Chronotype

A chronotype is 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. A chronotype is 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. Additional research has proposed an evolutionary link between chronotype and nighttime vigilance in ancestral societies. 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. Chronotypes have also been investigated in other species, such as fruit flies and mice. Normal variation in chronotype encompasses sleep–wake cycles that are two to three hours later in evening types than morning types. 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. The 20th century saw greatly increased interest in and research on all questions about sleep. Tremendous strides have been made in molecular, neural and medical aspects of biological rhythmicity. Physiology professor Nathaniel Kleitman's 1939 book Sleep and Wakefulness, revised 1963, summarized the existing knowledge of sleep, and it was he who proposed the existence of a basic rest-activity cycle. Kleitman, with his students including William C. Dement and Eugene Aserinsky, continued his research throughout the 1900s. O. Öquist's 1970 thesis at the Department of Psychology, University of Göteborg, Sweden, marks the beginning of modern research into chronotypes, and is entitled Kartläggning av individuella dygnsrytmer, or 'Charting Individual Circadian Rhythms'. Olov Östberg modified Öquist's questionnaire and in 1976, together with J.A. (Jim) Horne, he published the 19-item morningness–eveningness questionnaire, MEQ, which is still used and referred to in virtually all research on this topic. Researchers in many countries have worked on validating the MEQ with regard to their local cultures. A revision of the scoring of the MEQ as well as a component analysis was done by Jacques Taillard et al. in 2004, working in France with employed people over the age of 50. Previously the MEQ had been validated only for subjects of university age.