Chronobiology International The Journal of Biological and Medical Rhythm Research

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Actual versus preferred sleep times as a proxy of biological time for social jet lag Konrad S. Jankowski To cite this article: Konrad S. Jankowski (2017) Actual versus preferred sleep times as a proxy of biological time for social jet lag, Chronobiology International, 34:9, 1175-1176, DOI: 10.1080/07420528.2017.1356663 To link to this article: https://doi.org/10.1080/07420528.2017.1356663

Published online: 14 Sep 2017.

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Date: 07 December 2017, At: 12:38

CHRONOBIOLOGY INTERNATIONAL 2017, VOL. 34, NO. 9, 1175–1176 https://doi.org/10.1080/07420528.2017.1356663

LETTER TO THE EDITOR

Actual versus preferred sleep times as a proxy of biological time for social jet lag Konrad S. Jankowski

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Department of Psychometrics and Psychological Diagnosis, Faculty of Psychology, University of Warsaw, Warsaw, Poland

Social jet lag describes a misalignment between social and biological time and originally has been calculated as the difference between midsleep points on workdays and free days (Wittmann et al., 2006) derived from the Munich Chronotype Questionnaire (MCTQ; Roenneberg et al., 2003). In my recent paper (Jankowski, 2017), I indicated that the above formula provides a score reflecting not only this misalignment, but also variability in sleep duration resulting from a sleep debt accumulated over workdays, subsequently slept off on free days. To deal with this issue, I offered a method to reveal the score reflecting the misalignment, which would not be influenced by the sleep debt. This novel approach has been recently acknowledged by Groß and Fritsch (2017), who at the same time questioned how valid it is to conclude about biological time based on actual sleep times. They argued that social influences on sleep times are apparent all week long and gave an example of a person whose sleep times are restricted by work on workdays and parenting on free days. As a remedy they suggested using preferred sleep time as an indicator of biological time. The question of the extent to which people on their free days adapt their activities to their biological time or to social obligations still requires an answer from future research. I agree with Groß and Fritsch (2017) that there are some people for whom actual sleep times may not well reflect biological time. Existing studies, however, indicate that, in principle, actual sleep times do show associations with acknowledged markers of biological time. For instance, Kantermann et al. (2015) proved a high correlation between dim light melatonin onset (DLMO) and midsleep times on free days, which suggests that, in general, actual sleep CONTACT Konrad S. Jankowski © 2017 Taylor & Francis Group, LLC

[email protected]

times can be used for markers of biological time. I agree, however, that there are also individuals for whom such an approach can be invalid, but the MCTQ (Roenneberg et al., 2003), especially in its current form (MCTQ core, 2015), asks about social obligations potentially restricting sleep (e.g. alarm clock, taking care of children, pets, hobby); thus, it can be verified whether social impacts on sleep are substantial. In such cases the data can be either excluded from further analyses or (if this is the specificity of a sample or such individuals compose its majority) treated with an alternative approach. Here, the method proposed by Groß and Fritsch (2017) can be an interesting option, provided that preferred sleep times show associations with acknowledged markers of biological time (e.g. DLMO) that are not weaker than those observed for actual sleep times – an issue to be tested in future validation studies. The method of assessment of preferred sleep times for use in social jet lag calculations still needs to be specified; preferred sleep times could be measured (in addition to actual sleep times on workdays and free days) using the core MCTQ questions (those accompanied by images 1–6; MCTQ core, 2015) with a change to the question about wake-up time consisting of adding a complement imposing no restriction from the use of an alarm clock (e.g. “I wake up at __: __ without an alarm clock” referring to the MCTQ image 5; MCTQ core, 2015). The questions on preferred sleep times could be preceded with a heading “perfect day” and an instruction similar to that for the MCTQ (MCTQ core, 2015), but encompassing parts of the perfect day definition (italicized) proposed by Groß et al. (2017, p. 87), for instance: Faculty of Psychology, University of Warsaw, Stawki 5/7, Warsaw 00-183, Poland.

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K. S. JANKOWSKI

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Below you report on your preferred sleep behaviour during your perfect day. Imagine you have four weeks just for yourself, there is no other person influencing your day, no animal that has to be cared for at specific times, no work, no appointments, you can schedule all events just the way you want to. Please respond to the questions according to your perception of your perfect day at the end of these four weeks.

Groß and Fritsch (In press) suggested calculating two values of jet lag based on preferred sleep times – a difference in sleep timing between workdays and a perfect day, and between free days and a perfect day. Given that no clear physiological basis has been provided justifying differentiation between deviations from perfect day on workdays and free days, a single average weekly deviation can be used instead, reflecting the difference between actual sleep times during a week and preferred sleep times on a perfect day: SJLps ¼ jðnw  MSW þ nf  MSFÞ=7  MSPj; where SJLps denotes social jet lag based on preferred sleep; nw is the number of workdays during a week; nf is the number of free days during a week; MSW is the midsleep on workdays; MSF is the midsleep on free days; MSP is the midsleep on a perfect day. If the direction of the deviation is of interest, the relative value can be used instead of the absolute one. Furthermore, to remove the effects of chronic sleep deprivation (actual sleep is shorter than that on a perfect day) from the indicator, corrections can be used similar to those proposed for the original formula of social jet lag (Jankowski, 2017), resulting in the use of sleep onsets (if actual sleep offset is earlier than

that on a perfect day) or offsets (if actual sleep onset is later than that on a perfect day) instead of midsleep times. To sum up, in order to increase validity various formulas for social jet lag can be tailored to the sleep patterns predominating in different samples/ individuals. Calculations of social jet lag using actual sleep times seem valid, but in the case of subjects with sleep restricted all week long, the preferred sleep times could potentially be used as a proxy of biological time (chronotype). Funding The work has been conducted within the project No. IP2015 026774.

References Groß JV, Fritschi L. (In press). Complementing the social jetlag by considering pristine internal time. Chronobiol Int. doi:10.1080/07420528.2017.1356328. Groß JV, Fritschi L, Erren TC. (2017). Hypothesis: A perfect day conveys internal time. Med Hypotheses. 101:85–9. Jankowski KS. (2017). Social jet lag: Sleep-corrected formula. Chronobiol Int. 34:531–5. Kantermann T, Sung H, Burgess HJ. (2015). Comparing the morningness-eveningness questionnaire and Munich chronotype questionnaire to the dim light melatonin onset. J Biol Rhythms. 30:449–53. MCTQ Core. (2015, October 14). The worldwide experimental platform. Available from: https://www.thewep.org/doc umentations/mctq/item/english-mctq-core Roenneberg T, Wirz-Justice A, Merrow M. (2003). Life between clocks: Daily temporal patterns of human chronotypes. J Biol Rhythms. 18:80–90. Wittmann M, Dinich J, Merrow M, Roenneberg T. (2006). Social jetlag: Misalignment of biological and social time. Chronobiol Int. 23:497–509.