Chronotype

History

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".

Morningness–eveningness questionnaire

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.

Circadian Type Inventory

The Circadian Type Inventory, developed by Folkard (1987), is an improved version of the 20-item Circadian Type Questionnaire (CTQ).

Composite Scale of Morningness

Smith et al. (1989) analyzed items from MEQ, Diurnal Type Scale, and CTQ and chose the best ones to develop an improved instrument, the 13-item Composite Scale of Morningness (CSM or CS). CSM consists of 9 items from the MEQ and 4 items from the Diurnal Type Scale and is regarded as an improved version of MEQ. It currently exists in 14 language versions; the most recently developed are Polish, Russian and Hindi.

Others

Roberts, in 1999, designed the Lark-Owl Chronotype Indicator, LOCI. 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.

Characteristics

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. Paine et al. 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".

Sleep

Horne and Östberg 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, but 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. found differences in alertness between morning and evening types after a two-hour sleep reduction. Duffy et al. investigated "changes in the phase relationship between endogenous circadian rhythms and the sleep-wake cycle", and found that although evening types woke at a later clock hour than morning types, morning types woke at a later circadian phase. Zavada et al. 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. They also state that each chronotype category "contains a similar portion of short and long sleepers". Chung et al. 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".

Diurnal rhythms

Gibertini et al. 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, whereas amplitude was not. They note that morning types evidence a more rapid decline in melatonin levels after the peak than do evening types. Baehr et al. 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. Morning types had lower pain sensitivity throughout a day than evening types, but the two chronotype groups did not differ in the shape of diurnal variations in pain. There are some differences between chronotypes in sexual activity, with evening chronotypes preferring later hours for sex as compared to other chronotypes.

Personality

Chronotypes differ in many aspects of personality, but also in intellectual domains, like creative thinking. For example, eveningness preference has been related to unrestricted sociosexuality in females, but not in males.

Genetic variants associated with chronotype

Some studies show that there are 22 genetic variants associated with chronotype. These variants occur near genes known to be important in photoreception and circadian rhythms. 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. In mice, gene ablation of Rgs16 lengthens the circadian period of behavioural rhythm. 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.

PER2, is a well-known regulator of circadian rhythms and contains a variant recently shown to be associated with iris formation. This suggests a link between iris function and chronotype. Per2 knockout mice show arrhythmic locomotor activity.

Therefore, the chronotype is genetically heritable.

Chronotype and disease

Disrupted circadian rhythms are associated with several human diseases, for example, chronotype is genetically correlated with BMI (Body mass index). However, cause-and-effect is not yet determined.