A chronotype is an attribute of animals and human beings describing the time of the day their physical functions such as body temperature, cognitive faculties, eating and sleeping reach a certain level. This phenomenon refers to people as early birds or night owls, where morning people wake up early and are most alert in the first part of the day, and evening people are most alert in the late evening hours and prefer to go to bed late.
Humans are normally diurnal creatures that are active in the daytime. As with most other diurnal animals, human activity-rest patterns are endogenously controlled by circadian rhythms. 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.
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 early bird or night 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 Morningness-Eveningness Questionnaire, or MEQ, is used to conduct research on this topic. A short version can be found online. Several other assessment tools have been developed such as the Composite Scale of Morningness, the Lark-Owl Chronotype Indicator, and the Munich Chronotype Questionnaire. 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.