Chronobiology International The Journal of Biological and Medical Rhythm Research

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Social jet lag: Sleep-corrected formula Konrad S. Jankowski To cite this article: Konrad S. Jankowski (2017) Social jet lag: Sleep-corrected formula, Chronobiology International, 34:4, 531-535, DOI: 10.1080/07420528.2017.1299162 To link to this article: http://dx.doi.org/10.1080/07420528.2017.1299162

Published online: 20 Mar 2017.

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Date: 20 April 2017, At: 12:10

CHRONOBIOLOGY INTERNATIONAL 2017, VOL. 34, NO. 4, 531–535 http://dx.doi.org/10.1080/07420528.2017.1299162

SHORT COMMUNICATION

Social jet lag: Sleep-corrected formula Konrad S. Jankowski Department of Psychometrics and Psychological Diagnosis, Faculty of Psychology, University of Warsaw, Warsaw, Poland ABSTRACT

ARTICLE HISTORY

Social jet lag is a term describing misalignment between social and biological times. In this article, it is argued that the currently used formula for social jet lag captures not only this misalignment, but also sleep debt resulting from sleep deprivation during workdays. It is proposed to adopt the sleep-corrected formula for social jet lag, which takes the form of the difference between the sleep onset on free days and workdays in the case of subjects with longer sleep and later (or equal) sleep onset on free days compared to workdays; it takes the form of the difference between the sleep offset on free days and workdays for subjects with longer sleep and earlier (or equal) sleep offset on workdays compared to free days.

KEYWORDS

In 2003, Roenneberg and colleagues published their groundbreaking paper in the field of chronotype assessment, offering a strictly behavioral marker of chronotype cleared of subjective evaluations used when responding in traditional morningness–eveningness questionnaires (e.g. Morningness– Eveningness Questionnaire; Horne & Östberg, 1976). Their method, based on a new tool, the Munich ChronoType Questionnaire, operationalized the chronotype in physically measured units – time. To be precise, the chronotype was defined as a midsleep point expressed in local time, thus as the time of a halfway point between sleep onset and offset (Roenneberg et al., 2003). Specifically, an indicator of the chronotype was limited to midsleep on free days (MSF), to exclude any impact of social obligations on sleep (Roenneberg et al., 2003; Zavada et al., 2005). There appeared, however, a need to adjust the indicator to the fact that the majority of people sleep off the sleep debt accumulated during workdays (Roenneberg et al., 2004). The resultant formula, midsleep on free days sleep corrected (MSFsc), shifts the midsleep point back toward an earlier clock time, proportionally to the accumulated sleep debt (Roenneberg et al., 2004). Further development of chronotype-related indicators derived from the MCTQ resulted in the introduction of the concept of social jet lag CONTACT Konrad S. Jankowski © 2017 Taylor & Francis Group, LLC

[email protected]

Received 10 December 2016 Revised 4 February 2017 Accepted 21 February 2017 chronotype; circadian misalignment; MCTQ; morningness-eveningness; sleep; sleep debt; sleep deprivation; social jetlag

(Wittmann et al., 2006), describing a discrepancy between biological and social time coming from a conflict between chronotype-driven preference for sleep times and that required to fulfill social obligations (school, work, children, pets, etc.). This discrepancy is typically greater the more evening chronotype a person exhibits, as in most societies start times of school/work are set in the morning. Consequently, social jet lag has become a hypothetical link to explain why some people with evening chronotype experience adverse health outcomes. Amongst other things, social jet lag has been related to unhealthy behaviors such as smoking (Wittmann et al., 2006) or physical inactivity (Rutters et al., 2014), and has been indicated as a risk factor for obesity (Roenneberg et al., 2012) and metabolic dysfunctions that may predispose to diabetes and atherosclerotic cardiovascular disease (Parsons et al., 2015; Wong et al., 2015). A cardiovascular risk profile consisting of heightened cortisol levels, decreased sleep time during the week and increased resting heart rate, was also related to social jet lag (Kantermann et al., 2013; Rutters et al., 2014). When looking at the circadian system, social jet lag is associated with a lowered amplitude of the circadian rhythm of body temperature (Polugrudov et al., 2016). Social jet lag has also been indicated as a factor influencing psychological functioning. Faculty of Psychology, University of Warsaw, Stawki 5/7, 00-183 Warsaw, Poland.

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Specifically, it was related to depressive symptoms, seasonality (Levandovski et al., 2011; Polugrudov et al., 2016), and physical aggression (Randler & Vollmer, 2013). Social jet lag also seems to be a source of concern in education, as it has been related to lowered academic achievement both in undergraduates (Haraszti et al., 2014) and high school students (Díaz-Morales & Escribano, 2015), with the latter study indicating that social jet lag may also impact on basic cognitive abilities. Given the high prevalence of social jet lag in society (Roenneberg et al., 2012), it has emerged as a concern for public health. Thus, valid operationalization of the concept is vital to foster further research. Social jet lag (SJL) has been operationalized as the absolute difference between midsleep on free days (MSF) and midsleep on workdays (MSW) represented by formula 1 (Wittmann et al., 2006):

A

SJL ¼ jMSF  MSWj

(1)

MSF and MSW are expressed in local time and calculated as the mid-point between the sleep onset and sleep offset on free days and workdays, respectively. MSF represents here biological time, as on free days people are supposed to follow their circadian rhythm, whereas MSW represents social time, as on workdays people are supposed to adjust their sleep timing to social obligations (e.g. morning school/work starts; Wittmann et al., 2006). A model example of social jet lag is presented in Figure 1(A) – this person experiences 1 h of social jet lag. This model example may not be, however, the typical real-life situation, as people, particularly evening chronotypes, adjust their wake-up times via alarm clocks rather than bedtimes (Roenneberg et al., 2007; Wittmann et al.,

SJL=1h SJLsc=1h

Sleep duration on: -Free days=8h -Workdays=8h B

SJL=1h SJLsc=0h

Sleep duration on: -Free days=8h -Workdays=6h C

SJL=1h SJLsc=0.5h

Sleep duration on: -Free days=8h -Workdays=7h

Figure 1. Values of social jet lag computed using the original formula (SJL) and the proposed sleep-corrected formula (SJLsc) excluding effects of sleep deprivation during workdays, in three model situations: A – SJL is the result of workday-free day difference in sleep timing, thus, SJLsc provides the same value; B – SJL is the result of sleep deprivation during workdays, thus SJLsc indicates a lack of social jet lag; C – SJL is the result of both workday-free day difference in sleep timing and sleep deprivation during workdays, while SJLsc excludes effects of sleep deprivation and reflects the remaining difference in sleep timing.

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2006). An extreme form of this situation is presented in Figure 1(B). Person B falls asleep at the same time throughout the week, but on workdays wakes up at seven o’clock using an alarm clock and suffers sleep debt, which he/she tries to make up by sleeping longer on free days. Person B also exhibits SJL of 1 h, but this numerical value has a different physiological origin to that in Person A. Person B tries to sleep according to his/her biological time during the week and consequently experiences partial sleep deprivation leading to sleep debt. There are also intermediate cases – a person tries to advance their sleep onset on workdays, but the shift is not enough to prevent partial sleep deprivation (Roenneberg et al., 2007). Figure 1(C) represents just such a situation – SJL of one hour is the result of the workday-free day difference in both sleep timing and sleep duration. Looking at the above examples, the conclusion can be drawn that SJL, when computed using the original formula, may represent misalignment between biological and social time (as intended) in some people (Person A), but sleep debt in others (Person B). The relevance of the latter interpretation is strengthened when one looks at the results of research on sleep debt, which resemble findings on SJL. Amongst other things, sleep debt has been related to metabolic dysfunctions, risk of obesity, and diabetes (Knutson & Van Cauter, 2008; Spiegel et al., 1999), depressive symptoms (Regestein et al., 2010) and poorer academic performance (Shochat et al., 2014). A closer look at the currently used formula for SJL reveals that it captures two interrelated but different phenomena, and a mathematical correction can be offered to enable analysis of the effects of biological/social time misalignment, excluding those of sleep debt. The proposed correction is derived from the one introduced to MSF to address the very same problem of sleep debt. Based on the assumption that the average sleep duration designates individual sleep need (Roenneberg et al., 2004, 2015), the resultant MSFsc consists of adding half of the average weekly sleep duration to sleep onset. MSFsc provides midsleep point unbiased by work-free days alterations in sleep duration. MSW, however, remains biased by these alterations – the greater the sleep deprivation on workdays the earlier

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MSW becomes. In fact, the influence of sleep debt on MSW is much the same as on MSF, but in opposite direction (sleep debt advances MSW and delays MSF), given that the amount of sleep debt accumulated on workdays is subsequently slept off on free days in a proportional manner (Roenneberg et al., 2004, 2015). Consequently, to remove the effect of sleep debt from SJL, it has to be removed from both MSF and MSW. Correcting MSW by adding a half of average weekly sleep duration to sleep onset (instead of adding a half of sleep duration on workdays) provides MSWsc unbiased by sleep debt in the same manner as the MSFsc is. Based on the corrected indicators, a sleep-corrected formula for social jet lag is proposed below (SJLsc): SJLsc ¼ jMSFsc  MSWscj;

(2)

where MSFsc = sleep onset on free days + half of the average weekly sleep duration; MSWsc = sleep onset on workdays + half of the average weekly sleep duration. After mathematical simplification (i.e. removing half of the average weekly sleep duration because it is a constant here), formula 2 takes the following form: SJLsc ¼ jsleep onset on free days  sleep onset on workdaysj

(2:1) The effects of correction can be seen in Figure 1 – in Person A, SJL and SJLsc show the same value of social jet lag = one hour. In Person B, the value of SJL is only due to sleep debt, thus SJLsc, as expected, indicates a lack of social jet lag. In Person C, both a shift in sleep timing and sleep debt were apparent – after removing the effect of sleep debt, social jet lag decreases to the true value of half an hour. The simplified formula (2.1) indicates that it is sufficient to calculate social jet lag based solely on sleep onset times, which is in line with the assumption that in late chronotypes (that is, in those in whom the greatest social jet lag occurs), sleep onset is circadianly controlled (Wittmann et al., 2006). Formula 2.1 is relevant for the majority of people suffering from social jet lag, i.e. for those who sleep longer on free days than workdays and have later or the same sleep onset on free days than workdays. Wittmann and colleagues (2006) indicated that “social jetlag also exists in early types on free days

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when they stay up late into the night without the possibility of sleeping longer the next morning due to their normal circadian wake-up time”. Thus, in early chronotypes, i.e. those who sleep longer on workdays than free days and wake up at the same time or earlier on workdays than free days, the time of sleep offset would be the anchoring point for calculating midsleep. In such cases, the formula for social jet lag, cutting out the effects of sleep debt due to staying up late on free days, would take the following form: SJLsc ¼ jsleep offset on free days  sleep offset on workdaysj

(2:2) There are also possible other, presumably less prevalent patterns of sleep timing throughout a week. For instance, individuals who accumulate sleep debt on workdays and make it up on free days by both advancing bedtime and delaying wake up time. Such rather less typical patterns would be formally defined by longer sleep and earlier sleep onset on free days than workdays or longer sleep and later sleep offset on workdays than free days. For such cases, if at all they are included in analyses, the original formula 1 can be the most safely adopted, as it provides a value equal to the average of outcomes from formulas 2.1 and 2.2. To make the argumentation even more convincing a test of a hypothesis that indicators of sleep deprivation/debt correlate with SJL, but not with SJLsc could be offered. Below are results of a study with the indicators: weekly workday sleep loss = (mean weakly sleep duration – mean workday sleep duration) x number of workdays (MCTQ variables, 2015) and the total score of the Epworth Sleepiness Scale (ESS; Johns, 1991). In the sample of 412 university students analyzed here 74% met criteria for SJLsc formula 2.1, 14% for SJLsc formula 2.2, and 12% for SJL formula 1. For greater transparence, only the first subsample was analyzed using Pearson correlations between weekly sleep loss, ESS total score, SJLsc (formula 2.1), and SJL (formula 1). As expected, the results (Table 1) showed that both weekly sleep loss and daytime sleepiness are related to social jet lag calculated with the original formula 1, but not to the one calculated with the sleep corrected approach (formula 2.1). Next, it was tested whether or not SJL (formula 1) overlaps

Table 1. Pearson correlations between indicators of sleep debt and social jet lag. SJL SJLsc Sleep loss

SJLsc .73**

Sleep loss .62** –.05

ESS .17* .06 .19**

SJL: social jet lag calculated with the original formula 1; SJLsc: social jet lag calculated with the sleep corrected formula 2.1; ESS: total score of Epworth Sleepiness Scale; *p < .01; **p < .001

with sleep loss in predicting ESS scores. Linear regression analysis showed that when SJL and sleep loss are entered together as predictors of ESS then the association of SJL with ESS becomes weaker and statistically non-significant (Beta = .08, p = .26), whereas sleep loss remains significant (Beta = .14, p < .05) in the model (F 2 (2,301) = 6.58, p < .01, R = .04). These analyses indicate that SJL and sleep loss overlap and are redundant in predicting ESS, while SJLsc, as intended, is unrelated to the both indicators of sleep deprivation/debt. In summary, it is proposed to correct the formula for social jet lag to exclude the effects of sleep deprivation/debt on values resultant from the currently used equation (formula 1). To achieve this goal it is proposed to calculate SJLsc as the difference between sleep onset on free days and workdays (formula 2.1), or as the difference between sleep offset on free days and workdays (formula 2.2), depending on the weekly pattern of sleep timing. Acknowledgements The work has been conducted within the project No. IP2015 026774.

Declaration of interest The author reports no conflicts of interests. The author alone is responsible for the content and writing of this article.

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