Circadian Rhythm

The central clock in the brain's suprachiasmatic nucleus responds strongly to light information from the eyes. Clocks also operate in tissues throughout the body. Together they influence sleep timing, hormone release, body temperature, metabolism, alertness, and other recurring patterns. Circadian timing is related to sleep but is not the same as sleep duration or sleep quality.

Light exposure, meal timing, activity, travel, shift work, and social schedules can shift or challenge clock timing. Cortisol and melatonin follow circadian patterns, but a single measurement does not map the entire system. Individual chronotype also affects preferred timing, and normal differences should be distinguished from a clinically significant sleep-wake disorder.

Sleep architecture describes the pattern of stages within a sleep period, whereas circadian rhythm helps determine when sleep and wakefulness are biologically favoured. Consumer devices estimate aspects of both with varying accuracy. Persistent sleepiness, insomnia, or major schedule disruption deserves qualified assessment because many medical, behavioural, and environmental factors can contribute.

A circadian phase marker refers to where a person is within an internal cycle, not simply the clock time shown on a wall. Two people exposed to the same local time can have different biological timing because of schedule, travel, light history, or chronotype. This distinction is central when interpreting sleep timing.