circadian rhythm

Meaning: A circadian rhythm is an internal, self-sustaining cycle lasting about 24 hours. In mammals the master clock is the suprachiasmatic nucleus (SCN) of the hypothalamus. Retinal ganglion cells that detect light send input to the SCN, which entrains (synchronizes) the clock to the environment. The SCN signals the pineal gland, which releases melatonin in darkness, promoting sleepiness. Light suppresses melatonin release. Body temperature, cortisol, and alertness also follow circadian patterns.

On the MCAT: Passages describe shift work, jet lag, or a person kept in constant darkness or in a cave without clocks. In constant conditions the rhythm persists ("free-runs") with a period close to, but often not exactly, 24 hours, showing it is internally generated. Expect questions that predict how bright light in the evening or morning shifts sleep timing, or what happens to melatonin when light exposure changes. Jet lag is a mismatch between the internal clock and the local light-dark cycle. You may also be asked to link the rhythm to the sleep-wake cycle and its stages.

Common confusion: Students treat the rhythm as caused by light itself. Light is a zeitgeber that resets the clock, but the rhythm continues without it. Also, circadian rhythms (about 24 hours) are not the same as the roughly 90-minute cycle of sleep stages, which is an ultradian rhythm.