Circadian Rhythm

What is a Circadian Rhythm?

A circadian rhythm is an internally generated cycle lasting roughly 24 hours. These rhythms influence sleep timing, hormone secretion, body temperature, and metabolism. Light and other environmental cues help synchronize the body's clocks with the outside world.

Biological Basis

• Suprachiasmatic Nucleus (SCN)

  The suprachiasmatic nucleus (SCN), in the hypothalamus, coordinates the body’s circadian timing. The SCN consists of a group of neurons that receive input from specialized photoreceptive cells in the retina, which allows the SCN to synchronize with the external light-dark cycle.

• Peripheral Clocks

  Aside from the central clock in the SCN, peripheral clocks are found in various tissues and organs throughout the body. The SCN helps coordinate these clocks, which also regulate local processes in their tissues.

• Genetic Mechanisms

  Circadian rhythms are controlled by a complex network of genes and proteins that form transcriptional-translational feedback loops. Key clock genes include CLOCK, BMAL1, PER, and CRY. The expression of these genes and their protein products oscillate over a 24-hour period, driving the cellular and physiological processes that define circadian rhythms.

Functions

• Sleep-Wake Cycle

  One of the most prominent circadian rhythms is the sleep-wake cycle, which influences sleep timing alongside the homeostatic need for sleep. The SCN regulates the secretion of melatonin, a hormone that promotes sleep, in response to changes in light levels, helping to establish a stable sleep-wake cycle.

• Hormone Secretion

  Circadian rhythms also control the release of various hormones, including cortisol, insulin, and growth hormone. These hormones have essential roles in regulating metabolism, stress responses, and growth and development.

• Body Temperature

  Body temperature follows a daily rhythm. Its timing reflects circadian phase and can vary with a person’s schedule and circumstances.

• Metabolism

  Circadian rhythms influence metabolism by regulating the expression of genes involved in nutrient processing, energy storage, and energy expenditure. Disruptions in circadian rhythms can result in metabolic imbalances, contributing to obesity and related disorders.

Associated Disorders

• Sleep Disorders

  Disruptions in circadian rhythms can lead to sleep disorders such as insomnia, delayed sleep phase syndrome, and advanced sleep phase syndrome. These disorders are characterized by difficulty falling asleep, staying asleep, or waking up at appropriate times.

• Mood Disorders

  Alterations in circadian rhythms have been implicated in mood disorders, including major depressive disorder and bipolar disorder. These disorders are often associated with sleep disturbances and irregularities in hormone secretion, which may be related to dysregulated circadian rhythms.

• Metabolic Disorders

  Disturbances in circadian rhythms can contribute to metabolic disorders, such as obesity, diabetes, and metabolic syndrome. These conditions can result from imbalances in energy intake and expenditure, as well as dysregulated hormone secretion, which are influenced by circadian rhythms.

• Shift Work Disorder

  Shift work can create circadian misalignment. Shift work disorder involves insomnia or excessive sleepiness associated with the work schedule; not every shift worker has the disorder. The mismatch between working hours and biological timing can make it difficult to sleep when time is available or stay awake when work requires it.

Sources and evidence

• National Institute of General Medical Sciences: Circadian Rhythms. Rhythms are approximately 24 hours and timing varies across individuals and conditions.