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, respectively, to 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 19-item 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.
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 13 language versions; the most recently developed are Polish and Hindi.
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.
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."
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, 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.  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 while 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. The researchers 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."
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, while 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. 
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. 
- Logie, Bruce. "Larks and Owls". Retrieved 2007-11-02.
- American Academy of Sleep Medicine International Classification of Sleep Disorders, Revised Edition 2001.
- Kleitman, Nathaniel (1939, 1963). Sleep and Wakefulness. The University of Chicago Press. Check date values in:
- Horne, J.A.; Östberg, O. (1976). "A self-assessment questionnaire to determine morningness-eveningness in human circadian rhythms". Int J Chronobiol 4 (2): 97–110. PMID 1027738.
- Taillard, Jacques; et al. (2004). "Validation of Horne and Ostberg Morningness-Eveningness Questionnaire in a Middle-Aged Population of French Workers". Journal of Biological Rhythms 19 (1): 76–86. doi:10.1177/0748730403259849. PMID 14964706. Retrieved 2007-11-02.
- Jankowski, K.S. (2014) Composite Scale of Morningness: Psychometric properties, validity with Munich ChronoType Questionnaire and age/sex differences in Poland. European Psychiatry, doi: 10.1016/j.eurpsy.2014.01.004.
- Bhatia T, Agrawal A, Beniwal RP, Thomas P, Monk TH, Nimgaonkar VL, Deshpande SN. A Hindi version of the Composite Scale of Morningness. Asian J Psychiatr. 2013;6:581-4.
- Roenneberg, T.; Kuehnle, T.; Juda, M.; Kantermann, T.; Allebrandt, K.; Gordijn, M.; Merrow, M. (December 2007). "Epidemiology of the human circadian clock". Sleep Med Rev. 11 (6): 429–38. doi:10.1016/j.smrv.2007.07.005. PMID 17936039.
- Allebrandt, K.V.; Roenneberg, T. (2008). "The search for circadian clock components in humans: New perspectives for association studies". Brazilian Journal of Medical and Biological Research 41 (8). doi:10.1590/S0100-879X2008000800013.
- Schur, Carolyn (1994). "excerpt". Birds of a Different Feather. Saskatoon, Canada: Schur Goode Associates. ISBN 0-9698190-0-5. Retrieved 2009-07-20.
- Paine, Sarah-Jane; Gander, Philippa H.; Travier, Noemie (2006). "The Epidemology of Morningness/Eveningness: Influence of Age, Gender, Ethnicity, and Socioeconomic Factors in Adults (30–49 Years)". Journal of Biological Rhythms 21 (1): 68–76. doi:10.1177/0748730405283154. PMID 16461986. Retrieved 2007-11-02.
- Clodoré, M.; Foret, J.; Benoit, O. (1986). "Diurnal variation in subjective and objective measures of sleepiness: the effects of sleep reduction and circadian type". Chronobiol Int. 3 (4): 255–63. doi:10.3109/07420528609079543. PMID 3677208.
- Duffy, J.F.; Dijk, D.J.; Hall, E.F.; Czeisler, C.A. (1999). "Relationship of endogenous circadian melatonin and temperature rhythms to self-reported preference for morning or evening activity in young and older people". J Investig Med 47 (3): 141–50. PMID 10198570.
- Zavada, Andrei; Gordijn, Beersma; Daan, Roenneberg (2005). "Comparison of the Munich Chronotype Questionnaire with the Horne-Östberg's Morningness-Eveningness Score" (PDF). Chronobiol. Int. 22 (2): 267–78. doi:10.1081/CBI-200053536. PMID 16021843. Retrieved 2007-11-02.
- Chung, M.H.; Chang, F.M.; Yang, C.C.; Kuo, T.B.; Hsu, N. (January 2009). "Sleep quality and morningness-eveningness of shift nurses" (Abstract). Journal of Clinical Nursing 18 (2): 279–284. doi:10.1111/j.1365-2702.2007.02160.x. ISSN 1365-2702. PMID 19120754. Retrieved 2009-01-17.
- Gibertini, M.; Graham, C.; Cook, M.R. (1999). "Self-report of circadian type reflects the phase of the melatonin rhythm". Biol psychol. 50 (1): 19–33. doi:10.1016/S0301-0511(98)00049-0. PMID 10378437. Retrieved 2007-11-02.
- "Dictionary of Circadian Physiology". Circadian Rhythm Laboratory, University of South Carolina Salkehatchie, Walterboro campus.
- Baehr, E.K.; Revelle, W.; Eastman, C.I. (June 2000). "Individual differences in the phase and amplitude of the human circadian temperature rhythm: with an emphasis on morningness-eveningness". J Sleep Res 9 (2): 117–27. doi:10.1046/j.1365-2869.2000.00196.x. PMID 10849238.
- Mongrain, V.; Lavoie, S.; Selmaoui, B.; Paquet, J.; Dumont, M. (June 2004). "Phase relationships between sleep-wake cycle and underlying circadian rhythms in Morningness-Eveningness". J. Biol. Rhythms 19 (3): 248–57. doi:10.1177/0748730404264365. PMID 15155011.
- Jankowski, K.S. (2013). Morning types are less sensitive to pain than evening types all day long. European Journal of Pain, 17, 1068-1073.
- Jankowski, K.S., Díaz-Morales, J.F., Randler, C. (2014).Chronotype, gender, and time for sex. Chronobiology International, 31, 911-916.
- Giampietro, M.; Cavallera, G.M. (2006). "Morning and evening types and creative thinking". Elsevier Ltd. Retrieved 2007-11-02.
- Jankowski, K.S., Díaz-Morales, J.F., Vollmer, C., Randler, C. (2014). Morningness-eveningness and sociosexuality: evening females are less restricted than morning ones. Personality and Individual Differences, 10, 13-17.