Background
Obesity, which reflects energy imbalance related to increased dietary intake and low energy expenditure, has become one of the world’s most significant health problems in recent few decades [
1]. The effectiveness of obesity treatment based on reduction of food intake and increase in energy expenditure is low, both at population and patients’ levels [
2]. It results in an increase in the number of morbidly obese patients for whom bariatric surgery is the only method leading to a significant weight loss. Many patients, however, fail to maintain the achieved weight and become morbidly obese again. Therefore, a better understanding of factors that impede obesity treatment is needed to develop comprehensive and effective therapy. Eating and sleeping are two kinds of behavior that are essential for the survival of humans [
3]. Sleep is a major modulator of hormone release and glucose metabolism regulation. Food intake is controlled by the neuroendocrine system and the central nervous system [
4]. Previous studies have demonstrated that short sleep duration as well as experimental sleep deprivation in healthy humans increase hunger and appetite [
5,
6]. In a cross-sectional study in Japanese females poor sleep quality was found to be significantly associated with consumption of energy drinks and sugar-sweetened beverages, skipping breakfast, and eating irregularly [
7]. This finding suggests that unhealthy food habits may be associated with insomnia symptoms, and prospective weight gain in individuals with sleep disorders. Thus, the crosstalk between sleep quality and metabolism plays a key role in the regulation of food intake and energy balance, affects obesity development and should be taken into account in both obesity treatment and prevention. Insomnia is the most common sleep disorder [
8]. The insomnia diagnosis according to the fourth version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) includes difficulties in initiating or maintaining sleep or experiencing non-restorative sleep for a period of 1 month or more and the symptoms result in a significant impairment in daily functioning [
9]. In clinical practice to measure insomnia symptoms Athens Insomnia Scale (AIS) is commonly used [
10,
11]. Various risk factors for insomnia have been identified in the general population. Female gender and obesity were reported to increase the risk of chronic insomnia [
12]. Previous studies have indicated that individuals with obesity are significantly more likely to report insomnia [
13,
14], which could suggest that some individuals sleep worse because they are obese. This could be due to obstructive sleep apnea (OSA) that often coexists with obesity [
15]. It was shown that among individuals presenting complaints related to sleep apnea, the co-occurrence of insomnia varies between 6 and 84% [
16]. On the other hand, insomnia may predispose to overconsumption of energy or night eating, thus leading to weight gain [
14]. Unhealthy behaviors, including unhealthy food choices and consuming excessive amounts of food after sleepless night, are probably driven by coping mechanisms and hedonic stimuli processing in the brain. Thus snacking unhealthy food is not performed to satisfy hunger or thirst, but to compensate for sleep disturbances and to improve mood [
17]. There is an evidence for a combined relationship between unhealthy eating and depressive symptoms [
18,
19]. Snacking on food and/or beverages was associated with an increased odds ratio for depression in 24,697 Japanese adults after adjusting for sleep problems [
18]. While in a study in 376 Japanese adults, participants with more than two unhealthy eating behaviors had a higher incidence of depressive symptoms compared to those with fewer than two unhealthy eating behaviors [
19]. Also nocturnal snacking was associated with greater depressive symptoms and it was shown that the nocturnal snacking was higher among the bariatric surgery candidates than among participants from general community [
20].
Insomnia co-occurs with depression [
21] and can have serious consequences [
22]. Several recent studies have demonstrated that insomnia is associated with metabolic disorders. [
23‐
26]. It was shown that both sleep disturbances (difficulty in initiating or maintaining sleep) and sleep duration (< 5 h or more than 9 h) are risk factors for type 2 diabetes [
27,
28] and in patients with type 2 diabetes poor sleep quality as assessed by the Pittsburg Sleep Quality Index (PSQI) was associated with longer duration of diabetes [
29,
30] and poor glycemic control [
30]. On the other hand, insomnia symptoms were recognized to be associated with physical inactivity [
31,
32]. Reduced motivation to exercise may be caused partly by tiredness. There is some evidence supporting physical activity as a nonpharmacologic treatment for sleep disturbance [
33].
Improving sleep quality appears as an important point of intervention, which can enhance effectiveness of obesity therapy including effectiveness of bariatric surgery associated weight loss. The evidence of adverse effects of poor sleep on dietary intake is based mainly on studies of experimental reduction of sleep duration [
34,
35], and there is a lack of studies in which objective measures of sleep were used. For the present study, we hypothesized that sleep problems as assessed by Athens Insomnia Scale (AIS) are associated with unhealthy eating habits in bariatric surgery candidates. Moreover, the relationships between the insomnia and depressive symptoms, sedentary lifestyle, and obstructive sleep apnea (OSA) that often coexists with obesity may be of importance for effective obesity treatment.
Discussion
The present study shows that the participants reporting daily consumption of snack foods and eating in response to 3 or more emotions had the highest AIS and BDI-II scores. Unhealthy eating may be a coping mechanism for sleep deficit as well hard digestible food may deteriorate the quality of sleep. Therefore, the bidirectional relation between sleep disturbances and eating habits can be suggested. Nevertheless, it is not clear from our cross-sectional study whether sleep disturbances influence consumption of snack foods or vice versa. Very few previous findings suggest that unhealthy food habits are associated with insomnia symptoms. The relationship between dietary intake and sleep was examined in Japanese female workers. The results showed that low intake of vegetables, high intake of confectionary, and unhealthy eating habits were associated with poor sleep quality assessed using the Pittsburgh Sleep Quality Index (PSQI) [
7]. On the contrary, mainly weak and inconsistent associations between insomnia symptoms and poor food habits in Helsinki Health Study were found [
31]. However, the Finnish study was unable to confirm any associations probably due to general questions on food habits and suggestive categorization of food habits as healthy or unhealthy. On the other hand, an experimental reduction of sleep duration was accompanied by increased intake of calories from snacks [
34,
48], increased food purchasing in normal-weight men [
35], and increased hunger and appetite, especially for calorie-dense foods with high carbohydrate content [
5]. These associations seem to be related to the alterations in appetitive brain signaling [
49]. Neural activation was measured by functional magnetic resonance imaging (fMRI) in twenty-three healthy participants examined on two sessions: a night of normal sleep and a night of total sleep deprivation. Sleep deprivation significantly decreased activity in the anterior cingulate cortex, lateral orbital frontal cortex and anterior insular cortex (brain regions known to be instrumental in appetitive desire and food stimulus evaluation) and increased the amygdala responsivity to desirable food items. Additionally, increase in desire for high-calorie foods, which positively correlated with the severity of sleep deprivation, was demonstrated [
49]. Thus, deeper insight in potential determinants of eating behaviors is needed to identify new effective strategies for obesity prevention and treatment. The prevalence of insomnia in our study (47% participants scoring ≥6 on the AIS) is similar to insomnia frequency assessed in the population of 6079 Latin American women aged 40 to 59 years (43.6% had insomnia, AIS) [
50]. Also sedentary lifestyle, defined as fewer than three weekly 30-min periods of physical activity, was very common in surveyed American women (63.9% of them were self-defined as sedentary). Sedentary women had more depressive and insomnia symptoms as compared with non-sedentary women. As expected, in our report 65% studied patients with obesity described their physical activity as low or declared psychical inactivity and classified their lifestyle as sedentary. The statistical analysis revealed that these individuals had significantly higher AIS and BDI-II scores than those with self-reported “high” physical activity. It has been considered that cultivating exercise habits, reducing sedentary time and improving sleep quality may be important strategies for obesity prevention [
51‐
53]. Our present research indicates that insomnia and depression symptoms commonly coexist in obese individuals, and it is consistent with findings of a meta-analysis of 19 original papers reporting that sleep deprivation alters mood [
54] and the results of a large prospective population-based study — the Nord-Trøndelag Health Studies (HUNT2 and HUNT3) showing that insomnia is associated with more than twice the odds for depression [
22].
Sleep disturbances are considered as a risk factor of obesity [
55,
56]. However, available findings are inconsistent and high BMI was reported to be not associated with the insomnia symptoms or insomnia as a syndrome [
57,
58]. We also found no correlation between BMI and Athens Insomnia Scale scores. Nonetheless, it should be emphasized that participants in our study had extremely high BMI. In addition, insomnia might be directly associated with increased consumption of palatable food, but not with a general tendency to eat more. This suggests that insomnia symptoms may contribute to unhealthy eating patterns or increased amounts of energy consumed at inappropriate time points, i.e., at night. It is important to note that other studies revealed late-night eating in response to sleep restriction [
59], insomnia [
14] or depressive symptoms [
20]. Similarly, 2% of the studied bariatric surgery candidates reported daily episodes of eating at night, which was associated with the highest AIS scores (median = 9) and BDI-II scores (median = 19). However, interpretation of our findings is limited due to the small group of every night eating individuals.
In the present study, the AIS and BDI-II scores were high in participants who self-reported eating in response to more than 3 different emotions. Emotions may affect human eating behavior. Indeed, in the current study we demonstrated that depression was associated with a tendency to eat in response to more than 3 emotions as well as with unhealthy snack consumption among individuals with obesity. Our data support the previous findings showing that emotional eating can be associated with increased BMI [
60] and with depressive symptoms [
61].
Adequate sleep quality and duration are important for the appetite regulation and normal functioning of metabolic and hormonal processes [
62]. Moreover, many experimental and epidemiologic studies link sleep disturbance with alterations in glucose homeostasis [
63]. For instance, it was shown that short sleep time (4 h per night for 6 nights) in young, healthy men was associated with lower glucose tolerance than in the fully rested men [
64]. In healthy individuals, glucose tolerance varies throughout the day; glucose concentrations are markedly higher in the evening than in the morning, and glucose tolerance is reduced in the night [
65]. Circadian rhythm is important modulator of glucose homeostasis and sleep loss may result in metabolic alterations. In our study, high AIS scores were associated with high glucose and triglycerides concentrations in bariatric surgery candidates. This finding is consistent with the hypothesis that changes in the quantity or quality of sleep may affect carbohydrate and lipid metabolism. The amount of research investigating the relationship between insomnia symptoms and glucose and lipid abnormalities in obese patients is limited. However, prior reports demonstrated that shift workers from two plants of northern France had significantly higher levels of serum triglyceride than day workers, but there was no influence of shift work on total cholesterol and HDL cholesterol concentrations [
66].
Obstructive sleep apnea (OSA) is a highly prevalent respiratory disorder, characterized by recurrent episodes of upper airway obstruction occurring during sleep, and associated with recurrent cycles of desaturation and re-oxygenation [
67]. Although OSA is characterized by fragmented sleep, not all affected patients complain of insomnia [
12,
16,
68]. In our study, no correlation between AHI and insomnia symptoms as assessed with AIS was observed, however, 25% of the study participants had mild OSA and only 11% had severe OSA. Our results support the hypothesis that patients with OSA of mild to moderate severity may not necessarily report low quality of sleep, as it was previously suggested [
69].
The current findings should be considered in the context of several limitations. First, to assess insomnia we used the Athens Insomnia Scale, a self-report validated questionnaire, which is a subjective instrument to evaluate sleep disturbances. Unfortunately, we were not able to utilize PSG to measure objective electrophysiological parameters of sleep quantity and quality. Second, this study did not allow assessing insomnia symptoms in a relationship to amount of calories consumed, because we did not assess the overall caloric intake for each participant. We used self-report measurements of physical activity and there might be a concern that the recall of physical activity may be subjective in some cases. Our study also did not assess all the aspects of night eating, which may be relevant when diagnosing night eating syndrome (NES), i.e., consuming ≥25% of total daily calories after the evening meal, or the type of food consumed [
70]. Instead, we have assessed night eating frequency using the Eating Disorder Examination Questionnaire (EDE-Q) [
43]. Another limitation of our study may be related to a larger number of females than males. Therefore, future studies should focus upon identifying any potential gender differences. Our study participants had extremely high BMI and obesity-associated metabolic abnormalities, and some of them were under pharmacological treatment for dyslipidemia and/or type 2 diabetes. Despite taking medications that improve lipid or glucose profile we still could demonstrate in our patients the influence of sleep disturbances on studied biochemical parameters.
The dramatic increase in obesity prevalence overlaps the reduced sleep duration and quality, which are hallmarks of the modern society. Sleep disturbances and depressive symptoms may be considered as a risk factor, which contributes to unhealthy eating behaviors that might result in the development of obesity.