Chronotype
« Back to Glossary IndexA chronotype is our biological timing preference for sleep, wakefulness, and peak cognitive or physical performance across the 24-hour day. It reflects the phase position of our internal circadian clock relative to the external light–dark cycle. In research, chronotype is plotted along a morningness – eveningness continuum:
- Morningness (early bird) reflects an earlier circadian phase position: earlier melatonin onset, earlier sleep timing, and earlier peak alertness.
- Eveningness (night owl) reflects a later circadian phase position: delayed melatonin onset, later sleep timing, and later peak alertness.
Objective chronotype markers include dim-light melatonin onset (DLMO), lowest point of core body temperature, and the midpoint of sleep on weekend days (correcting for sleep debt). Chronotype is shaped by genetic variation in core clock genes — PER, CLOCK, ARNTL — as well as age, sex, light exposure, and social structure. It’s not simply a habit or preference, but reflects underlying circadian phase biology. However, when chronotype is chronically misaligned with environmental demands (work schedules, school start times, social obligations) — sometimes called social jet lag — sleep restriction and circadian strain can occur. In bipolar disorder research, stronger eveningness is associated with greater mood episode frequency, sleep irregularity, and markers of circadian vulnerability. Chronotype itself is not pathological. But pronounced eveningness, particularly when combined with irregular routines, may increase risk in susceptible individuals.
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