Chapter 10: Biological Rhythms and Sleep

Biological rhythms

• body temperature, hormone secretion, sleep-wake cycle, energy use, and metabolism, just to mention a few, are influenced by a biological cycle

• fluctuations of life processes and act with “clocklike” regulatory to regulate aspects of physiology and behavior

Circadian rhythms

the body's natural 24-hour cycles that regulate physical, mental, and behavioral changes, primarily the sleep-wake cycle

• include the sleep-wake cycle, daily variations in body temperature, and daily fluctuations in hormones like melatonin and cortisol.

Breeding seasons in animals appear to be triggered by changes in

day/night cycles

Circadian rhythm =

the pattern (what happens every 24 hours)

Endogenous clock =

• the internal mechanism that produces that pattern

What happens when jet lag or shift work occurs?

: They desynchronize internal circadian rhythms from the external environment.

What causes internal circadian rhythms to get out of sync?

Abrupt changes in daily rhythms or external cues.

What are possible effects of differences between internal and external rhythms?

Sleep disturbances, mood changes, and difficulty functioning normally during waking hours.

How long does jet lag typically last?

Usually only a few days, as the body adjusts over time.

Why is shift work harder to overcome than jet lag?

Because shift schedules often change continually, preventing stable synchronization.

How can a person’s internal clock be synchronized with upcoming external changes?

By exposing them to cues (like light or activity timing) that help synchronize biological rhythms.

The __________ acts as an internal clock to regulate many circadian rhythms

suprachiasmatic nucleus

Pineal gland

• secretes melatonin, plays a role in biological rhythms by regulating various hormones, physiological processes, and behaviors.

SCN cells can “keep time” on their own.

Even when SCN cells are taken out of the body and grown in a lab (in vitro), they still show rhythmic activity.

What part of the brain acts as the body’s “master clock” for circadian rhythms?

The suprachiasmatic nucleus (SCN) in the hypothalamus.

What happens when the SCN is damaged or removed?

Circadian rhythms such as drinking, movement, and hormone secretion are lost or disrupted.

Do endocrine glands alone control circadian rhythms?

No. They play roles in hormone release but do little to maintain circadian timing without the SCN.

What does it mean that SCN cells show rhythmicity “in vitro”?

Even outside the body, SCN cells maintain their own rhythmic activity, showing that the rhythm is generated internally.

What happens when SCN tissue is transplanted into another animal whose SCN was damaged?

The recipient’s circadian rhythms return, proving the SCN generates the rhythm.

What hormone does the pineal gland secrete, and what is its role?

It secretes melatonin, which helps regulate biological rhythms by influencing hormones, physiological processes, and behavior.

How do the SCN and pineal gland work together?

The SCN sets the body’s master rhythm, and the pineal gland releases melatonin to help synchronize body functions to environmental light-dark cycles.

¡It appears that typical rods and cones ______needed for  entrainment to light (as observed in transgenic mice lacking rods and cones)

are not

Melanopsin

• releasing ganglion cells appear to be absent or dysfunctional in persons who are blind (these individuals often display free running circadian rhythms)

What helps set the circadian rhythms 

visual light and melatonin

Where is melanopsin found

• Melanopsin is located in intrinsically photosensitive retinal ganglion cells (ipRGCs).

• These cells are a third type of photoreceptor, separate from rods (for dim light) and cones (for color and bright light).

What special cells of the retina help to set (entrain) circadian cycles with light?

Intrinsically photosensitive retinal ganglion cells (ipRGCs) that contain melanopsin.

Do rods and cones play a large role in circadian entrainment?

No. Entrainment to light can still occur without rods and cones, as shown in mice lacking these cells.

What is melanopsin, and what cells use it?

Melanopsin is a light-sensitive pigment found in certain retinal ganglion cells that detect light levels to help regulate circadian rhythms.

What brain structure releases melatonin?

The pineal gland.

Melatonin is synthesized from what molecule?

Serotonin.

What does melatonin do regarding sleep?

It helps signal the body that it’s time to sleep and assists in synchronizing the body’s internal clock with the external light–dark cycle.

Which retinal cells send light information directly to the SCN?

Melanopsin-containing ganglion cells, which inform the hypothalamus about light to help entrain circadian rhythms.

Why do some blind individuals have free-running circadian rhythms?

Because their melanopsin-containing ganglion cells are absent or nonfunctional, preventing light from entraining their internal clock.

How does melatonin supplementation help with sleep and circadian rhythm?

It helps sighted people fall asleep and helps blind individuals synchronize their internal clocks to the 24-hour day.

How does the brain normally keep circadian rhythms synchronized?

By using both light cues detected by melanopsin cells and melatonin signals to entrain the internal clock to the external light–dark cycle.

What do the Clock/Cycle proteins regulate?

They regulate the transcription of other genes, including per and cry, which are important for maintaining the circadian rhythm.

How do Clock and Cycle function?

Clock and Cycle form a dimer (pair up) and act as a transcription factor to turn on genes like per and cry.

What are “cry” and “per”?

They are genes whose proteins act as inhibitors in the circadian feedback loop. Their proteins accumulate and eventually inhibit the Clock/Cycle dimer, helping to regulate the 24-hour cycle.

How does Clock/Cycle act to inhibit the circadian cycle via the SCN?

Indirectly. Clock/Cycle activates per and cry, and the proteins produced by these genes feed back to inhibit Clock/Cycle. Once the inhibitory proteins degrade, Clock/Cycle becomes active again, restarting the cycle.

What is the difference between “cry” and “Cry”?

• “cry” refers to the gene itself.

• “Cry” (capitalized) usually refers to the protein produced by the cry gene.

Circadian Cycle steps

1. Clock and Cycle proteins are produced in the SCN and form a dimer.

2. The Clock/Cycle dimer activates the per and cry genes.

3. PER and CRY proteins accumulate over time.

4. PER and CRY inhibit Clock/Cycle, stopping their own production.

5. PER and CRY degrade, lifting inhibition, and the cycle restarts, taking ~24 hours.

Alert wakefulness:

Beta waves (high frequency, low amplitude), desynchronized EEG.

Relaxed wakefulness:

Alpha waves (low frequency, regular oscillations), occurs when eyes are closed and relaxed.

Non-REM Sleep

has three stages

Stage 1 (N1)

• Transition from wakefulness to sleep.

• Alpha activity decreases; EEG shows smaller amplitudes and irregular frequencies.

• Lasts several minutes.

Stage 2 (N2)

Characterized by sleep spindles (brief bursts of activity) and K-complexes.

Stage 3 (N3 / Slow Wave Sleep, SWS)

• Delta waves dominate (large amplitude, low frequency).

• Cortical activity is synchronized.

• Very restorative sleep stage.

REM Sleep (Rapid Eye Movement / Paradoxical Sleep)

• Occurs about 1 hour after sleep onset.

• EEG shows small amplitude, high frequency activity (similar to wakefulness).

• Rapid eye movements occur.

• Muscle tone is lost (atonia), but brain is highly active.

• Breathing and pulse are irregular, like wakefulness.

• Most vivid dreams occur during REM.

Sleep is divided into what two classes?

• Non-REM (NREM) sleep

• REM (Rapid Eye Movement) sleep

What are the stages of non-REM sleep?

• Stage 1 (N1): Light sleep, transition from wakefulness, decreasing alpha activity.

• Stage 2 (N2): Characterized by sleep spindles and K-complexes.

• Stage 3 (N3 / SWS): Deep sleep, slow-wave sleep, dominated by delta waves.

What is SWS?

• SWS (Slow Wave Sleep) is stage 3 NREM sleep, marked by large-amplitude, low-frequency delta waves and synchronized cortical activity.

• It is the most restorative stage of sleep.

REM sleep characteristics

• postural tension = eliminated

• cerebral cortex activity= increased firing rates

Non-REM sleep characteristics

• Postural tension= progressively reduced

• Cerebral cortex activity= many cells reduced

At puberty, the circadian rhythm of sleep shifts so that _________get up later in the day

teenagers

Dreams during REM

• visual imagery

• nightmares

Dreaming during NON-REM

• more “thinking” type dreams.

• night terrors= sudden awakening or arousal from stage 3 sleep

Various medications tend to ________ the frequency of nightmares, but such dreaming (nightmares) typically occur without any medication.

increase

Dreaming occurs during both non-rem and REM sleep

yes

We tend to sleep more during_

How much REM sleep do infant mammals show?

Similar to human babies; about half of sleep in the first two weeks is REM sleep.

How much REM sleep can premature infants have?

As much as 80% of their sleep, likely reflecting growth and nervous system maturation.

Can human infants go directly from awake to REM sleep?

Yes, unlike most adults, human infants can go directly from the awake state to REM sleep.

How much REM sleep do killer whales and bottlenose dolphins get in their first month?

Little or no REM sleep during the first month of life.

Do we know the exact function of REM sleep?

No, the exact function of REM sleep is still not known.

People tend to sleep less and less as we age

true

Sleep Changes in Older Adults

• Exactly why older individuals have trouble sleeping is not fully known.

• Older individuals have no trouble falling asleep, but have trouble staying asleep.

Stage 3 Sleep (Slow-Wave Sleep)

• These individuals spend much less time in stage 3 sleep (half as much as they did at 20 y/o).

• By 90 years of age, there is no stage 3 sleep.

Possible Causes of Decline

• The decline in stage 3 sleeping may be due to reduced cognitive functioning, as is observed in those with senile dementia.

• The cognitive deficits may be due to reduced secretion of growth hormone, as occurs during SWS.

Sleep deprivation leads to

decreased mental capacity and functioning, decreased immune response, increased lethargy, and tiredness

Various ____systems work together to regulate and orchestrate the sleep process

neural

The forebrain

¡promotes SWS (slow wave sleep) by releasing GABA in the tuberomammillary nucleus of the hypothalamus (these same GABA_A receptors are used by various anesthetics to induce unconsciousness)

The reticular formation

(part of the brainstem- also called the reticular activating system) acts to wake up the forebrain (promoting wakefulness)

The pontine system

(in the pons of the brainstem) triggers REM sleep, in part, by inhibiting motoneurons, leading to increased muscle relaxation

The hypothalamus

integrates information to determine the current stage of sleep: it sends projections to the forebrain region, brainstem reticular formation, and the pontine nuclei

Sudden infant death syndrome (SIDS) is thought to arise from

sleep apnea

SIDS

• crib death; is thought to be due to relaxation of chest muscles and diaphragm (and perhaps muscle in the throat) or from decreased respiratory neuron activity in the brain stem

• the serotonin in the brainstem may be involved

• it is now recommended that babies be placed on their backs to sleep and not their stomachs