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
When all external time cues, such as light, meals, and social schedules, are removed, the human circadian clock free-runs at a period of about 24.2 hours. Because this differs from exactly 24 hours, daily light exposure is essential to reset (entrain) the clock. Without this entrainment the internal clock drifts roughly 12 minutes later each day.
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.
Was this article helpful?
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.
Related Articles
Jet Lag Recovery Strategies - Science-Based Methods for Faster Adjustment
Discover evidence-based strategies to minimize jet lag when crossing multiple time zones, including light exposure timing, meal scheduling, and pre-trip adjustment techniques.
Resetting the Circadian Rhythm - The Science of Light, Meals, and Movement
The body's internal clock is not exactly 24 hours, so it must be reset daily. This article explains the suprachiasmatic nucleus, how light, meal timing, and exercise act as zeitgebers, and how shift workers and travelers can apply the science to recover faster.
Health Effects of Chronic Time Differences - Long-Term Risks and Prevention
Frequent international travel and shift work expose people to chronic circadian disruption, which carries cardiovascular, metabolic, immune, and cognitive risks. This article surveys the epidemiological evidence and offers practical strategies for reducing the impact.