Entrainment

Entrainment is the synchronization of an endogenous biological rhythm to a periodic external cue, such as light or temperature.

Zeitgeber: A periodic environmental cue that synchronizes an organism’s biological rhythms. Zeitgebers provide the recurring signals to which biological rhythms become synchronized.
Light therapy: A treatment that uses timed exposure to bright light to alter biological timing or mood. Timed light can shift circadian phase and help realign rhythms with the day.
Biological rhythm: A recurring pattern in biological activity with a characteristic period. Entrainment acts on an existing recurring pattern rather than creating the general category of rhythm.
Masking (chronobiology): A direct effect of an environmental stimulus on measured behavior that does not require shifting the biological clock. Light can change activity immediately through masking without entraining the underlying rhythm.
Phase response curve: A graph showing how a stimulus at different points in a cycle shifts an oscillator’s phase. It predicts whether a cue advances or delays a rhythm at a given biological time.
Jet lag: A temporary mismatch between the circadian system and local time after rapid travel across time zones. Recovery depends on the clock re-entraining to the destination’s light-dark cycle.
Circadian rhythm: A biological rhythm with a period of approximately 24 hours. Daily entrainment is most commonly discussed in relation to circadian rhythms.
Forced desynchrony: A research protocol that schedules sleep and wake on a cycle outside the range of circadian entrainment. It separates circadian timing from sleep-wake behavior rather than synchronizing them.
Circadian oscillator: An internal timekeeping system that generates rhythms with periods near 24 hours. Its self-sustained cycle is the rhythm that daily cues synchronize.
Shift work: Employment requiring work schedules outside conventional daytime hours. Night work can place behavioral demands out of step with light-entrained circadian rhythms.

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