Sleep is generated by the brain, but sleep certainly does not only impact brain function. Several epidemiological studies and interventional studies have provided evidence for an association between sleep parameters and sleep duration, and outcomes such as body mass index and cardiovascular risk factors. However, how strong and clinically significant are these associations, and what happens to sleep if we change energy balance? Are there associations between what we eat and how we sleep? These questions are addressed in several papers concerning the interactions of sleep and metabolism in the first issue of the Journal of Sleep Research in 2014. In a prospective population-based cohort study in 1481 children, Berentzen and colleagues analysed the association between school-week sleep measures such as sleep duration and sleep timing, sleep quality as derived from indicators of nocturnal awakenings and daytime functioning, and cardiometabolic markers (Berentzen et al., 2014). The latter included anthropometry measurements (height, weight, etc.) as well as total cholesterol, high-density lipoprotein cholesterol and glycated haemoglobin (HbA1c), which is an indicator of the average blood glucose levels in the preceding months. The main findings of the study are that there are associations between sleep measures and some metabolic measures, and primarily so in girls, and not in boys. The associations observed in the girls were in the expected direction. Thus, longer sleep was associated with lower body mass index and smaller waist circumference and going to sleep late and rising early was associated with higher cholesterol. Importantly, and as pointed out by the authors, the effect sizes were relatively small, and the clinical significance of the associations is questioned by the authors. Profound sex differences are also reported in a study of anthropometric and metabolic markers as predictors of severity of obstructive sleep apnea (Mazzuca et al., 2014). These two studies, together with the well-documented sex differences in lipid metabolism (Sugiyama and Agellon, 2012) and sleep and circadian rhythms (Mong et al., 2011), highlight the importance of considering sex differences in studies in which the associations between sleep and metabolic markers are considered. If there are important interactions between sleep and metabolism it may be expected that not only how much we eat but also what we eat influences our sleep. Gradner and colleagues address this question in an analysis of the intake of specific dietary nutrients and aspects of sleep in a representative sample of adults (n = 4548) in the United States (Grandner et al., 2014). The aspects of sleep assessed included difficulties initiating and maintaining sleep as well as non-restorative sleep and daytime sleepiness. Diet and nutrient data were based on a 24-h recall procedure. A number of dietary components/nutrients were identified as contributing to the variance explained for the various sleep variables, i.e. difficulty falling asleep, difficulty maintaining sleep, etc. These include well-known contributors to a healthy diet, such as selenium, alpha carotene, calcium and vitamin D. How these associations will hold up in subsequent studies and whether associations will also be observed for objectively assessed sleep remains to be investigated. Nevertheless, the study provides a starting-point for other studies in this important area, for which there is also a substantial interest in the general population. We are often asked: ‘is it true that what you eat in the evening affects your sleep?’. The closely related question, ‘does it matter when we eat?’, is addressed in a study on the effects of intermittent fasting, such as occurs during Ramadan. In this study, food and water intake was restricted to the hours of darkness, while the sleep schedule and caloric intake were controlled (Bahammam et al., 2014). It is reported that whereas total sleep time and non-rapid eye movement (NREM) sleep were not significantly affected, a reduction in REM% was observed. The authors offer several explanations for this effect of a change in the ‘circadian’ pattern of food and water intake on sleep, and this study provides initial evidence that the timing of food and water intake also affects sleep in humans. Overall, these studies highlight the interactions between sleep, metabolism and circadian rhythmicity, an area of research in which, at a basic level, considerable progress has been made in the identification of the molecular elements contributing to these interactions (Bass, 2012).
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