Activity in the posterior areas of the cerebral cortex correlates with the circadian rhythm.
Flying squirrels in total darkness maintain a consistent activity and sleep rhythm.
The self-generated cycle may be slightly shorter or longer than 24 hours.
Adjusting to Different Schedules
Humans generate 24-hour wake-sleep rhythms, modifiable only a little.
Astronauts on Mars (24 hours and 39 minutes) need to adjust. Engineers monitoring the Phoenix mission lived on a Martian schedule.
Most adapted, but they slept less and some experienced alertness loss.
Circadian rhythm cannot easily adjust to departures from a 24-hour schedule.
Submariners face months without sunlight, living under artificial light.
Their bodies generate rhythms averaging over 24 hours, and alertness suffers.
Circadian Rhythms Affect Many Bodily Functions
Circadian Rhythms include hunger, thirst, liver activity, insulin secretion, DNA repair, and intestinal activity.
Body temperature fluctuates, reaching a low at night and a peak in the late afternoon.
Mood also follows a circadian rhythm.
Positive mood increases from waking until late afternoon, then declines until bedtime.
In an unchanging environment, people report the most pleasant mood around 5 p.m. and the least around 5 a.m., regardless of when they started a 30-hour period of wakefulness.
Setting and Resetting the Biological Clock
Circadian rhythm is close to 24 hours but not perfect, so it requires resetting.
Zeitgeber: German term for “time-giver,” the stimulus that resets the circadian rhythm.
Light is the dominant zeitgeber for land animals; tides are important for marine animals.
Other Zeitgebers include exercise, arousal, meals, and environmental temperature.
Social stimuli alone are ineffective as zeitgebers unless they induce physical activity.
In Antarctica's constant darkness, people maintain a 24-hour rhythm but with variations, causing difficulties in working together.
Astronauts in orbit, with alternating 45-minute periods of daylight and darkness, experience constant dim light.
They are less alert during wakeful periods and don't sleep deeply during rest, leading to depression and impaired performance.
Setting wake-sleep cycles by the clock is influenced by sunlight.
Daylight Saving Time causes people to be ill-rested for days after the shift, especially those already sleep-deprived.
Staying up late on weekends leads to a biological clock that is misaligned on Monday mornings.
Importance of Sunlight
Sunrise occurs earlier in eastern Germany than in western Germany.
People at the eastern edge have a sleep, midpoint about 30 minutes earlier than those at the west.
Blind people, need to set rhythms by non-light zeitgebers, but their rhythms are a little longer than 24 hours if they are not sensitive to the secondary zeitgebers.
This can cause insomnia at night and sleepiness during the day.
Most blind people report frequent sleep problems.
Jet Lag
A disruption of circadian rhythms after crossing time zones.
Travelers experience sleepiness during the day, sleeplessness at night, and impaired concentration.
Easier to adjust to westward travel (phase-delaying) rather than eastward (phase-advancing).
Adjusting to jet lag is stressful, raising cortisol levels.
Flight attendants on frequent long-distance flights have smaller hippocampus volumes and memory impairments.
Night Work
Night-shift workers sleep during the day but often remain groggy and have poor sleep.
Body temperature peaks during daytime sleep instead of nighttime work.
Night-shift workers have more accidents than day-shift workers.
Artificial lighting (150 to 180 lux) is not as effective in resetting the rhythm.
Adjustment is better with very dark rooms during the day and bright lights at night.
Short-wavelength (bluish) light resets the circadian rhythm more strongly.
Morning People and Evening People
Morning people (larks) awaken early and peak early.
Evening people (owls) warm up slowly, peaking late.
Morning people are most impaired on the night shift, and evening people are most impaired on the morning shift.
People differ by age; adolescents tend to stay up later and wake up later.
This trend occurs in every culture and in other species, triggered by sex hormones.
Speculation exists as to why staying up late and waking up late might be advantageous for adolescents.
Factors Affecting Circadian Rhythms
Being a “morning person” or an “evening person” depends partly on age, genetics, and environmental factors.
In low-tech societies, people sleep about three hours after sunset and wake at sunrise.
Electric lights cause people to stay awake later and get less sleep.
City dwellers are more likely to stay up late than rural people.
High school classes start early, causing drowsy teenagers to have lower test scores.
Morning types perform better in morning classes.
Morning people report being happier, as their biological rhythms align with school or work schedule.
Mechanisms of the Biological Clock
Curt Richter demonstrated that the brain generates its own rhythms.
Biological clock is insensitive to interference (blindness, deafness, food/water deprivation, X-rays, tranquilizers, alcohol, anesthesia, lack of oxygen, most brain damage or removal of endocrine organs).
Biological clock is a hardy mechanism.
Suprachiasmatic Nucleus (SCN): Main driver of sleep and body temperature rhythms, located in the hypothalamus.
Damage to the SCN causes erratic body rhythms.
The Suprachiasmatic Nucleus (SCN)
SCN generates circadian rhythms automatically.
SCN neurons produce a circadian rhythm of action potentials even when disconnected or in tissue culture.
Interactions among neurons and astrocytes sharpen the rhythm's accuracy.
SCN transplants from hamsters with a 20-hour rhythm into adults with SCN lesions resulted in the recipients producing a 20-hour rhythm.
SCN rhythms come from the SCN itself.
How Light Resets the SCN
A retinohypothalamic path from the retina to the SCN alters the SCN’s settings.
Input does not come from normal retinal receptors.
Blind mole rats respond to light by resetting their biological clocks.
Light resets their circadian rhythms, enabling them to be awake only at night.
Retinohypothalamic path comes from retinal ganglion cells with melanopsin.
These cells receive input from rods and cones but respond directly to light.
They respond slowly to the overall average amount of light, which the SCN needs to gauge the time of day.
These ganglion cells respond mainly to short-wavelength (blue) light.
Evening exposure to television, video games, computers, and eReaders phase-delays the circadian rhythm.
Wearing glasses that filter out blue light might help.
Biochemistry of the Circadian Rhythm
Research on circadian rhythm began in Drosophila (fruit flies).
Genes period (PER) and timeless (TIM) produce proteins PER and TIM, which promote sleep.
Concentration oscillates over a day based on feedback interactions.
Messenger RNA levels for PER and TIM start low, increase during the day, and increase protein synthesis.
PER and TIM proteins inhibit the genes producing the RNA molecules.
During the night, PER and TIM concentrations are high, promoting sleep, but messenger RNA concentrations are declining.
By next morning, PER and TIM protein levels have declined, the fly awakens, and the cycle starts again.
Light activates a chemical that breaks down the TIM protein about as fast as it forms during the day, keeping the fly awake and synchronizing the internal clock to the external world.
Relevance to Mammals
Analyzing the mechanism in flies told researchers what to look for in humans and other mammals.
Mammals have variants of the PER and other proteins found in flies.
People with PER mutations have circadian rhythms shorter than 24 hours, consistently getting sleepy early and awakening early, while other genetic mutations can cause the reverse condition.
Melatonin and the Pineal Gland
The suprachiasmatic nucleus controls activity levels in other brain areas, including the pineal gland.
The pineal gland releases melatonin.
Melatonin is released mostly at night in all animals.
In diurnal animals like humans, it increases sleepiness.
Melatonin also helps control the onset of puberty and adjustments to changes of season.
Melatonin secretion starts to increase about 2 or 3 hours before bedtime.
Taking a melatonin pill earlier can hasten the onset of sleepiness and shift the circadian rhythm.
Melatonin pills are helpful when people travel across time zones if they need to sleep at an unaccustomed time.
Sleep–Wake Cycles Summary
Animals have circadian rhythms, internally generated rhythms of activity and sleep lasting about 24 hours.
Timing of sunrise influences circadian rhythm.
It is easier for most people to follow a cycle slightly longer than 24 hours (westward travel).
The shifting of the circadian rhythm can be achieved with bright lights at night and darkness during the day.
There are differences in alertness at different times of the day. The greatest preference for staying awake late and sleeping late the next morning is most apparent in young adults.
The Suprachiasmatic Nucleus generates the body’s circadian rhythms for sleep and temperature.
Light resets the biological clock.
Genes controlling the circadian rhythm are similar in mammals as in insects, with circadian rhythms resulting from genes producing proteins that feed back to inhibit the genes that produce them.
Some people have genetic variations causing circadian rhythms to run faster or slower than average, affecting their schedule
The Suprachiasmatic Nucleus controls the body’s rhythm partly by directing the release of melatonin by the pineal gland, with the hormone increasing sleepiness and resetting the circadian rhythm.
Key Terms
Endogenous Circadian Rhythms
Endogenous Circannual Rhythm
Jet Lag
Melatonin
Pineal Gland
Suprachiasmatic Nucleus
Zeitgeber
Thought Question
Why would evolution have enabled blind mole rats to synchronize their SCN activity to light if they cannot see well enough to make any use of the light?
End-of-Module Quiz
Migratory birds know when to fly north for spring because of endogenous circannual rhythm.
Submariners on a 6-hour work/relax/sleep schedule continue producing the usual 24-hour rhythm.
People in Antarctica find it difficult to work together because their circadian rhythms drift out of phase.
For most young adults, mood tends to be most pleasant in the late afternoon or early evening.
Evidence of sunlight resetting humans’ circadian rhythm: People awaken earlier in eastern Germany than in western Germany.
High school students perform worse in the morning because teenagers tend to stay up late and awaken late.
Blind people experience something similar to jet lag because their circadian rhythms drift out of phase.
SCN produces the Circadian Rhythm itself: SCN cells isolated from the body continue to produce a circadian rhythm.
Light Resets the SCN even after damage: SCN receives input from ganglion cells that respond to light.
To get to sleep on time, avoid short-wavelength light late in the evening.
After high levels of TIM and PER during day, their level decreases at night: High levels of the proteins inhibit the genes that produce these proteins.
Melatonin increases sleepiness.