When animals continue to maintain their own cycle even when they aren't provided external cues about time of day, it is called free-running. The cycle during free-running is not exactly 24 hours. 
- changes in circadian rhythm
- hypothalamus 

Free running

The process by which circadian rhythms can be reset by light. 

* Phase delay: time of sleep and wake move to later in the day, due to light exposure in the first half of the night.
* Phase advance: time of sleep and wake move to earlier in the day, due to light exposure in the second half of the night..

Entrainment

The suprachiasmatic nucleus (SCN) of the hypothalamus regulates circadian rhythms. Light activates intrinsically photosensitive retinal ganglion cells, causing the release of glutamate onto SCN neurons. This eventually leads to the transcription of the circadian regulating genes Per1 and Per2. The SCN is referred to as the "circadian pacemaker" because it oscillates in a circadian fashion.

Suprachiasmatic nucleus in circadian rhythms

An internal biological clock that allows animals to proceed with their circadian rhythms. Usually, the internal clock is dependent on light. However, animals will still continue their circadian rhythm even in the absence of external cues. For example, even if it is constantly light or dark, animals will proceed with their circadian rhythm. 
- free running

University of Rochester

Google

A pattern centered around behavioral, biochemical, or physiological fluctuation that repeats every 24 hours. Major value of a circadian rhythm is synchronizing behaviors and body states with environmental changes, providing temporal organization. 

Circadian Rhythms

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

Endogenous clock and circadian rhythm

Irregular sleepers had less exposure to light during the day compared to regular sleepers. Irregular sleepers also had a smaller difference between the amount of light they were exposed to during the day and during the night. This suggests that the circadian rhythms of irregular sleepers is delayed.

Irregular Sleep and Light/Dark Cycle

Circadian Rhythems during the COVID-19 Pandemic

Psychiatry tends to use the circadian cycle to explain illness genesis and illness vulnerability. This is most prominant for mood disorders where a disruption of the 24-hour system occurs before, during and after an episode.

There is also evidence that circadian abnormalities are part of mood disorders functioning as either a cause or a result.

Circadian rhythem involvement in mood disorders

Zeitgebers are environmental cues that help align biological clocks with the external environment. Light is an important cue for setting this clock.

Zeitgebers (time givers)

The sleep-wake cycle is a fundamental circadian rhythm that is internally regulated by the body's biological clock. This process governs the daily pattern of sleeping and waking, ensuring it is synchronized with the 24-hour light-dark cycle of the environment.

Sleep-Wake Cycle

A collection of symptoms that arises from a mismatch between the body's internal circadian cycles and the external environment. A common cause for this desynchronization is traveling across multiple time zones.

Jet Lag

Based on the following case study, analyze the most likely cause of the nurse's symptoms. Explain how the change in their work schedule leads to these specific physiological and psychological effects, referencing the body's internal 24-hour cycle.

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