Introduction

Obesity (OB) is a metabolic disorder caused by several factors. Is characterized by the excess of body fat which can be harmful to health. Since 1975, its prevalence worldwide has almost tripled. Currently, more than 650 million adults over 18 years old have OB, which is why the World Health Organization (WHO) classifies this disease as a pandemic global public health problem. Latest studies in Latin America from 2021 indicate that OB affects 24.2 % of the Latin American population, which means that 1 in 4 adults suffer from this disease. In Uruguay, 27.6 % of the population has OB, which means that 3 out of every 10 adults are obese. ^1,2,3,4,5

Within this context of multicausality, inadequate sleep -both in quantity, quality and regularity- is one of the factors that influences energy metabolism through two mechanisms. On the one hand, it affects energy expenditure and on the other hand it affects caloric consumption. The influence on energy expenditure is linked to the presence of drowsiness, and as a consequence of this alteration, there is a decrease in movement and therefore energy expenditure. Regarding caloric consumption, it is altered by the imbalance of the hormones that mediate appetite and satiety. ⁶

Lack of sleep or interrupted sleep has been associated with negative health effects, in addition to those related to obesity, type 2 diabetes, development of cardiovascular diseases (CVD), weakening of the immune system, cognitive impairment and mental health problems (such as anxiety and depression). This is why sleep fulfills an essential biological function in our body. ⁷

According to current recommendations, healthy adults should sleep an average of 7 to 9 hours per night. It is estimated that 35.2 % of the American adult population sleeps on average less than 7 hours, according to the American Sleep Association (ASA). Of the general proportion of adults with OB, 3 to 5 % could be attributable to lack of sleep. Current lifestyle is one of the main factors that has affected the quantity and quality of sleep, altering circadian rhythms (CR) and causing repercussions on health. Sleep is considered one of the main markers of these rhythms, which are responsible for helping certain body processes to be optimized at different times during the 24 hours of a day. ^{(8,9,10,11,12,13}

If the duration and/or quality of sleep are altered, and therefore the CR are also affected, then the processes coordinated by this system will be compromised. It is observed that sleep plays a significant role in the modulation of weight gain, in neuroendocrine signals, and consequently in the homeostatic regulation of glucose and insulin. As a result, alterations in the duration and/or quality of sleep are considered risk factors for the development of OB. ^12,13

In Uruguay, government documents issued by the Ministry of Public Health (MPH) mention short sleep duration as a risk factor for the development of OB, indicating that this essential component in people's health should be considered within the pillars of the treatment of this disease. ¹⁴

Obesity in its treatment must be managed due to the characteristics it presents: a chronic, epidemic, complex, heterogeneous and stigmatized health problem. In Latin America, there is the “Argentine Clinical Practice Guide on Diagnosis and Treatment of Obesity” that provides updated information regarding the management of this disease. This document proposes an approach that faces the problem from the paradigm of complexity, understanding that it cannot be treated with isolated measures. Its treatment requires a multidisciplinary view and an interdisciplinary approach. Reversing the high numbers of OB will have an impact on the morbidity it presents and the health costs associated with it. ¹⁵

Worldwide, 2.8 million people die each year due to comorbidities associated with OB. Therefore, since it is a non-communicable disease (NCD) and at the same time a risk factor for the development of other NCDs, treating it may indirectly affect the morbidity and mortality of these others, such as: type 2 diabetes mellitus, high blood pressure, CVD, dyslipidemia, among others. ^14,16

Previous reviews have already established a relationship between sleep and OB. According to Chamorro et al. and Noh, both the alteration of the quantity and quality of sleep are considered factors that influence the development of OB and other NCDs. As expressed by Cooper et al., in individuals whose sleep duration is regularly less than 7 hours per night there is a greater probability of developing OB or having a higher Body Mass Index (BMI), compared to individuals who have more hours of sleep per night. ^6,17,18

According to the reviews mentioned above, they share the following results that reveal the presence of a link between sleep disturbance and OB. Firstly, numerous studies share in their findings anthropometric variations related to increases in BMI, waist circumference (WC) and weight, when the quantity and/or quality of sleep are affected. In this way, it has been seen that a greater state of fatigue, due to a decrease in hours of sleep, can reduce physical activity levels, causing lower energy expenditure. Another factor that can contribute to a positive energy balance is the alteration that occurs at the endocrine level in the hormones ghrelin (orexigenic hormone) and leptin (anorexigenic hormone), where ghrelin is increased and leptin is decreased. On the other hand, in order to balance a longer time awake, it has been observed that there is an increase in intake, related to hedonic appetite, which is especially linked to a greater desire for foods with high energy density. Likewise, it is observed that a greater food intake during the biological night is associated with unwanted weight gain. ^6,17,18

The objective of this review is to understand the recent evidence about the underlying mechanisms that make a possible relationship between the quantity and/or quality of sleep with the development of OB in adults.

Method

The bibliography presented in this narrative review was extracted from different virtual bibliographic database platforms such as: PubMed, Medline Plus, Scielo, Timbó, MDPI, Mendeley and Elsevier. Regarding the type of publications, articles from scientific journals, reviews (narrative and systematic), clinical trials, randomized clinical trials and meta-analysis were used. Likewise, to extract specific data or figures, publications and files issued by health entities such as: WHO, Food and Agriculture Organization of the United Nations (FAO), Uruguayan Ministry of Health, Official Site of Uruguay (GUB), World Bank, Sleep Foundation, National Sleep Foundation, American Academy of Sleep Medicine (AASM), Sleep Research Society (SRS), ASA, Argentine Ministry of Health.

The selected bibliography must have a range of no more than five years to the current date (2017 to 2022).

The information search was filtered by full text, English and Spanish languages, studies conducted in humans, male and/or female sex, and adult age from 18 to 64 years, or the average in that range.

As a condition, articles that present studies that evaluate sleep quality must apply the Pittsburgh Sleep Quality Index (PSQI). While the studies that evaluated chronotype had to apply Horne and Ostberg’s Morningness-Eveningness Questionnaire or Munich Chronotype Questionnaire. These evaluation instruments have been validated for their application in the adult population.

Controlled vocabulary was applied to search information, using health science descriptors (DeCS and MeSH terms), such as: “circadian dysregulation”, “circadian clocks”, “chronobiology rhythm disorder”, “sleep disturbance”, “sleep deprivation”, “obesity”, “central obesity”, “body mass index”, “weight gain”, “adults”, “circadian rhythm sleep disorders”, “chronobiology disorder”. Likewise, these terms were implemented to search studies in Spanish language, using their respective translations in these idioms. Finally, the articles selected were those whose titles and abstracts qualified as potentially relevant were selected, meeting the inclusion criteria.

Results

After the bibliographic search, it was decided to subdivide the evidence found into different sections, based on the results obtained.

Discussion

In Western societies, sleep has been considerably modified in relation to its 24-hour context. Light is one of the most important synchronizing stimuli for proper development of the CR, with the sleep/wake cycle being a component thereof. During the night, exposure to light can be a disruptive factor for the circadian system. Within the results section, referring to lifestyle and urbanization, according to the study by Pilz et al., the incorporation of electricity and artificial light were events that modified nighttime sleep. On the other hand, Kolovos et al. found that shorter sleep time was associated with higher levels of BMI due to greater time spent using screens and a lack of physical activity. On the other hand, poorer sleep quality was related to higher levels of stress according to Almojali et al. Finally, presenting an accumulated sleep debt, known as the SJL, added to a poor quality of diet and poor quality of sleep as expressed by Bodur et al. In their study, they are factors that were associated with weight gain and possible development of OB. ^6,17,19)F2

Figure 1: Scheme of factors that influence the quantity and/or quality of sleep for the development of obesity in the adult population.  

Through the section referring to chronotype, it can be concluded that a tendency is observed in the evening chronotype to delay energy intake to later times at night and presenting a dietary pattern of lower quality, which together would increase the probability of developing OB due to weight gain. Food is a powerful synchronizer of the CR, commanded by the central biological clock (or pacemaker), located in the suprachiasmatic nucleus in the area of the hypothalamus. It is characterized that eating food during the biological night can affect metabolic and digestive processes. However, while evidence shows the importance of the timing of energy intake in metabolic health, the role of chronotype within this association needs further study.^6,24,25

According to the results in the chrononutrition section, eating at night is a predictive factor for OB if it is maintained chronically. It is interesting to note that eating food at unusual times can result in the development of OB. Chrononutrition encompasses the effects of timing (time of day) in food and the composition of intake (energy and macronutrients), in relation to how the CRs are organized throughout the day. The modern lifestyle has led to 40 % of humans currently delaying their energy intake towards the evening. Therefore, according to the study by Ha and Song, allocating a greater consumption of total daily energy in the morning, in conjunction with adequate hours of sleep (7-8 hours), may be a useful recommendation to reduce the risk of OB and metabolic syndrome. ^6,17,27

In relation to diet quality and dietary pattern, several results have been highlighted in this section. Within the therapeutic pillars of the OB, diet plays an essential role. Not only should caloric intake be reduced to promote negative energy expenditure, but it is also essential to improve dietary quality. Regarding sleep, diet plays a substantial role in its regulation. It has been seen that a diet high in CARBS can unfavorably impact sleep quality. Thus, according to Tavakoli et al., reducing its consumption would reduce the probability of poor quality sleep by 43 %. Likewise, it is highlighted that a high intake of CARBS before going to bed increases sleep latency (component of sleep quality). The findings by Hudson et al. maintain that a healthy American-style dietary pattern high in protein improves the subjective quality of sleep and daytime sleepiness in adults with OW and OB. Specifically, regarding the protein, it stands out that tryptophan, being a precursor of melatonin and serotonin, manages to improve the quality of sleep. Its consumption depends on dietary protein intake as it is an essential amino acid. As obtained in the studies of Muscogiuri et al. and Godos et al., the presence of proteins rich in tryptophan in the Mediterranean diet may result in a good mechanism to promote better quality of sleep. Regarding fats, no studies are mentioned since the results are inconsistent and controversial, requiring more research on the matter. ^{15,17,33,34,35,36}

Sleep is highlighted as an important modulator within neuroendocrine and metabolic functions, and alterations in its architecture can trigger hormonal modifications, such as ghrelin and leptin, hormones involved in the regulation of appetite and energy expenditure. Leptin is secreted by adipose tissue and acts on receptors in the hypothalamus, inhibiting appetite and generating a feeling of satiety. Ghrelin is secreted by the stomach and pancreas, and is responsible for stimulating appetite. Ghrelin levels circulating in the blood vary throughout the day and depend on food intake. It stands out that people who sleep little have low levels of leptin, high levels of ghrelin and greater body weight, unlike individuals who sleep between 7 and 9 hours. As a result, sleep restriction can lead to greater food consumption due to lack of appetite control, increasing the risk of developing OB. The above coincides with the results obtained by Qian et al., where individuals with altered CR presented increased postprandial active ghrelin. ^17,18,26,42

On the other hand, sleep disruption can cause alterations in glucose tolerance. It has been seen, as confirmed in two different studies by Leung et al., that glucose metabolism follows a circadian rhythm. It is observed that there is a greater tolerance to glucose during the day than at biological night, where it decreases. The same happens with insulin, which decreases its sensitivity towards the night. This mechanism can be explained since the lower insulin sensitivity at night, the lower glucose tolerance will also be. This is how it was seen in the study by Hashemipour et al. that subjects with OB and OW associated worse sleep quality with IR. This suggests that improvements in the quantity and/or quality of sleep can promote metabolic health. ^{(17,18,27,28,29}

In order to combat NCDs, developing prevention policies and programs that consider other elements than diet and exercise are necessary. Aspects such as sleep hygiene and the obesogenic environment are some of the nutritional considerations to integrate into the approach in relation to sleep and OB. There are several studies that show a relationship between poor quality and/or lack of sleep with weight gain, Sweatt et al. shows in its findings that the lower the quality of sleep, the greater association of abdominal adiposity. The importance of this statement lies in the fact that abdominal OB increases the risk of mortality and suffering from cardiometabolic diseases. Likewise, González et al. observed that a sleep restriction of less than 6 h can lead to an increase in total energy and macronutrient intake. Therefore, this association suggests that sleep restriction increases total daily energy intake, but not energy expenditure. ^38,39,42

As mentioned above, metabolic alterations occur after sleep restriction. Metabolic behavior differs depending on sex, so Park et al. noted that lower sleep quality is associated more in women with OB than in men. Likewise, Hur et al. observed that women with OB had poorer sleep quality, enhanced by poor diet quality. The origin of this gender disparity is not clear, however it could be explained due to inequalities in metabolic hormones involved in the sleep-wake cycle, such as leptin, as well as in eating behavior between different genders. ^37,40

Finally, shift workers are a population strongly affected due to their work schedules, which necessarily modifies their CR, altering their sleeping and eating hours. In the study by Vidafar et al., it was found that 84 % of individuals reported weight gain since they began shift working, in whom the quality and duration of sleep decreased. On the other hand, Herrera et al. found similar results in his research. Undoubtedly incorporating sleep as one of the therapeutic pillars of OB will be useful in patients with this disease, or in people exposed to a circadian imbalance such as these workers. ^6,41,42

It is necessary to highlight that OB per se causes alterations in the organization of sleep, being a risk factor for the development of respiratory disorders such as sleep apnea. Therefore, this relationship reflects a bidirectionality between OB and the quantity and/or quality of sleep. ^6,14,15

As the main limitation, it is highlighted that the present review mainly presents studies with a cross-sectional design, which does not allow us to establish a causal correlation between the variables. Despite the methodological differences between the various studies used, similar results are obtained, which consolidate a relationship between the quantity and/or quality of sleep with OB.

Conclusions

It is concluded that there is current evidence that relates the quantity and/or quality of sleep with OB in the adult population. There are several underlying mechanisms that explain this link. The reduction in the quantity and/or quality of sleep results in an alteration in neuroendocrine and metabolic functions. About neuroendocrine considerations, hormonal changes in ghrelin and leptin affect the control of appetite and satiety. On the other hand, at the metabolic level, an increase in IR and a decrease in glucose tolerance is generated. Sleeping between 7 and 9 hours per night would be recommended in order to maintain homeostasis in these processes. Other factors such as increased stress, SJL, a poor quality dietary pattern, delaying meals towards the evening, high exposure to artificial light at night, shift work and presenting an evening chronotype, could alter the quantity and/or quality of sleep and result in weight gain. Still, more research is required, in particular clinical trials to elucidate a causal relationship between sleep and OB. In our country, due to the high prevalence of OB, it is important to carry out studies that evaluate the topic of the present review. Having data on the sleeping habits of the Uruguayan population will allow health professionals to integrate sleep as another pillar to be addressed within the prevention and treatment of OB at a public health level.