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

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.

Shift Work and Biological Rhythms

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. 

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- Circadian rhythms
- Infradian rhythms
- Ultradian rhythms
- Free-running

Types of biological rhythms and clocks

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

A pattern centered around behavioral, biochemical, or physiological fluctuation whose period is longer than a circadian rhythm. Frequency of occurrence is less than once per day. Examples include the menstrual cycle, or seasonal shifts in body weight.

- Circannual 

Infradian rhythms

A pattern centered around behavioral, biochemical, or physiological fluctuation whose period is shorter than a circadian rhythm. These periods usually range from several minutes to hours. Ultradian rhythms can be a part of a circadian rhythm. For example, humans have a 90 minute ultradian cycle of within the circadian sleep-wake cycle. 

Ultradian rhythms

A rhythm of behavior shown by an animal deprived of external cues about time of day, where the animal maintains its own cycle. Not exactly 24 hours long. Otherwise known as the natural rhythm, it is not just a reflection of external cues. 

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