Chronotype is an attribute of human beings, reflecting at what time of the day their physical functions (hormone level, body temperature, cognitive faculties, eating and sleeping) are active, change or reach a certain level. This phenomenon is commonly reduced to sleeping habits only, referring to people as "larks" and "owls" which refer to, respectively, morning people (those who wake up early and are most alert in the first part of the day) and evening people (those who are most alert in the late evening hours and prefer to go to bed late).
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 period.
Normal variation in chronotypes 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.
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 Morningness-eveningness questionnaire, MEQ, which still is 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.
Several other assessment tools have been developed: the Circadian Type Inventory (Folkard 1987); the Composite Scale of Morningness (Smith 1989); the Lark-Owl Chronotype Indicator, LOCI (Roberts 1999); and the Munich Chronotype Questionnaire, MCTQ (Roenneberg 2003). Some of these are designed with particular situations in mind, such as shift work scheduling, travel fatigue and jet lag, athletic performance or best timing of medical procedures.
Most people are neither evening nor morning types but lie somewhere in between. Estimates vary, but up to half are either morning or evening people. People who share a chronotype, morningness or eveningness, have similar activity-pattern timing: sleep, appetite, exercise, study etc. Researchers in the field of chronobiology look for objective markers by which to measure the chronotype spectrum.
- Horne and Östberg, 1976, found that morning types had a higher daytime temperature with an earlier peak time than evening types and that they went to sleep and awoke earlier, while no differences in sleep lengths were found. They also note that age should be considered in assessments of morningness and eveningness, noting how a "bed time of 23:30 may be indicative of a Morning type within a student population, but might be more related to an Evening type in the 40–60 years age group" (Horne & Östberg, 1976, p. 109).
- Clodoré et al., France, 1986, found differences in alertness between morning and evening types after a two hour sleep reduction.
- Gibertini et al., US, 1999, assessed blood levels of the hormone melatonin, finding that the melatonin acrophase (the time at which the peak of a rhythm occurs) was strongly related to circadian type, while amplitude was not. They note that morning types evidence a more rapid decline in melatonin levels after the peak than do evening types.
- Duffy et al., US, 1999, investigated "changes in the phase relationship between endogenous circadian rhythms and the sleep-wake cycle," and found that while evening types woke at a later clock hour than morning types, morning types woke at a later circadian phase.
- Baehr et al., US, 2000, found that, in young adults, the daily body temperature minimum occurred at about 4 a.m. for morning types but at about 6 a.m. for evening types. This minimum occurred at approximately the middle of the eight hour sleep period for morning types, but closer to waking in evening types. Evening types had a lower nocturnal temperature. The temperature minimum occurred about a half hour earlier in women than in men. Similar results were found by Mongrain et al. in Canada, 2004.
- Zavada et al., The Netherlands, 2005, show that the exact hour of mid-sleep on free (non-work) days may be the best marker for sleep-based assessments of chronotype; it correlates well with such physiological markers as Dim-Light Melatonin Onset (DLMO) and the minimum of the daily cortisol rhythm. The researchers also state that each chronotype category "contains a similar portion of short and long sleepers."
- Giampietro and Cavallera, Italy, 2006, refer to many studies in their examination of the relationship among chronotypes, personality and creative thinking.
- Paine et al., New Zealand, 2006, conclude that "morningness/eveningness preference is largely independent of ethnicity, gender, and socioeconomic position, indicating that it is a stable characteristic that may be better explained by endogenous factors."
- Chung et al., Taiwan, 2009, studied sleep quality in shift-working nurses and found that "the strongest predictor of sleep quality was morningness-eveningness not the shift schedule or shift pattern," as "evening types working on changing shifts had higher risk of poor sleep quality compared to morning types."
- American Academy of Sleep Medicine International Classification of Sleep Disorders, Revised Edition 2001.