Social time pressure, sleep, and eating rhythms—lessons from a pandemic “natural experiment”

The COVID-19 pandemic forced millions of people worldwide into an unplanned natural experiment: a world with fewer alarms, fewer commutes, and more flexible schedules. Once we were less constrained by these external pressures, our sleep and eating patterns appeared to become more interconnected. In their recent study, published in SLEEP, Korman and colleagues [1] suggested that when social time constraints are relaxed, sleep and meal timing become closely linked, and the daily fasting duration lengthens. This work not only highlights the plasticity of human behavioral cycles in response to changing conditions but also raises important questions about how societal structures influence our biological rhythms.

Authors analyzed a large multinational epidemiological dataset of 5,747 participants (average age 37.17 ± 13.66 years, 67.1% female) to explore whether the COVID-19 lockdown (April to May 2020) could relieve the human circadian clock from social time pressures, allowing the observation of an interrelationship between sleep-wake and eating-fasting cycles (partially) independent of modern societal influences. Korman and colleagues [1] demonstrated that reducing social time pressures during COVID-19 resulted in a 34-min delay in the midpoint of fasting and a 42-min extension of the fasting duration, primarily caused by a delay of the first meal without a change in the last meal. This was further linked to increased sleep duration, delayed sleep midpoint, and a longer presleep fasting duration. Moreover, the relaxation of social time constraints resulted in a stronger correlation between the midpoint and duration of fasting with individuals’ chronotype, as assessed by the Munich Chronotype Questionnaire. Taken together, these findings point to a daily sleep–fasting structure, where the timing of sleep not only shapes but also anchors the fasting interval. The stability of this relationship under reduced social time pressure suggests that sleep and eating are more tightly coordinated than previously recognized. This raises important questions about how such coupling links to circadian regulation, metabolic function, and long-term health.

The observed strong connection between fasting and sleep during the pandemic highlights the intrinsic coupling of these behaviors, which are both influenced by the circadian system [2–4]. Circadian clocks, governed by the central pacemaker neurons of the suprachiasmatic nucleus in the hypothalamus, orchestrate a wide range of physiological and behavioral processes, including hormone secretion, energy metabolism, and immune function [5]. When the coupling between the sleep-wake and eating-fasting rhythms become weakened, this may lead to circadian misalignment, and subsequently metabolic disorders, inflammation, and an increased risk of cardiometabolic disease [6–10]. The finding that fasting duration is interconnected with sleep timing suggests that the body organizes its daily metabolic cycles around this paired structure. Notably, in Korman et al.’s study [1], the midpoint of fasting consistently precedes the midpoint of sleep on free days (MSFsc, mid-sleep on free days, corrected for sleep debt; chronotype), suggesting that the timing of fasting may serve as an independent yet complementary marker of circadian phase (eating/fasting chronotype), as well as highlighting the importance of the mediating role of societal obligations.

Social jetlag (SJL), the discrepancy in sleep timing between work or school days and free days, is believed to reflect the misalignment between our circadian system and our behavioral and environmental routines, caused by the weekly waxing and waning of timed social demands such as work hours and school start times [11, 12]. Korman and colleagues [1] indicated that following social demand reduction due to COVID-19, SJL reduced by ~29 min, indicating a substantial alignment of the circadian and behavioral cycles. This matters because higher SJL has been associated with cardiometabolic risk [13, 14]. Therefore, reducing SJL through later school start times, flexible work hours, or other strategies may provide broad health benefits.

Beyond population-level patterns, Korman and colleagues [1] uncovered important behavioral nuances. For example, after social demands relaxed, fasting duration decreased and the fasting midpoint advanced in those who regularly skipped breakfast, opposite to the pattern observed in habitual breakfast eaters. This shift was primarily driven by breakfast skippers decreasing their postsleep fasting duration, effectively becoming more likely to eat in the morning. In addition, they showed that those who were working/studying from home and stopped using an alarm during COVID-19 exhibited a larger increase in fasting duration and delay in the midpoint of fasting compared to those who continued to use an alarm. These changes were primarily driven by longer presleep fasting duration and extended sleep duration, while postsleep fasting duration remained largely unchanged. These findings highlight how social structures can influence or shape individual eating patterns, and how alleviating these pressures reveals latent flexibility in human behavior.

The observed tight connection between sleep and fasting during the lockdown illustrates how these behaviors are inherently coupled and shaped by social constraints. This relationship is of particular interest because mounting evidence links the timing of sleep and eating to metabolic regulation and health outcomes [15, 16]. For example, sleep restriction was shown to alter appetite-regulating hormones, including leptin and ghrelin, leading to increased hunger and higher energy intake [17, 18]. Research on SJL offers an additional perspective on its connection with unhealthy eating habits [19] and a higher risk of overweight/obesity [14]. Moreover, previous studies have also shown that late eating, in particular late-night caloric intake, is associated with metabolic disorders, including obesity [20, 21]. What Korman et al. [1] add is the suggestion that relaxing social constraints allows a stronger alignment between sleep and eating timing to emerge, and allows the sleep timing to be more regular throughout the week.

From a translational perspective, this study highlights the opportunity to modify the current social structure in order to reduce circadian misalignment. Circadian-friendly policies, including later school start times, more flexible work hours, and protected mealtimes, might help synchronize biological and behavioral rhythms, leading to improved cardiometabolic outcomes. However, not all individuals have the privilege of adjusting their schedules, and low-income and shift work populations may be disproportionately affected by these tight work schedules, benefiting less from the relaxation of social constraints even during the COVID-19 pandemic.

Korman et al.’s [1] study is notable for its large and multinational sample size and for using the pandemic context as a rare, large-scale “natural experiment.” However, several limitations warrant consideration. Both sleep and eating times were self-reported, which could introduce recall bias into the data. Despite the large sample size, the study population may not be fully representative of the general population, as online survey respondents tend to have higher levels of education and socioeconomic status. Moreover, the lockdown itself was accompanied by a variety of exposures, including changes in dietary intake, altered physical activity, and emotional stress, all of which could confound the observed association.

The study of Korman and colleagues [1] highlights the critical next steps and provides the foundation for future studies. Objective measures, such as actigraphy (sleep), continuous glucose monitoring (meal timing), and circadian phase assessment (e.g. dim light melatonin onset, blood/fibroblast/hair follicle-based omics or wearables), are needed to validate and extend these findings in future large-scale, population-based, and/or in-depth physiological studies. Intervention trials that manipulate work schedules/school start times could test whether relaxing social constraints can improve metabolic outcomes. Finally, translational research should address heterogeneity in response, for example, why habitual breakfast skippers shifted their fasting pattern differently than breakfast eaters, and also determine how personalized recommendations based on chronotype might maximize health benefits.

The study of Korman and colleagues [1] revealed how the social schedules and constraints shape and sometimes mask the temporal alignment of sleep and eating behaviors. Their results suggest that some modest changes to our social routines (flexible hours, later school start times, protected meal breaks) may change the alignment between different behavioral rhythms and regularize their timing; however, translating these population-based manipulations into better metabolic health will need mechanistic randomized controlled trials.

Disclosure statement

Financial disclosure: F.A.J.L.S. received consulting fees from the University of Alabama at Birmingham, Morehouse School of Medicine, and Salk Institute for Biological Studies. F.A.J.L.S. interests were reviewed and managed by Brigham and Women’s Hospital and Mass General Brigham in accordance with their conflict of interest policies. F.A.J.L.S. consultancies are not related to the current work. The other authors report no conflicts of interest.

Non-financial disclosure: F.A.J.L.S. served on the Board of Directors for the Sleep Research Society. F.A.J.L.S. interests were reviewed and managed by Brigham and Women’s Hospital and Mass General Brigham in accordance with their conflict of interest policies. F.A.J.L.S. consultancies are not related to the current work. The other authors report no conflicts of interest.