Circadian rhythm

Diagram of the human circadian rhythm.

The circadian rhythm is a natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours. It is driven by a circadian clock, and the term "circadian" comes from the Latin "circa diem," meaning "around a day." This rhythm is found in most living organisms, including animals, plants, and many tiny microbes.

Biological Basis

The circadian rhythm is controlled by the suprachiasmatic nucleus (SCN), a group of cells located in the hypothalamus of the brain. The SCN receives direct input from the retina of the eye, allowing it to synchronize with the external environment, particularly the light-dark cycle.

Molecular Mechanisms

At the molecular level, circadian rhythms are generated by a set of core clock genes and proteins that form feedback loops. These include genes such as CLOCK, BMAL1, PER, and CRY. These genes and their protein products interact in a complex network to produce oscillations in gene expression and protein activity that cycle over approximately 24 hours.

Functions of Circadian Rhythms

Circadian rhythms influence various physiological processes, including:

• Sleep: The sleep-wake cycle is one of the most well-known circadian rhythms. The SCN signals the pineal gland to produce melatonin, a hormone that promotes sleep, in response to darkness.
• Hormone Secretion: Hormones such as cortisol and growth hormone are released in a circadian pattern.
• Body Temperature: Body temperature fluctuates throughout the day, typically peaking in the late afternoon and reaching its lowest point in the early morning.
• Metabolism: Circadian rhythms regulate metabolic processes, influencing appetite, digestion, and energy expenditure.

Disruption of Circadian Rhythms

Disruptions to circadian rhythms can have significant health implications. Common causes of disruption include:

• Jet Lag: Rapid travel across time zones can desynchronize the internal clock from the local time.
• Shift Work: Working at night or irregular hours can lead to chronic misalignment of circadian rhythms.
• Sleep Disorders: Conditions such as insomnia and delayed sleep phase disorder are associated with circadian rhythm disturbances.

Health Implications

Chronic disruption of circadian rhythms has been linked to various health issues, including:

• Cardiovascular Disease
• Obesity
• Diabetes
• Mood Disorders: Such as depression and bipolar disorder.

Circadian Rhythm in Other Organisms

Circadian rhythms are not unique to humans. In plants, they regulate processes such as photosynthesis and flowering. In animals, they influence behaviors such as migration and hibernation.

Related Pages

• Sleep
• Suprachiasmatic nucleus
• Melatonin
• Jet lag
• Shift work sleep disorder