Chronotype

A chronotype is the behavioral manifestation of underlying circadian rhythm's myriad of physical processes. A person's chronotype is the propensity for the individual to sleep at a particular time during a 24-hour period. Eveningness (delayed sleep period; most active and alert in the evening) and morningness (advanced sleep period; most active and alert in the morning) are the two extremes with most individuals having some flexibility in the timing of their sleep period. However, across development there are changes in the propensity of the sleep period with pre-pubescent children preferring an advanced sleep period, adolescents preferring a delayed sleep period and many elderly preferring an advanced sleep period.

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.^[1] There are candidate genes (called CLOCK genes) that exist in most cells in the body and brain, referred to as the circadian system that regulate 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.^[2]

Humans are normally diurnal creatures that are active in the daytime.^{[citation needed]} As with most other diurnal animals, human activity-rest patterns are endogenously regulated by biological clocks with a circadian (~24-hour) period.^{[citation needed]} Chronotypes have also been investigated in other species, such as fruit flies^[3] and mice.^[4]

Normal variation in chronotype encompasses sleep–wake cycles that are two to three hours later in evening types than morning types.^[5] 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.^[6]

^ Walker RJ, Kribs ZD, Christopher AN, Shewach OR, Wieth MB (2014). "Age, the Big Five, and time-of-day preference: A mediational model". Personality and Individual Differences. 56: 170–174. doi:10.1016/j.paid.2013.09.003. S2CID 18325145.
^ Samson DR, Crittenden AN, Mabulla IA, Mabulla AZ, Nunn CL (July 2017). "Chronotype variation drives night-time sentinel-like behaviour in hunter-gatherers". Proceedings. Biological Sciences. 284 (1858): 20170967. doi:10.1098/rspb.2017.0967. PMC 5524507. PMID 28701566.
^ Zakharenko LP, Petrovskii DV, Putilov AA (21 June 2018). "Larks, owls, swifts, and woodcocks among fruit flies: differential responses of four heritable chronotypes to long and hot summer days". Nature and Science of Sleep. 10: 181–191. doi:10.2147/NSS.S168905. PMC 6016586. PMID 29950910.
^ Refinetti R, Wassmer T, Basu P, Cherukalady R, Pandey VK, Singaravel M, et al. (July 2016). "Variability of behavioral chronotypes of 16 mammalian species under controlled conditions". Physiology & Behavior. 161: 53–59. doi:10.1016/j.physbeh.2016.04.019. PMID 27090227.
^ Lack L, Bailey M, Lovato N, Wright H (2009). "Chronotype differences in circadian rhythms of temperature, melatonin, and sleepiness as measured in a modified constant routine protocol". Nature and Science of Sleep. 1: 1–8. doi:10.2147/nss.s6234. PMC 3630920. PMID 23616692.
^ The international classification of sleep disorders (PDF) (revised: diagnostic and coding manual ed.). Rochester, MN: American Sleep Disorders Association. 2001. ISBN 0-9657220-1-5. Archived from the original (PDF) on 27 September 2007.

[1] Walker RJ, Kribs ZD, Christopher AN, Shewach OR, Wieth MB (2014). "Age, the Big Five, and time-of-day preference: A mediational model". Personality and Individual Differences. 56: 170–174. doi:10.1016/j.paid.2013.09.003. S2CID 18325145.

[2] Samson DR, Crittenden AN, Mabulla IA, Mabulla AZ, Nunn CL (July 2017). "Chronotype variation drives night-time sentinel-like behaviour in hunter-gatherers". Proceedings. Biological Sciences. 284 (1858): 20170967. doi:10.1098/rspb.2017.0967. PMC 5524507. PMID 28701566.

[3] Zakharenko LP, Petrovskii DV, Putilov AA (21 June 2018). "Larks, owls, swifts, and woodcocks among fruit flies: differential responses of four heritable chronotypes to long and hot summer days". Nature and Science of Sleep. 10: 181–191. doi:10.2147/NSS.S168905. PMC 6016586. PMID 29950910.

[4] Refinetti R, Wassmer T, Basu P, Cherukalady R, Pandey VK, Singaravel M, et al. (July 2016). "Variability of behavioral chronotypes of 16 mammalian species under controlled conditions". Physiology & Behavior. 161: 53–59. doi:10.1016/j.physbeh.2016.04.019. PMID 27090227.

[5] Lack L, Bailey M, Lovato N, Wright H (2009). "Chronotype differences in circadian rhythms of temperature, melatonin, and sleepiness as measured in a modified constant routine protocol". Nature and Science of Sleep. 1: 1–8. doi:10.2147/nss.s6234. PMC 3630920. PMID 23616692.

[ICSD-6] The international classification of sleep disorders (PDF) (revised: diagnostic and coding manual ed.). Rochester, MN: American Sleep Disorders Association. 2001. ISBN 0-9657220-1-5. Archived from the original (PDF) on 27 September 2007.

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