Sleep medicine reviewsOct 16, 2007

Patterns of the Human Body Clock in Populations

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Mar 31, 2026

Abstract

More than 55,000 people have completed the Munich ChronoType Questionnaire (MCTQ) to assess individual chronotypes.

Humans exhibit significant differences in their sleep and wake timing, forming a near-Gaussian distribution across populations.
Extreme early risers wake when extreme night owls are going to sleep, highlighting the variability in circadian rhythms.
The relationship between circadian systems and different chronotypes is well established across various experimental organisms.
Sleep duration varies significantly based on chronotype when analyzed separately for work and free days.
Chronotype is influenced by both age and sex, indicating demographic factors play a role in sleep behavior.

Simplified

Humans show large inter-individual differences in organising their behaviour within the 24-h day-this is most obvious in their preferred timing of sleep and wakefulness. Sleep and wake times show a near-Gaussian distribution in a given population, with extreme early types waking up when extreme late types fall asleep. This distribution is predominantly based on differences in an individuals' circadian clock. The relationship between the circadian system and different "chronotypes" is formally and genetically well established in experimental studies in organisms ranging from unicells to mammals. To investigate the epidemiology of the human circadian clock, we developed a simple questionnaire (Munich ChronoType Questionnaire, MCTQ) to assess chronotype. So far, more than 55,000 people have completed the MCTQ, which has been validated with respect to the Horne-Østberg morningness-eveningness questionnaire (MEQ), objective measures of activity and rest (sleep-logs and actimetry), and physiological parameters. As a result of this large survey, we established an algorithm which optimises chronotype assessment by incorporating the information on timing of sleep and wakefulness for both work and free days. The timing and duration of sleep are generally independent. However, when the two are analysed separately for work and free days, sleep duration strongly depends on chronotype. In addition, chronotype is both age- and sex-dependent.

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Patterns of the Human Body Clock in Populations

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