As with most other diurnal animals, human activity-rest patterns are endogenously regulated by biological clocks with a circadian (~24-hour) period.
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".
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.
A revision of the scoring of the MEQ as well as a component analysis was done by Jacques Taillard et al. in 2004,[10] working in France with employed people over the age of 50.
An 18-item version was used as part of the larger Standard Shiftwork Index (SSI) in a study conducted by Barton and colleagues.
[17] Till Roenneberg's Munich Chronotype Questionnaire (MCTQ) from 2003 uses a quantitative approach; his many thousands of subjects have answered questions about their sleep behavior.
Estimates vary, but a 2007 survey of over 55,000 people by Roenneberg et al. showed that morningness–eveningness tends to follow a normal distribution.
[18] People who share a chronotype, morningness or eveningness, have similar activity-pattern timing: sleep, appetite, exercise, study etc.
Paine et al.[20] 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".
[9]: 109 Clodoré et al. found differences in alertness between morning and evening types after a two-hour sleep reduction.
[37] The variant most strongly associated with chronotype occurs near RGS16, which is a regulator of G-protein signalling and has a known role in circadian rhythms.
By temporally regulating cAMP signalling, Rgs16 has been shown to be a key factor in synchronising intercellular communication between pacemaker neurons in the suprachiasmatic nucleus (SCN), the centre for circadian rhythm control in humans.
[37][38] PER2 is a well-known regulator of circadian rhythms and contains a variant recently shown to be associated with iris formation.
[37][39][40] The gene ASB1, associated with eveningness and a tendency to day-napping is a result of interbreeding between archaic and modern humans and is originally a Neanderthal trait, possibly linked to a more crepuscular lifestyle in this species.