chronotype

{{More citations needed|section|date=July 2018}}

Physiology professor Nathaniel Kleitman's 1939 book Sleep and Wakefulness, revised 1963,{{Cite book |title=Sleep and Wakefulness |vauthors=Kleitman N |publisher=The University of Chicago Press |year=1963 |orig-year=1939}} summarized the existing knowledge of sleep and 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".{{Cite thesis |title=Kartläggning av individuella dygnsrytmer. |degree=PhD |publisher=Göteborgs Universitet, Psykologiska Institutionen |language=Swedish |vauthors=Öquist O |trans-title=Charting Individual Circadian Rhythms |year=1970}}

The causes and regulation of chronotypes, including developmental change, individual propensity for a specific chronotype, and flexible versus fixed chronotypes have yet to be determined. However, research is beginning to shed light on these questions, such as the relationship between age and chronotype.{{Cite journal |vauthors=Walker RJ, Kribs ZD, Christopher AN, Shewach OR, Wieth MB |year=2014 |title=Age, the Big Five, and time-of-day preference: A mediational model. |journal=Personality and Individual Differences |volume=56 |pages=170–174 |doi=10.1016/j.paid.2013.09.003|s2cid=18325145 }} There are candidate genes (called CLOCK genes) that exist in most cells in the body and brain, referred to as the circadian system that regulates physiological phenomena (hormone levels, metabolic function, body temperature, cognitive faculties, and sleeping). With the exception of the most extreme and rigid chronotypes, regulation is likely due to gene-environment interactions. 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.{{Cite journal |vauthors=Samson DR, Crittenden AN, Mabulla IA, Mabulla AZ, Nunn CL |date=July 2017 |title=Chronotype variation drives night-time sentinel-like behaviour in hunter-gatherers |journal=Proceedings. Biological Sciences |volume=284 |issue=1858 |pages=20170967 |doi=10.1098/rspb.2017.0967 |pmc=5524507 |pmid=28701566}}

In 2016, clinical psychologist Michael J. Breus proposed four main chronotype categories: lions, bears, wolves, and dolphins. These categories correspond respectively to larks, middle-of-the-road people, night owls, and people with irregular sleep schedules.{{cite web |url=https://www.psychologytoday.com/us/blog/sleep-newzzz/202104/the-four-chronotypes-which-one-are-you |title= The Four Chronotypes: Which One Are You? |last=Breus |first=Michael J. |date=10 April 2021 |website=Psychology Today |publisher=Psychology Today |access-date=10 May 2025}}{{cite book |last=Breus |first=Michael J. |date=13 September 2016 |title=The Power of When |publisher=Little, Brown Spark |language=English |isbn=978-0316391269}}

Normal variation in chronotype encompasses sleep–wake cycles that are two to three hours later in evening types than morning types.{{Cite journal |vauthors=Lack L, Bailey M, Lovato N, Wright H |year=2009 |title=Chronotype differences in circadian rhythms of temperature, melatonin, and sleepiness as measured in a modified constant routine protocol |journal=Nature and Science of Sleep |volume=1 |pages=1–8 |doi=10.2147/nss.s6234 |pmc=3630920 |pmid=23616692 |doi-access=free }} 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 normally considered to have a circadian rhythm sleep disorder.{{Cite book |url=http://www.absm.org/PDF/ICSD.pdf |title=The international classification of sleep disorders |date=2001 |publisher=American Sleep Disorders Association |isbn=0-9657220-1-5 |edition=revised: diagnostic and coding manual |location=Rochester, MN |archive-url=https://web.archive.org/web/20070927034423/http://www.absm.org/PDF/ICSD.pdf |archive-date=27 September 2007}}

{{Main|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,{{Cite journal |vauthors=Horne JA, Ostberg O |year=1976 |title=A self-assessment questionnaire to determine morningness-eveningness in human circadian rhythms |journal=International Journal of Chronobiology |volume=4 |issue=2 |pages=97–110 |pmid=1027738}} 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,{{Cite journal |vauthors=Taillard J, Philip P, Chastang JF, Bioulac B |date=February 2004 |title=Validation of Horne and Ostberg morningness-eveningness questionnaire in a middle-aged population of French workers |journal=Journal of Biological Rhythms |volume=19 |issue=1 |pages=76–86 |doi=10.1177/0748730403259849 |pmid=14964706 |s2cid=23248143}} working in France with employed people over the age of 50. Previously the MEQ had been validated only for subjects of university age.

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

The CTI was initially developed to identify individuals capable of adapting to shift work. Thus, the scale assesses two factors that influence a person's ability to alter his or her sleeping rhythms: rigidity/flexibility of sleeping habits and ability/inability to overcome drowsiness. Since its creation, the scale has undergone a number of revisions to improve its psychometric properties. An 18-item version was used as part of the larger Standard Shiftwork Index (SSI) in a study conducted by Barton and colleagues. This shorter scale was then reduced and altered to make an 11 item scale by De Milia et al.{{Cite book |title=STOP, THAT and One Hundred Other Sleep Scales |vauthors=Shahid A, Wilkinson K, Marcu S, Shapiro CM |date=2011 |publisher=Springer |isbn=978-1-4419-9893-4 |veditors=Shahid A, Wilkinson K, Marcu S, Shapiro CM |location=New York, NY |pages=123–126 |chapter=Circadian Type Inventory (CTI). |doi=10.1007/978-1-4419-9893-4_22}}

Smith et al. (1989){{Cite journal |last1=Smith |first1=Carlla S. |last2=Reilly |first2=Christopher |last3=Midkiff |first3=Karen |year=1989 |title=Evaluation of three circadian rhythm questionnaires with suggestions for an improved measure of morningness. |journal=Journal of Applied Psychology |volume=74 |issue=5 |pages=728–738 |doi=10.1037/0021-9010.74.5.728 |pmid=2793773}} analyzed items from MEQ, Diurnal Type Scale (DTS),{{Cite journal |last1=Torsvall |first1=Lars |last2=Åkerstedt |first2=Torbjörn |year=1980 |title=A diurnal type scale: construction, consistency and validation in shift work. |journal=Scandinavian Journal of Work, Environment & Health |volume=6 |issue=4 |pages=283–290 |doi=10.5271/sjweh.2608 |issn=0355-3140 |pmid=7195066 |doi-access=free}} 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{{By whom|date=July 2018}} as an improved version of MEQ. It currently exists in 14 language versions{{citation needed|date=July 2018}}; the most recently developed are Polish,{{Cite journal |vauthors=Jankowski KS |date=January 2015 |title=Composite Scale of Morningness: psychometric properties, validity with Munich ChronoType Questionnaire and age/sex differences in Poland |journal=European Psychiatry |volume=30 |issue=1 |pages=166–71 |doi=10.1016/j.eurpsy.2014.01.004 |pmid=24630377 |s2cid=22020964}} Russian{{Cite journal |vauthors=Kolomeichuk S |year=2015 |title=Psychometric properties of the Russian version of the Composite Scale of Morningness |journal=Biological Rhythm Research |volume=45 |issue=6 |pages=725–737 |doi=10.1080/09291016.2015.1048963 |bibcode=2015BioRR..46..725K |s2cid=218602418}} and Hindi.{{Cite journal |vauthors=Bhatia T, Agrawal A, Beniwal RP, Thomas P, Monk TH, Nimgaonkar VL, Deshpande SN |date=December 2013 |title=A Hindi version of the Composite Scale of Morningness |journal=Asian Journal of Psychiatry |volume=6 |issue=6 |pages=581–4 |doi=10.1016/j.ajp.2013.09.001 |pmc=4026194 |pmid=24309877}}

Roberts, in 1999, designed the Lark-Owl Chronotype Indicator, LOCI.{{Cite web |year=1999 |title=Construction and validation of the Lark-Owl (Chronotype) Indicator (LOCI): status report |url=http://www.psych.usyd.edu.au/difference5/papers/locistatus.html |url-status=dead |archive-url=https://web.archive.org/web/20120326105534/http://www.psych.usyd.edu.au/difference5/papers/locistatus.html |archive-date=26 March 2012 |access-date=21 June 2018 |website=University of Sydney |vauthors=Roberts RD}} 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.{{Cite journal |vauthors=Roenneberg T, Kuehnle T, Juda M, Kantermann T, Allebrandt K, Gordijn M, Merrow M |date=December 2007 |title=Epidemiology of the human circadian clock |journal=Sleep Medicine Reviews |volume=11 |issue=6 |pages=429–38 |doi=10.1016/j.smrv.2007.07.005 |pmid=17936039 |hdl-access=free |hdl=11370/65d6f03a-88cd-405c-a067-4afbc1b9ba9d |s2cid=11628329|url=https://pure.rug.nl/ws/files/6712602/2007SleepMedRevRoenneberg.pdf }}{{Cite journal |vauthors=Allebrandt KV, Roenneberg T |date=August 2008 |title=The search for circadian clock components in humans: new perspectives for association studies |journal=Brazilian Journal of Medical and Biological Research |volume=41 |issue=8 |pages=716–21 |doi=10.1590/S0100-879X2008000800013 |pmid=18797707 |doi-access=free}}

Most people are neither evening nor morning types but lie somewhere in between. 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. 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.{{Cite journal |author-link2=Philippa Gander |vauthors=Paine SJ, Gander PH, Travier N |date=February 2006 |title=The epidemiology of morningness/eveningness: influence of age, gender, ethnicity, and socioeconomic factors in adults (30-49 years) |journal=Journal of Biological Rhythms |volume=21 |issue=1 |pages=68–76 |doi=10.1177/0748730405283154 |pmid=16461986 |s2cid=20885196}} 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, 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".{{Rp|109}} Clodoré et al. found differences in alertness between morning and evening types after a two-hour sleep reduction.{{Cite journal |vauthors=Clodoré M, Foret J, Benoit O |year=1986 |title=Diurnal variation in subjective and objective measures of sleepiness: the effects of sleep reduction and circadian type |journal=Chronobiology International |volume=3 |issue=4 |pages=255–63 |doi=10.3109/07420528609079543 |pmid=3677208}} 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.{{Cite journal |vauthors=Duffy JF, Dijk DJ, Hall EF, Czeisler CA |date=March 1999 |title=Relationship of endogenous circadian melatonin and temperature rhythms to self-reported preference for morning or evening activity in young and older people |journal=Journal of Investigative Medicine |volume=47 |issue=3 |pages=141–50 |pmc=8530273 |pmid=10198570}} 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.{{Cite journal |vauthors=Zavada A, Gordijn MC, Beersma DG, Daan S, Roenneberg T |year=2005 |title=Comparison of the Munich Chronotype Questionnaire with the Horne-Ostberg's Morningness-Eveningness Score |url=http://dissertations.ub.rug.nl/FILES/faculties/science/2007/a.zavada/c2.pdf |url-status=dead |journal=Chronobiology International |volume=22 |issue=2 |pages=267–78 |doi=10.1081/CBI-200053536 |pmid=16021843 |archive-url=https://web.archive.org/web/20081217060051/http://dissertations.ub.rug.nl/FILES/faculties/science/2007/a.zavada/c2.pdf |archive-date=17 December 2008 |access-date=2 November 2007 |hdl-access=free |hdl=11370/4baad537-bed0-4161-8729-b5da5b22808d |s2cid=17928135}} 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".{{Cite journal |vauthors=Chung MH, Chang FM, Yang CC, Kuo TB, Hsu N |date=January 2009 |title=Sleep quality and morningness-eveningness of shift nurses |journal=Journal of Clinical Nursing |volume=18 |issue=2 |pages=279–84 |doi=10.1111/j.1365-2702.2007.02160.x |pmid=19120754}}

{{Main|Diurnal cycle}}

Gibertini et al.{{Cite journal |vauthors=Gibertini M, Graham C, Cook MR |date=May 1999 |title=Self-report of circadian type reflects the phase of the melatonin rhythm |journal=Biological Psychology |volume=50 |issue=1 |pages=19–33 |doi=10.1016/S0301-0511(98)00049-0 |pmid=10378437 |s2cid=12654232}} assessed blood levels of the hormone melatonin, finding that the melatonin acrophase (the time at which the peak of a rhythm occurs{{Cite web |title=Dictionary of Circadian Physiology |url=http://www.circadian.org/dictionary.html |website=Circadian Rhythm Laboratory, University of South Carolina Salkehatchie, Walterboro campus}}) 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.{{Cite journal |vauthors=Baehr EK, Revelle W, Eastman CI |date=June 2000 |title=Individual differences in the phase and amplitude of the human circadian temperature rhythm: with an emphasis on morningness-eveningness |journal=Journal of Sleep Research |volume=9 |issue=2 |pages=117–27 |doi=10.1046/j.1365-2869.2000.00196.x |pmid=10849238 |s2cid=6104127 |doi-access=}} 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.{{Cite journal |vauthors=Mongrain V, Lavoie S, Selmaoui B, Paquet J, Dumont M |date=June 2004 |title=Phase relationships between sleep-wake cycle and underlying circadian rhythms in Morningness-Eveningness |journal=Journal of Biological Rhythms |volume=19 |issue=3 |pages=248–57 |doi=10.1177/0748730404264365 |pmid=15155011 |s2cid=41561035}} 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.{{Cite journal |vauthors=Jankowski KS |date=August 2013 |title=Morning types are less sensitive to pain than evening types all day long |journal=European Journal of Pain |volume=17 |issue=7 |pages=1068–73 |doi=10.1002/j.1532-2149.2012.00274.x |pmid=23322641 |s2cid=7308674}} There are some differences between chronotypes in sexual activity, with evening chronotypes preferring later hours for sex as compared to other chronotypes.{{Cite journal |vauthors=Jankowski KS, Díaz-Morales JF, Randler C |date=October 2014 |title=Chronotype, gender, and time for sex. |url=http://eprints.ucm.es/32633/1/Jankowski_Diaz_Morales_Randler_2014_Chronotype_gender_timeforsex_CBI.pdf |journal=Chronobiology International |volume=31 |issue=8 |pages=911–6 |doi=10.3109/07420528.2014.925470 |pmid=24927370 |s2cid=207469411}}

Chronotypes differ in many aspects of personality, such as grit,{{Cite journal |display-authors=6 |vauthors=Allee MF, Anderson SE, Bloom MJ, Jost SR, Keating III DP, Lang AS, Mankin NV, Mast ZW, Nelson PP, Spear EM, Valderrama EF |year=2020 |title=The Influence of Chronotype and Grit on Lifestyle and Physical Activity |journal=Building Healthy Academic Communities Journal |volume=4 |issue=2 |pages=57–70 |doi=10.18061/bhac.v4i2.7617 |issn=2573-7643 |doi-access=free}} but also in intellectual domains, like creative thinking.{{Cite web |year=2006 |title=Morning and evening types and creative thinking |url=http://avalon.cuautitlan2.unam.mx/materialesdidacticos/gerardo_sa/articulos/mayo2007/morning |url-status=dead |archive-url=https://web.archive.org/web/20090728043458/http://avalon.cuautitlan2.unam.mx/materialesdidacticos/gerardo_sa/articulos/mayo2007/morning |archive-date=28 July 2009 |access-date=2 November 2007 |publisher=Elsevier Ltd |vauthors=Giampietro M, Cavallera GM}} Experimental studies on discomfort glare perception and chronotype have shown that early chronotypes can tolerate more discomfort glare in the morning compared to late chronotypes.{{cite journal | vauthors = Kent MG, Altomonte S, Wilson R, Tregenza PR | title = Temporal variables and personal factors in glare sensation | journal = Lighting Research and Technology | date = 2016 | volume = 48 | issue = 6 | pages = 689–710 | doi = 10.1177/1477153515578310 | s2cid = 56096900 | url = https://journals.sagepub.com/doi/10.1177/1477153515578310?icid=int.sj-full-text.similar-articles.1 }}

A meta-analysis found a small positive association between an evening chronotype and intelligence;{{Cite journal |vauthors=Preckel F, Lipnevich AA, Schneider S, Roberts RD |date=October 2011 |title=Chronotype, cognitive abilities, and academic achievement: A meta-analytic investigation |journal=Learning and Individual Differences |volume=21 |issue=5 |pages=483–492 |doi=10.1016/j.lindif.2011.07.003}} similar results were subsequently found in a large sample using a standardized battery.{{Cite journal |display-authors=6 |vauthors=Kyle SD, Sexton CE, Feige B, Luik AI, Lane J, Saxena R, Anderson SG, Bechtold DA, Dixon W, Little MA, Ray D, Riemann D, Espie CA, Rutter MK, Spiegelhalder K |date=October 2017 |title=Sleep and cognitive performance: cross-sectional associations in the UK Biobank |journal=Sleep Medicine |volume=38 |pages=85–91 |doi=10.1016/j.sleep.2017.07.001 |pmc=5930168 |pmid=29031762}}

Studies show{{efn| For example, see Kalmbach, et al. for discussion of three landmark genome-wide association studies{{Cite journal |vauthors=Kalmbach DA, Schneider LD, Cheung J, Bertrand SJ, Kariharan T, Pack AI, Gehrman PR |date=February 2017 |title=Genetic Basis of Chronotype in Humans: Insights From Three Landmark GWAS |journal=Sleep |volume=40 |issue=2 |doi=10.1093/sleep/zsw048 |pmc=6084759 |pmid=28364486}}}} that there are 22 genetic variants associated with chronotype. These variants occur near genes known to be important in photoreception and circadian rhythms.{{Cite journal |display-authors=6 |vauthors=Jones SE, Tyrrell J, Wood AR, Beaumont RN, Ruth KS, Tuke MA, Yaghootkar H, Hu Y, Teder-Laving M, Hayward C, Roenneberg T, Wilson JF, Del Greco F, Hicks AA, Shin C, Yun CH, Lee SK, Metspalu A, Byrne EM, Gehrman PR, Tiemeier H, Allebrandt KV, Freathy RM, Murray A, Hinds DA, Frayling TM, Weedon MN |date=August 2016 |title=Genome-Wide Association Analyses in 128,266 Individuals Identifies New Morningness and Sleep Duration Loci |journal=PLOS Genetics |volume=12 |issue=8 |pages=e1006125 |doi=10.1371/journal.pgen.1006125 |pmc=4975467 |pmid=27494321 |doi-access=free}} 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.{{Cite journal |display-authors=6 |vauthors=Doi M, Ishida A, Miyake A, Sato M, Komatsu R, Yamazaki F, Kimura I, Tsuchiya S, Kori H, Seo K, Yamaguchi Y, Matsuo M, Fustin JM, Tanaka R, Santo Y, Yamada H, Takahashi Y, Araki M, Nakao K, Aizawa S, Kobayashi M, Obrietan K, Tsujimoto G, Okamura H |date=1 January 2011 |title=Circadian regulation of intracellular G-protein signalling mediates intercellular synchrony and rhythmicity in the suprachiasmatic nucleus |journal=Nature Communications |volume=2 |pages=327 |bibcode=2011NatCo...2..327D |doi=10.1038/ncomms1316 |pmc=3112533 |pmid=21610730 |doi-access=free}}

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.{{Cite journal |vauthors=Preitner N, Damiola F, Lopez-Molina L, Zakany J, Duboule D, Albrecht U, Schibler U |date=July 2002 |title=The orphan nuclear receptor REV-ERBalpha controls circadian transcription within the positive limb of the mammalian circadian oscillator |journal=Cell |volume=110 |issue=2 |pages=251–60 |doi=10.1016/S0092-8674(02)00825-5 |pmid=12150932 |doi-access=free}}{{Cite journal |display-authors=6 |vauthors=Larsson M, Duffy DL, Zhu G, Liu JZ, Macgregor S, McRae AF, Wright MJ, Sturm RA, Mackey DA, Montgomery GW, Martin NG, Medland SE |date=August 2011 |title=GWAS findings for human iris patterns: associations with variants in genes that influence normal neuronal pattern development |journal=American Journal of Human Genetics |volume=89 |issue=2 |pages=334–43 |doi=10.1016/j.ajhg.2011.07.011 |pmc=3155193 |pmid=21835309 |doi-access=free}} 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.{{Cite journal |vauthors=Dannemann M, Kelso J |date=October 2017 |title=The Contribution of Neanderthals to Phenotypic Variation in Modern Humans |journal=American Journal of Human Genetics |volume=101 |issue=4 |pages=578–589 |doi=10.1016/j.ajhg.2017.09.010 |pmc=5630192 |pmid=28985494 |doi-access=free}}

Therefore, chronotypes are genetically heritable.{{Cite journal |display-authors=6 |vauthors=von Schantz M, Taporoski TP, Horimoto AR, Duarte NE, Vallada H, Krieger JE, Pedrazzoli M, Negrão AB, Pereira AC |date=March 2015 |title=Distribution and heritability of diurnal preference (chronotype) in a rural Brazilian family-based cohort, the Baependi study |journal=Scientific Reports |volume=5 |pages=9214 |bibcode=2015NatSR...5.9214V |doi=10.1038/srep09214 |pmc=4363835 |pmid=25782397 |doi-access=free}}

Disrupted circadian rhythms are associated with several human diseases. For example, chronotype{{which|date=February 2025}} is genetically correlated with BMI (body mass index).{{Cite journal |display-authors=6 |vauthors=Kohsaka A, Laposky AD, Ramsey KM, Estrada C, Joshu C, Kobayashi Y, Turek FW, Bass J |date=November 2007 |title=High-fat diet disrupts behavioral and molecular circadian rhythms in mice |journal=Cell Metabolism |volume=6 |issue=5 |pages=414–21 |doi=10.1016/j.cmet.2007.09.006 |pmid=17983587 |doi-access=free}}{{Cite journal |display-authors=6 |vauthors=Turek FW, Joshu C, Kohsaka A, Lin E, Ivanova G, McDearmon E, Laposky A, Losee-Olson S, Easton A, Jensen DR, Eckel RH, Takahashi JS, Bass J |date=May 2005 |title=Obesity and metabolic syndrome in circadian Clock mutant mice |journal=Science |volume=308 |issue=5724 |pages=1043–5 |bibcode=2005Sci...308.1043T |doi=10.1126/science.1108750 |pmc=3764501 |pmid=15845877}} However, cause-and-effect is not yet determined. Evening chronotype has been reported to be a risk factor for bipolar disorder{{Cite journal |last1=Scott |first1=Jan |last2=Etain |first2=Bruno |last3=Miklowitz |first3=David |last4=Crouse |first4=Jacob J. |last5=Carpenter |first5=Joanne |last6=Marwaha |first6=Steven |last7=Smith |first7=Daniel |last8=Merikangas |first8=Kathleen |last9=Hickie |first9=Ian |date=April 2022 |title=A systematic review and meta-analysis of sleep and circadian rhythms disturbances in individuals at high-risk of developing or with early onset of bipolar disorders |journal=Neuroscience & Biobehavioral Reviews |language=en |volume=135 |pages=104585 |doi=10.1016/j.neubiorev.2022.104585 |pmc=8957543 |pmid=35182537}} and depression.{{Cite journal |last1=Crouse |first1=Jacob J |last2=Carpenter |first2=Joanne S |last3=Song |first3=Yun Ju C |last4=Hockey |first4=Samuel J |last5=Naismith |first5=Sharon L |last6=Grunstein |first6=Ronald R |last7=Scott |first7=Elizabeth M |last8=Merikangas |first8=Kathleen R |last9=Scott |first9=Jan |last10=Hickie |first10=Ian B |date=September 2021 |title=Circadian rhythm sleep–wake disturbances and depression in young people: implications for prevention and early intervention |url=https://linkinghub.elsevier.com/retrieve/pii/S2215036621000341 |journal=The Lancet Psychiatry |language=en |volume=8 |issue=9 |pages=813–823 |doi=10.1016/S2215-0366(21)00034-1|pmid=34419186 |url-access=subscription }}

Evening chronotypes are genetically linked to ADHD, according to genetic polymorphism studies that have linked genes involved in ADHD to genes involved in circadian rhythm.{{cite journal | vauthors = Kissling C, Retz W, Wiemann S, Coogan AN, Clement RM, Hünnerkopf R, Conner AC, Freitag CM, Rösler M, Thome J | display-authors = 6 | title = A polymorphism at the 3'-untranslated region of the CLOCK gene is associated with adult attention-deficit hyperactivity disorder | journal = American Journal of Medical Genetics. Part B, Neuropsychiatric Genetics | volume = 147 | issue = 3 | pages = 333–8 | date = April 2008 | pmid = 17948273 | doi = 10.1002/ajmg.b.30602 | s2cid = 17357730 | url = https://mural.maynoothuniversity.ie/16461/1/AC_A%20Polymorphism.pdf }}{{cite journal | vauthors = Baird AL, Coogan AN, Siddiqui A, Donev RM, Thome J | title = Adult attention-deficit hyperactivity disorder is associated with alterations in circadian rhythms at the behavioural, endocrine and molecular levels | journal = Molecular Psychiatry | volume = 17 | issue = 10 | pages = 988–95 | date = October 2012 | pmid = 22105622 | doi = 10.1038/mp.2011.149 | doi-access = free }} A high proportion of people with ADHD have delayed sleep phase disorder.{{cite journal | vauthors = Van der Heijden KB, Smits MG, Van Someren EJ, Gunning WB | title = Idiopathic chronic sleep onset insomnia in attention-deficit/hyperactivity disorder: a circadian rhythm sleep disorder | journal = Chronobiology International | volume = 22 | issue = 3 | pages = 559–70 | year = 2005 | pmid = 16076654 | doi = 10.1081/CBI-200062410 | s2cid = 24044709 }}

Chronotype is also associated with differential response to some treatments. For example, higher evening chronotype is associated with more side effects and lower efficacy of some selective serotonin reuptake inhibitors and serotonin norepinephrine reuptake inhibitors in adults with depression.{{Cite journal |last1=Crouse |first1=Jacob J. |last2=Park |first2=Shin Ho |last3=Byrne |first3=Enda M. |last4=Mitchell |first4=Brittany L. |last5=Chan |first5=Karina |last6=Scott |first6=Jan |last7=Medland |first7=Sarah E. |last8=Martin |first8=Nicholas G. |last9=Wray |first9=Naomi R. |last10=Hickie |first10=Ian B. |date=July 2024 |title=Evening Chronotypes With Depression Report Poorer Outcomes of Selective Serotonin Reuptake Inhibitors: A Survey-Based Study of Self-Ratings |journal=Biological Psychiatry |language=en |volume=96 |issue=1 |pages=4–14 |doi=10.1016/j.biopsych.2023.12.023|pmid=38185236 |doi-access=free }}

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• {{Cite journal |last1=Panjeh |first1=Sareh |last2=Pompeia |first2=Sabine |last3=Archer |first3=Simon N. |last4=Pedrazzoli |first4=Mario |last5=von Schantz |first5=Malcolm |last6=Cogo-Moreira |first6=Hugo |date=1 February 2021 |title=What are we measuring with the morningness–eveningness questionnaire? Exploratory factor analysis across four samples from two countries |journal=Chronobiology International |volume=38 |issue=2 |pages=234–247 |doi=10.1080/07420528.2020.1815758 |pmid=32993374 |doi-access=free}}
• {{Cite book |last1=Shahid |first1=Azmeh |last2=Wilkinson |first2=Kate |last3=Marcu |first3=Shai |last4=Shapiro |first4=Colin M. |title=STOP, THAT and One Hundred Other Sleep Scales |chapter=Composite Morningness Questionnaire |year=2011 |chapter-url=https://www.med.upenn.edu/cbti/assets/user-content/documents/Composite%20Morningness%20Questionnaire.pdf |pages=137–140 |doi=10.1007/978-1-4419-9893-4_25 |isbn=978-1-4419-9892-7}}

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Category:Sleep physiology

Category:Circadian rhythm

Category:Sleep