Sleep duration and energy intake: timing matters

Dear Sir:

It was with great interest that we read the article by Kant and Graubard (1). The authors examined both self-reported sleep duration and eating patterns of ∼15,000 adult Americans of the NHANES cohort. The main finding was that short sleepers, ie, those reporting sleeping ≤6 h/night, were found to consume breakfast earlier and consume fewer main meals but more snacks compared with average sleepers (habitual sleep duration between 7 and 8 h). In contrast, self-reported eating behavior did not differ between average and long sleepers (ie, ≥9 h). Finally, the overall self-reported 24-h energy intake was not different between short, average, and long sleepers.

Notwithstanding their elegant demonstration of the association between short sleep and altered eating patterns, Kant and Graubard did not investigate whether short sleepers also eat at times of the day when the circadian system is not metabolically adjusted to process ingested nutrients or stimulants such as caffeine (ie, primarily during the night). Indeed, their article does not report how many of those who were self-reported short sleepers were also shift workers. This is an important aspect of the analysis of food intake in relation to sleep and circadian timing, because shift workers are often forced to eat, and not only sleep, at odd times of their 24-h day, including during the night (2), times during which the body is more insulin resistant and generally disadvantageously adapted to handling food intake. Not only did previous studies link shift work with the development of obesity but they also showed that under conditions of experimental circadian disruption, in which sleep is misaligned in much the same way as experienced by many shift workers, this alters the hormonal profile, increases inflammation, and reduces insulin sensitivity (3, 4). Hormones responsible for determining the appetitive and metabolic response to food intake, such as the hunger-promoting hormone ghrelin and the satiety-enhancing adipokine leptin, are known to be perturbed by circadian misalignment (4, 5). Although with ad libitum food intake such paradigms can lead to increased food intake, circadian misalignment can even promote weight loss under apparently isocaloric conditions, ie, if food intake is equal between groups, matched to the calculated 24-h energy expenditure (5). In an intervention aiming for weight loss, the timing at which food intake occurs was also linked to the success of weight loss (6), such that late lunch eaters lost less weight than did early lunch eaters during the 20-wk intervention.

At the molecular level, when the circadian machinery was disrupted in mice due to genetic mutation of the gene Clock, this resulted in obesity (7). Forcing wild-type mice under ad libitum high-fat diet conditions to flip their circadian eating time by 12 h, effectively eating during their “physiological” night, led to a 48% increase in body weight, as opposed to a 20% increase for the mice with a normal meal pattern (8). Importantly, these differences were noted despite no significant differences in energy intake between the 2 groups of mice.

Importantly, if food is provided at the incorrect time of day, this may in the long run not only cause an increase in body weight but also predispose individuals to develop type 2 diabetes—the risk of which a recent meta-analysis once more confirmed to be elevated for individuals who perform shift work (9). The importance of circadian misalignment for such ultimate consequences of shift work was highlighted by the fact that workers with rotating shifts, in which the timing of both sleep and food intake will keep occurring at time points to which the body cannot adapt to quickly enough from a chronobiological viewpoint, had the highest risk of developing type 2 diabetes. Shift work, or the experimental version thereof, with desynchronizing extended sleep-wake cycles or day lengths (2, 3, 9), are known to lead to reduced average sleep duration and sleep quality.

Although Kant and Graubard did not find any differences in after-dinner meals between short, average, or long sleepers, they found a higher percentage of 24-h energy intake at or after 2000 h in short sleepers (1). This suggests that this sleep group may still consume significant amounts of energy at inappropriate time points of the day (ie, night), perhaps in combination with metabolically equally challenging mistimed sleep. Given that eating at the “wrong” time from a circadian perspective may increase the risk of weight gain, even under neutral energy balance conditions, an important next step would be to examine to what extent specifically misaligned sleep or rotating sleep schedules—habits typically seen in those who “work on shifts”—favor energy surplus and metabolic consequences in cohorts such as NHANES.