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Review
. 2019 Jul 12;8(3):54.
doi: 10.3390/biology8030054.

Chronotype and Social Jetlag: A (Self-) Critical Review

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Free PMC article
Review

Chronotype and Social Jetlag: A (Self-) Critical Review

Till Roenneberg et al. Biology (Basel). .
Free PMC article

Abstract

The Munich ChronoType Questionnaire (MCTQ) has now been available for more than 15 years and its original publication has been cited 1240 times (Google Scholar, May 2019). Additionally, its online version, which was available until July 2017, produced almost 300,000 entries from all over the world (MCTQ database). The MCTQ has gone through several versions, has been translated into 13 languages, and has been validated against other more objective measures of daily timing in several independent studies. Besides being used as a method to correlate circadian features of human biology with other factors-ranging from health issues to geographical factors-the MCTQ gave rise to the quantification of old wisdoms, like "teenagers are late", and has produced new concepts, like social jetlag. Some like the MCTQ's simplicity and some view it critically. Therefore, it is time to present a self-critical view on the MCTQ, to address some misunderstandings, and give some definitions of the MCTQ-derived chronotype and the concept of social jetlag.

Keywords: circadian clock; entrainment; light; period; phase; sleep-wake timing.

Conflict of interest statement

T.R. is the founder of the company Chronsulting UG and consults several other companies (Vanda Pharmaceuticals, Chiesi GmbH, jetlite GmbH, Condor Instruments, Salzgitter AG); none of these activities created conflicts with the content of this paper. L.K.P, G.Z., and E.C.W. declare no conflicts of interest.

Figures

Figure 1
Figure 1
Distributions of midsleep on free days (MSF) (left panel) and midsleep on free days sleep corrected (MSFsc) (right panel) in the Munich ChronoType Questionnaire (MCTQ) database (as of July 2017). The distribution is based on half-hourly bins. MCTQ entries were only included in these distributions if all questions of the core-MCTQ were answered, no alarm clocks were used on free days, and values were within a ± 3 σ range. The resulting population sizes were 221,480 for MSF and 185,333 for MSFsc (note that the latter requires information about work status and regular work schedules and is therefore smaller). Color-coding is arbitrary and classifies the population into the seven groups indicated in the legends. The left y-axis shows the local times of the midsleep values, and the right y-axis indicates the sleep window of the respective MSF group (in local time), assuming a sleep duration of eight hours.
Figure 2
Figure 2
Differences in light conditions for the Biological Clock in the pre-industrialized (a) and the industrialized (b) eras. Historically (a), it was the strong difference between natural daylight and darkness that was perceived by the eyes and relayed to the central pacemaker in the suprachiasmatic nucleus (SCN). The SCN neurons entrain to this zeitgeber and transmit this information about day and night to the biological clocks in the rest of the body. Sleep is the major physiological behavior that is under the control of the biological clock interacting with the homeostatic component [67]. The light conditions of the industrialized/urban human environment (see text for details) have resulted in more or less constant light throughout the 24-hour day (b), except for the time when we close our eyes during sleep. This situation can also be described as a short circuit between the inputs and the outputs of the circadian system.
Figure 3
Figure 3
This sleep-log example clearly shows the weekly structure in both sleep timing and duration, which we use as the basis for quantifying social jetlag (SJL). Workday-sleep episodes are dark gray and free day episodes are drawn in light gray. The difference between the average of the midsleep points on workdays (red dots) and those on work-free days (green triangles) is defined as SJL and used as a measure for circadian misalignment (see text for details).
Figure 4
Figure 4
Distributions of social jetlag (SJL) in the Munich ChronoType Questionnaire (MCTQ) database (as of July 2017). The distribution is based on half-hourly bins (population as described in Figure 1 for midsleep on free days sleep corrected (MSFsc)). Color-coding is arbitrary and classifies the population into the six SJL groups indicated in the legends. To signify the distribution of the absolute version of SJL (see text for details), the negative SJL categories are mirrored as light blue extensions of the respective positive SJL categories.
Figure 5
Figure 5
Interrelationship between chronotype (MSFsc) and social jetlag (SJL). (a) On average, increasing SJL is associated with increasing lateness in chronotype; color-coding is chosen according to the distribution shown in Figure 4. (b) Inversely, the later the chronotype, the stronger the SJL. Color-coding is chosen according to the distribution shown in Figure 1.
Figure 6
Figure 6
Social jetlag computation. This schematic illustrates the calculations of social jetlag (SJL) [5] and that for social jetlag sleep-corrected (SJLsc) as recently suggested by Jankowski [98]. Bars illustrate sleep episodes, as well as their timing and duration, on work- and work-free days, and dots/triangles are the respective midsleep times either including or excluding the dashed parts of the sleep episode. SJL is based on uncorrected, actual midsleep times, thus representing the change of mid-darkness between workdays and free days. SJLsc uses midsleep times that were corrected for a potential oversleep or undersleep in an attempt to remove homeostatic confounders from the sleep schedule. The schematic is drawn to scale and is based on the scenario given in the box assuming a late chronotype with early work schedules in a week with five workdays and two work-free days. Abbreviations: SDw/f/week, sleep duration on workdays/on free days/as the daily average across a week; SOw/f, sleep onset on workdays/on free days; MSF, midsleep on free days; MSW, midsleep on workdays; SJL, social jetlag; XXsc, sleep corrected.

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