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Life Between Clocks: Daily Temporal Patterns of Human Chronotypes

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Life Between Clocks: Daily Temporal Patterns of Human Chronotypes 10.1177/0748730402239679JOURNALRoenneberg OF et BIOLOGICALal. / LIFE BETWEENARTICLE RHYTHMS CLOCKS / February 2003 Life between Clocks: Daily Temporal Patterns of Human Chronotypes Till Roenneberg,*,1 Anna Wirz-Justice,† and Martha Merrow* *Centre for Chronobiology, Institute for Medical Psychology, Munich, Germany, †Centre for Chronobiology, Psychiatric University Clinic, Basel, Switzerland Abstract Human behavior shows large interindividual variation in temporal organization. Extreme “larks” wake up when extreme “owls” fall asleep. These chronotypes are attributed to differences in the circadian clock, and in animals, the genetic basis of similar phenotypic differences is well established. To better understand the genetic basis of temporal organization in humans, the authors developed a questionnaire to document individual sleep times, self-reported light exposure, and self-assessed chronotype, considering work and free days separately. This report summarizes the results of 500 questionnaires completed in a pilot study. Individual sleep times show large differences between work and free days, except for extreme early types. During the workweek, late chronotypes accumulate considerable sleep debt, for which they compensate on free days by lengthening their sleep by several hours. For all chronotypes, the amount of time spent outdoors in broad daylight significantly affects the timing of sleep: Increased self-reported light exposure advances sleep. The timing of self- selected sleep is multifactorial, including genetic disposition, sleep debt accumu- lated on workdays, and light exposure. Thus, accurate assessment of genetic chronotypes has to incorporate all of these parameters. The dependence of human chronotype on light, that is, on the amplitude of the light:dark signal, fol- lows the known characteristics of circadian systems in all other experimental organisms. Our results predict that the timing of sleep has changed during industrialization and that a majority of humans are sleep deprived during the workweek. The implications are far ranging concerning learning, memory, vigi- lance, performance, and quality of life. Key words circadian, clock, human, light, sleep, chronotype Our daily life is organized by three different clocks: ings time. When shielded from the solar and the social a solar clock, providing light and warmer tempera- clock (constant conditions), the biological clock “runs tures during the day; a social clock, which we see or free.” In real life, however, circadian clocks are usually hear first thing on a working day; and a biological synchronized (entrained) to the 24 h of the solar clock clock, which we sense most vividly when jet lagged, (Rajaratnam and Arendt, 2001; Roenneberg and Fos- during shift work, or when adjusting to daylight sav- ter, 1997). The major environmental signal for entrain- 1. To whom all correspondence should be addressed: Centre for Chronobiology,Institute for Medical Psychology,Munich, Ger- many; e-mail: [email protected]. JOURNAL OF BIOLOGICAL RHYTHMS, Vol. 18 No. 1, February 2003 80-90 DOI: 10.1177/0748730402239679 © 2003 Sage Publications 80 Roenneberg et al. / LIFE BETWEEN CLOCKS 81 ment (zeitgeber) is light that, in mammals, can reach tally measured timing of the individual’s circadian the biological clock only through the eyes (for refer- rhythms, for example, temperature, melatonin, or ences, see Roenneberg and Merrow, 2002). cortisol (Bailey and Heitkemper, 2001; Duffy et al., The circadian clock controls physiology at many 1999, 2001), the genetics behind “morningness” and levels, from gene expression to complex behaviors “eveningness” are far from straightforward (Duffy (e.g., sleep and performance). As with other genetic and Czeisler, 2002; Duffy et al., 1999; Vink et al., 2001). traits, circadian properties depend on specific alleles We have developed a new questionnaire to quanti- of genes (clock genes; for review, see Young and Kay, tatively assess the timing of sleep within the 24-h day 2001). In a given population, free-running periods are (sleep phase) using simple questions (see the Methods distributed around a species-specific mean both in section). The results of this pilot study show that animals (Pittendrigh and Daan, 1976) and in humans although indicated sleep times correlate well with (e.g., Dijk and Lockley, 2002; Klerman, 2001; Wever, chronotype self-assessment, they are very different 1979). These circadian differences can also be seen between workdays and free days. Most important, when the clock is entrained because the phase angle phase and duration of sleep on free days strongly between dawn and activity onset depends on the indi- depend on (1) the amount of sleep obtained during the vidual free-running period (Aschoff, 1979; Duffy et al., workweek and (2) the amount of time subjects spend 2001; Roenneberg and Merrow, 2000). Human prefer- outdoors in daylight. Thus, timing of sleep is a good ences in the timing of sleep and wake (called “chrono- indicator for the underlying chronotype when cor- types,” such as “larks” and “owls”) are, at least partly, rected for these factors. based on genetics (Ebisawa et al., 2001; Jones et al., 1999; Katzenberg et al., 1998; Toh et al., 2001). Chronotypes have been assessed mainly by ques- METHODS tionnaires designed to associate individuals to ten- dencies that were coined “morningness” or This article describes the results of an ongoing “eveningness” (Horne and Östberg, 1976). The ques- study, based on the first 500 completed questionnaires tions used are mostly subjective, relating sleep and (see Fig. 1), which were distributed in Germany and activity times to a personal “feeling best rhythm” Switzerland in high schools, universities, and the gen- (Horne and Östberg, 1976) or to the habits of others eral population mainly between March and June (shift (e.g., “I get up later than most people”) (Smith et al., workers were excluded; for age and gender distribu- 2002), or they ask subjects to assess hypothetical situa- tion, see Table 1). All questionnaires were filled in vol- tions (e.g., “Approximately what time would you get untarily,and subjects were informed about confidenti- up if you were entirely free to plan your day?”) ality and data handling on a cover page (not shown). (Terman and White, 2001). These questionnaires yield We continue to distribute and evaluate the question- plausible results, yet oddly enough, in terms of “real” naire in a project that searches for extreme behavior, they do not explicitly assess free days and chronotypes and their families, and the results pre- workdays separately nor do they ask for actual sleep sented here prove to be extremely stable. This first times (Putilov, 2000) or exposure to outdoor light. report concentrates on the temporal distribution of These details have, so far, been addressed only in a daily behavior and therefore does not address all questionnaire assessing seasonality (Wirz-Justice questions. The temporal structure of daily life was et al., 1996, 2003). assessed separately for workdays and free days. Sub- A growing number of studies investigate the jects are explicitly asked to describe their behavior for genetic basis of the human circadian clock and its rela- normal circumstances, that is, without partying. In tionship to abnormal sleep times using the existing addition, subjects self-rated their chronotype category questionnaires (e.g., Ebisawa et al., 2001; Jones et al., based on a description (0 = extreme early,1=moderate 1999; Katzenberg et al., 1998). However, determina- early,2=slight early,3=normal,4=slight late,5=moder- tion of circadian genes in experimental organisms ate late, 6 = extreme late; see questionnaire in Fig. 1). stems from the excellent ability to quantify circadian A preliminary validation was carried out in a ran- outputs (e.g., period length). Thus, successful genetic dom sample of subjects (N = 30) who kept a sleep log research into the human clock also requires a quantita- for 5 weeks after completing the questionnaire. Actual tive description of circadian traits. Although and indicated sleep times on both workdays and free “morningness” scores correlate with the experimen- days correlated highly (p < 0.0001) with slopes around 82 Self assessment Munich ChronoType Questionnaire (MCTQ) After you have answered the preceding questions, you should have a feeling to which chronotype (time-of-day-type) you belong to. If for example, you like (and manage) to sleep quite a bit longer on free days than on workdays, or if you cannot get out of bed on Monday mornings, even Please enter your age, gender, etc.. This information is important for our evaluations without a Sunday-night-party, then you are more a late type. If, however, you regularly wake up and feel perky once you jump out of bed, and if you would rather go to bed early than to an eve- Age: _________ female male Height ______ Weight _______ ning concert then you are an early type. In the following questions, you should categorise your- self and your family members. On work days … Please tick only one possibility! I have to get up at… _________o ’ c l o c k Description of categories: extreme early type = 0 I need… _________m i n t o w a k e u p moderate early type = 1 I regularly wake up… before the alarm with the alarm slight early type = 2 From… _________o ’ c l o c k I am fully awake normal type = 3 At around… _________o ’ c l o c k , I have an energy dip slight late type = 4 moderate late type = 5 On nights before workdays, I go to bed at _________o ’ c l o c k … extreme late type = 6 … and it then takes me… _________m i n to fall asleep If I get the chance, I like to take a siesta/nap ... I am… 0 1 2 3 4 5 6 correct I then sleep for…_________ m i n as a child, I was … 0 1 2 3 4 5 6 not correct I would feel terrible afterwards On free days (please only judge normal free days, i.e., without parties etc.) … as a teenager, I was …0 1 2 3 4 5 6 My dream would be to sleep until… _________o ’ c l o c k In case you are older than 65: in the middle of my life, I was … I normally wake up at… _________o ’ c l o c k 0 1 2 3 4 5 6 If I wake up at around the normal (workday) alarm time, I try to get back to sleep..
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