What is "chronotype"

n. The totality of factors, associated with circadian rhythms, that determine if an individual is most alert in the morning or evening

Chronotype refers to the behavioral manifestation of underlying circadian rhythms of myriad physical processes. A person's chronotype is the propensity for the individual to sleep at a particular time during a 24-hour period. 'Eveningness' (delayed sleep period) and 'morningness' (advanced sleep period) are the two extremes with most individuals having some flexibility in the timing of their sleep period. However, across development there are changes in the propensity of the sleep period with pre-pubescent children preferring an advanced sleep period, adolescents preferring a delayed sleep period and many elderly preferring an advanced sleep period.

The causes and regulation of chronotypes, including developmental change, individual propensity for a specific chronotype, and flexible versus fixed chronotypes have yet to be determined. However, research is beginning to shed light on these questions, such as the relationship between age and chronotype. There are candidate genes (called clock genes) that exist in most cells in the body and brain, referred to as the circadian system that regulate physiological phenomena (hormone levels, metabolic function, body temperature, cognitive faculties, and sleeping). With the exception of the most extreme and rigid chronotypes regulation is likely due to gene-environment interactions. Important environmental cues ( zeitgebers) include light, feeding, social behavior, and work and school schedules.

Humans are normally diurnal creatures, that is to say they are active in the daytime. As with most other diurnal animals, human activity-rest patterns are endogenously controlled by biological clocks with a circadian (~24-hour) period.

Normal variation in chronotype encompasses sleep–wake cycles that are from about two hours earlier to about two hours later than average. Extremes outside of this range can cause a person difficulty in participating in normal work, school, and social activities. If a person's " lark" or (more commonly) " owl" tendencies are strong and intractable to the point of disallowing normal participation in society, the person is considered to have a circadian rhythm sleep disorder.