lecture 3 - Circadian Rhythms

Study Notes

Circadian Rhythms

Circadian rhythms are biological processes with an approximately 24-hour cycle length that are generated internally (endogenous).
Zeitgebers are external cues that synchronize the body's internal clock, such as the suprachiasmatic nucleus (SCN).
Circadian rhythms regulate sleep-wake cycles, eating, drinking, body temperature, and hormone release.

Why We Have Circadian Rhythms

Circadian rhythms allow living species to sync their behavior and internal biology with the Earth's 24-hour light/dark rotation.

History of Circadian Rhythms

Jean-Jacques d'Ortous de Mairan (1729) demonstrated circadian rhythms in plants by studying leaf movements over a 24-hour period, even in the absence of light.
Kleitman and Richardson (1938) spent six weeks in Mammoth Cave, Kentucky, demonstrating the endogenous nature of human circadian rhythms.
They found that the human circadian clock generates a rhythm slightly longer than 24 hours in the absence of external cues.
The average circadian rhythm for most humans is estimated to be approximately 24 hours and 10 minutes.

Body Temperature and Circadian Rhythm

Body temperature reaches its lowest point for the day about two hours after sleep onset and peaks approximately six hours before sleep onset.

Master Clock and Regulation

The suprachiasmatic nucleus (SCN), located in the hypothalamus, is the master clock that regulates circadian rhythms.
The SCN controls brain and body activity levels to regulate wake and sleep patterns.
The SCN uses melatonin, a chemical messenger released from the pineal gland, to regulate circadian rhythms.

Melatonin

Melatonin is released from the pineal gland, beginning about two hours before bedtime.
Light suppresses melatonin secretion, while darkness triggers its release.
Melatonin remains high throughout the night and is suppressed during the day.

Zeitgebers and Circadian Rhythm Resetting

Zeitgebers are any stimuli that reset our daily circadian rhythm to precisely 24 hours.
Light is the most important zeitgeber due to its reliable 24-hour cycle.

Circadian Rhythm Disruptions

Jet Lag is caused by a mismatch between the internal circadian clock and external time due to crossing time zones.
Melatonin pills can help phase-advance the internal clock, making it easier to sleep in a new time zone.

Two-Process Model of Sleep-Wake Regulation

The circadian factor process (Process C) is one of the two processes governing sleep-wake rhythms.
As the circadian rhythm increases, our drive to be awake increases and vice versa.

Description

Explore the fascinating world of circadian rhythms and their significance in biological processes. Understand how internal clocks and external cues, like light and darkness, influence behaviors such as sleep, eating, and hormone release. Learn about the historical studies that shaped our knowledge of these rhythms.