Chronotype

Chronotype is an individual's characteristic timing preference for sleep and activity relative to the day–night cycle. It reflects the interaction of circadian biology, age, and environmental cues.

Circadian rhythm: A roughly 24-hour cycle in biological processes, synchronized chiefly by light and darkness. The circadian system provides the daily timing framework in which chronotypes differ.
Social jetlag: The mismatch between biological sleep timing and sleep schedules imposed by social obligations. Late chronotypes often accumulate this mismatch when work or school begins early.
Sleep: A recurring, reversible state of reduced responsiveness and altered brain activity essential to health. Chronotype concerns sleep timing, not sleep itself or the amount required.
Adolescent sleep phase delay: A developmental tendency for sleep timing to shift later during adolescence. This age-related shift helps explain why eveningness is common among teenagers.
Morningness–eveningness: A continuum describing preference for activity and alertness earlier or later in the day. This is the common preference dimension used to describe chronotype.
Suprachiasmatic nucleus: A hypothalamic brain region that coordinates circadian timing across the body. This central clock helps align sleep timing with the external day-night cycle.
Chronotherapy: A treatment approach that schedules interventions according to biological rhythms. Circadian timing can guide when light or sleep schedules are used to shift phase.
Circadian phase: The position of a biological rhythm within its recurring cycle at a given time. Phase specifies where an individual's internal day falls relative to clock time.
PER3: A gene encoding a component of the circadian clock, with variants studied in relation to sleep timing and other traits. Genetic variation in clock-related biology contributes to differences in timing preference.
Lark and owl: Informal labels for people who prefer earlier or later daily schedules, respectively. These familiar metaphors simplify chronotype into early and late patterns.

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