The Circadian rhythm is an intrinsic biochemical cycle in most mammals that is responsible for the integration of sleep time with external cues, such as light and darkness (day and night).

It is naturally 25 or 26 hours, but the coincidental 24-hour cycle of light and darkness compresses the circadian rhythm into 24 hours. While the circadian rhythm typically integrates sleep with day and night, promoting sleep during nighttime and consciousness during day time, suppression of optical somatosensory input and lack of knowledge about current day/night status will force the circadian rhythm to use other sensory cues to coordinate sleep times.

The circadian rhythm is coordinated by a biological "clock" nervous system structure, the Super-chiasmic nucleus (SCN). The SCN is a GABAergic structure that inhibits downstream structures. It is maximally active during darkness, inhibiting more downstream targets during darkness than during periods of light, suggesting that it plays a role in sleep and sleep's parasympathetic effects.