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    • Another relevant factor that contributes to the development

    2018-11-05

    Another relevant factor that contributes to the development of metabolic disturbances associated with sleep restriction is energy expenditure. Individuals who sleep less are more likely to experience fatigue and sleepiness during the day, which may discourage them from daytime physical activity and promote sedentary behaviors [81,82]. However, the literature presents varied results, in part due to differences in sleep protocols, either total sleep deprivation [83,84] or partial sleep restriction [85,86], and measurement type (doubly-labeled water [86,87], indirect calorimetry [84], metabolic chamber [83,88], or actigraphy [85,86]). The majority of studies have enrolled small samples with only young, normal-weight men [83,84,88]. Thus, more studies are necessary to determine if sleep duration does affect total energy expenditure and if there is difference between men and women, normal weight and overweight/obese, and young and older individuals. It is possible that improving physical activity could improve sleep, which in turn would impact other components of energy balance, since individual variability in sleep onset latency is reduced by regular physical activity [89]. Not only can sleep affect metabolism, but metabolic changes also can affect sleep architecture [73]. There is an association between late-night-snack intake and sleep patterns [56]. Rodents adjust their sleep onset to match food availability during their sleep–wake mth1 inhibitor [90]. Inversely, food restriction can increase sleep onset latency and reduce total SWS [91]. The common behavior of overeating during a period of sleep deprivation may be a physiological attempt to restore sleep, as it is known that higher food intake promotes sleep [92]. The impact of sleep duration on energy expenditure is less clear due to the multiple factors involved, such as sleeping metabolic rate, thermic effect of food, physical activity, non-exercise activity thermogenesis, etc. [66]. In summary, current literature shows a pattern of increased food intake during periods of sleep loss, mostly in lean and normal sleepers. To date, studies looking for the influence of sleep duration on energy expenditure have produced disparate results due to methodological issues. Due to individual variability, future research assessing whether improving physical activity would improve sleep is also desired for a better understanding.
    Stress and metabolism Similarly to sleep, stress is also connected to metabolism. Basal HPA axis activity seems to be dysregulated and overactive both in humans with diabetes and in animal models of type 1 and type 2 diabetes, underlining the neuroendocrine abnormalities common to diabetes-related risk factors such as depression, obesity, hypertension, and cardiovascular diseases [93,94]. Exposure to stressful events leads to increased release of glucocorticoids by activation of the HPA axis [95]. Prolonged activation of the HPA axis may result in maladaptive changes [96], affecting puberty, stature, body composition, as well as leading to obesity, metabolic syndrome, and type 2 diabetes mellitus [97]. Excesses in glucocorticoids increase glucose and insulin and decrease adiponectin levels [98]. Stress exposure alters food intake [99], increasing or decreasing it, depending upon the type of stress [100]. For instance, Ely and colleagues [99] showed that rats subjected to repeated stress by restraint presented increased ingestion of sweet food, while models of chronic variable stress demonstrated a decrease in appetite for sweet food or palatable solutions [101]. There is evidence that glucocorticoids stimulate appetite [102] and increase body weight through the orexigenic effect of NPY [103], an effect that is inhibited by leptin and insulin [103]. Clinical studies also reveal high food consumption, specifically of palatable food, during periods of psychological stress [104]. The increase in palatable food intake is induced by glucocorticoids [105] and is associated with reward-based eating, as a way to reduce the stress response [106].