Daylight Saving Time
dst
The practice of advancing clocks by one hour during warmer months to extend evening daylight.
Circadian Rhythm
circadian rhythm
Overview
Circadian rhythm (from the Latin circa, "about," and dies, "day") is an internal clock mechanism that oscillates with a period of roughly 24 hours. The master clock resides in the suprachiasmatic nucleus (SCN) of the brain and orchestrates sleep-wake cycles, daily body temperature variation, cortisol and melatonin secretion rhythms, and digestive activity patterns throughout the body.
Free-Running Period
The period the human circadian clock keeps on its own - its free-running period - averages about 24.2 hours rather than exactly 24 when measured under laboratory conditions that control for light and behavioral schedules. Because that period differs slightly from 24 hours, daily light exposure is essential to reset (entrain) the clock. Without this entrainment the internal clock drifts later by roughly 12 minutes a day, the difference between 24.2 and 24 hours.
Effects of Disruption
Circadian disruption manifests as jet lag, shift-work disorder, and social jet lag. In the short term it causes sleep disturbance, impaired concentration, and digestive problems. Chronic disruption has been linked by epidemiological studies to elevated risks of cardiovascular disease, metabolic disorders, depression, and cancer. Crossing time zones and daylight saving time transitions are external perturbations that challenge circadian stability.
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Related Terms
Time Zone
timezone
A region of the globe that observes a uniform standard time for legal, commercial, and social purposes.
JST (Japan Standard Time)
jst
JST (Japan Standard Time) is the standard time of Japan at UTC+9, based on the 135th meridian east. Japan does not observe daylight saving time.
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