Class 20: Rhythms of Waking and Sleeping

Chronobiology

how time and seasons affect the biological functioning of living organisms

Each year=

circannual

Each month=

circamensual

Each day=

circadian

endogenous

If changes happen because of mechanisms in the body("born from within")

exogenous

If changes happen because of mechanisms outside the body("born from outside")

Zeitgeber

1. daily environmental signals or stimuli that reset the circadian rhythm

2. seasonal environmental signals that set the circannual rhythm (e.g., migration, mating, hibernation) of an animal

"entrainment"

the process of resetting the biological clock of an animal

Light

plays a critical role in setting/resetting the biological clock of land animals, especially humans

fluctuations in the body's internal temperature is normally regulated by the suprachiasmatic nucleus (SCN)…

—> multiple cells in the body, in turn, respond to these fluctuations with increased or decreased levels of activity

less important stimuli (Zeitgeber)

• noise, activity level (exercise), and environmental/ambient temperature

• food: feeding and metabolism affects circadian rhythm

Jet Lag

= disruption of circadian rhythms due to travel across time zones

Jet Lag: Problems

• stress, elevated cortisol levels,

• prolonged exposure to cortisol: harms memory & immune system

• most people find it easier to travel from east to west across time zones rather than west to east

Shift Work

• frequent changes in sleep times →trouble sleeping in the afternoon or early evening

• may not experience enough bright light at work to reliably shift their biological clock

Jet Lag & shift work may be causing…

obesity and other metabolic problems by disrupting the daily rhythms of gut microbe (microbiome tends to lose its rhythmicity)

Social jetlag (SJL)

the time we MUST get up for work, school, etc. is often different and much shorter than the time our bodies would prefer (chronic stress factor)

"morning people" or "larks"

as day progresses, they tend to become less productive, report being happier

"evening people" or "owls"

• reach their peak in the late afternoon or evening

• lower grades → often in school earlier than their circadian rhythm prefers

• higher level of alcohol use, overeating, other risky behaviors

main control for internal body temperature and sleep resides in…

the superchiasmatic nucleus (SCN) of the hypothalamus

Cells of the SCN

• depend upon a genetically-based control system to follow a 24-hour cycle

• sends a signal which ends up at the pineal gland (controls activity level of this gland in the posterior of the thalamus)

melanopsin

• light hits special retinal ganglion cells which use this photopigment

• retina near nose

• maximally sensitive to light of ca. 460-480 nm wavelengths, that is, violet to blue

melanopsin ganglion (M-G) cells

• intrinsically-photosensitive Retinal Ganglion Cells or ipRGCs)

• transmit info about ambient light along the retinohypothalamic tract to the SCN

• not sensitive to rapid or small changes, only overall light level (changes gradually)

The pineal gland

releases melatonin

Melatonin

• a hormone that increases sleepiness

• secreted at night about 1-3 hours before we normally go to bed

• highest amount secreted in the middle of the night

Melatonin Pills

• (one moderate dose of 0.5 mg) in the afternoon → reset the biological clock by advancing it → person sleeps earlier and wakes up earlier the next morning

• "short term" use of melatonin (1-2 months) is safe for problems with sleep (should not be used in chronic insomnia)

Studies on Melatonin have found…

• helpful in adults with respiratory, metabolic, and sleep disorders

• little effect in adults who are coping with mental disorders, neurodegenerative diseases, breast cancer, migraine, alcohol use disorder, IBS, or brain injury

High doses of melatonin

• in rats, cause a Parkinsonian-like movement disorder, lowers fertility, and harms developing fetus

• many over-the-counter capsules contain very high single doses (10 times normal level)