circadian rhythm

1. A daily cycle of biological activity based on a 24-hour period and influenced by regular variations in the environment, such as the alternation of night and day. Circadian rhythms include sleeping and waking in animals, flower closing and opening in angiosperms, and tissue growth and differentiation in fungi.

2. See also biological clock

circadian rhythm

1. An activity cycle lasting twenty-four hours. Many living things, including humans, follow a circadian rhythm. (See biological clock.)

A Closer Look

The circadian rhythm, present in humans and most other animals, is generated by an internal clock that is synchronized to light-dark cycles and other cues in an organism's environment. This internal clock accounts for waking up at the same time every day even without an alarm clock. It also causes nocturnal animals to function at night when diurnal creatures are at rest. Circadian rhythms can be disrupted by changes in daily schedule. Biologists have observed that birds exposed to artificial light for a long time sometimes build nests in the fall instead of the spring. While the process underlying circadian rhythm is still being investigated, it is known to be controlled mainly by the release of hormones. In humans, the internal clock is located within the brain's hypothalamus and pineal gland, which releases melatonin in response to the information it receives from photoreceptors in the retina. Nighttime causes melatonin secretion to rise, while daylight inhibits it. Even when light cues are absent, melatonin is still released in a cyclical manner.

Example Sentences

"These things are not rocket science, but they're really fundamental to having a good, healthy circadian rhythm and a good healthy physiology through the winter."

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A team of researchers at Washington University School of Medicine in St. Louis has now demonstrated, in mouse models, that Alzheimer's disease disrupts circadian rhythms within specific brain cells.

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Horner noted that future studies could test whether reducing screen exposure in the hours before bedtime, when screen light may disrupt circadian rhythms and delay sleep onset, helps lower cardiometabolic risk.

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She notes the importance of keeping waking and sleeping times similar throughout the year to regulate your circadian rhythm.

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“When the bioenergetics are damaged, you can imagine that it has detrimental effects … impacting your immune system and circadian rhythm.”

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