All humans are synchronised to the rhythmic light-dark changes that occur on a daily basis. Rhythms in physiological and biochemical processes and behavioural patterns persist in the absence of all external 24-hour signals from the physical environment, with a period that is close to 24 hours. These rhythms are referred to as 'circadian', from the Latin 'circa diem' ('about a day'), and are attributable to internal biological clocks, driven by a major circadian pacemaker in the brain. The circadian pacemaker is entrained each day to the 24-hour solar cycle, which is the major ´zeitgeber´ (literally time-giver). Other zeitgebers are food intake, activity, or social cues, e.g. the alarm clock.
Good temporal entrainment allows for optimal performance at the right time of the day, because being able to anticipate future tasks allows the appropriate physiological and psychological preparation. However, our modern society often imposes deviations from the natural light-dark cycle which results in problems with entrainment.
Failure to adapt to environmental and societal time cues (being out of rhythm with nature) leads to misalignment of internal biological clocks. This disentrainment comes with enhanced risk of errors and accidents, loss of productivity, and health risks such as increased propensity for cancer, depression, sleep disturbances, gastrointestinal, metabolic and cardiovascular disorders, decreased immune responses and even life span.