The rhythm is entrained by external cues known as zeitgebers
18: By the time you notice all of the following zeitgebers
(a German word for time keeper or marker), Early Winter could well be here: Budding Christmas Cacti, the opening of climbing bittersweet, juncos at the bird feeder, the fall of the last maple and the burning bush leaves, a killing frost on your tomatoes, poinsettias in the supermarket, sparrow hawks on the high wires, scouting the fields for prey.
This brain region controls the internal circadian pacemaker, which is synchronized to the hour of the day by both external zeitgebers
(time givers, or cues) and internal cues.
The circadian system is acutely sensitive to environmental photic cues, with natural light being one of the universal and predominant environmental time cues, or zeitgebers
Entrainment in calorie-restricted mice: conflicting zeitgebers
and free-running conditions.
Frank said based on her clinical experience--she has been following some patients for 10-12 years--people with bipolar disorder are vulnerable to disruptions in their social zeitgebers
The light-dark cycle is the most important zeitgeber
or time-cue for resetting the sleep/wake rhythm to the 24-hour day, with other environmental cues such as regular social interactions and meals acting as secondary zeitgebers