This ability to align behaviors such as feeding and activity to the external environmental cycle is a process called entrainment.
[5] Biological rhythms, including cycles related to sleep and wakefulness, mood, and cognitive performance, are synchronized with the body's internal circadian clock.
[6] Normally however, external cues like light-dark cycles and social interactions also exert an influence on the body's rhythms.
Early research into circadian rhythms demonstrated that, when humans are without zeitgebers, or in constant lighting conditions, they have a "free running" period of 24.9 hours.
Such zeitgeber disruptions can also lead to decreased cognitive performance, negative mood, and in some cases, episodes of mental illness.
Researchers have shown that the 24-hour circadian clock also influences cognitive performance in a wide variety of paradigms, including serial search, verbal reasoning, working memory tasks, suppressing wrong answers, and manual dexterity.
[11] This variation in the performance of various tasks is attributable to a number of factors, including relative working memory load, change in strategy, hemispheric dominance, ability to suppress wrong answers, age, level of practice, and morningness-eveningness, many of which fluctuate according to time of day.
[1] Recent work has also demonstrated that interventions like light therapy, sleep deprivation, and some pharmacological antidepressants may be effective in treating depression by reordering these rhythms to their natural state.
This adjustment can be difficult and may lead to disruptions in sleep quality and quantity, and possibly increase risk for depression as a result.
Researchers have attempted to explore the effect life events that disturb social rhythms might have on depressive symptoms in a number of ways.
As of the year 2000, studies have also found a connection between the disruption of social rhythms and the triggering of manic episodes in bipolar disorder.
[15] Because circadian clocks synchronize human sleep-wake cycles to coincide with periods of the day during which reward potential is highest – that is, during the daytime[16] – and recent studies have determined that daily rhythms in reward activation in humans are modulated by circadian clocks as well,[16] external influences on those rhythms may influence an individual's mood.