Introduction

Sleep is a physiological and behavioral state which plays an essential role in the homeostasis, energy metabolism, and cognitive function [1]. Circadian rhythms coordinate most biological processes [2], requiring endogenous and peripheral clocks [3], which regulate the rhythm of physiological functions such as heart rate, gastrointestinal motility, hormonal secretion, macronutrient metabolism, energy metabolism [4], and gastrointestinal functions [5].

Since the beginning of the pandemic, the population has been exposed to a substantial period of social isolation, also called self-quarantine [6]. Quarantine requires people to stay at home [7], changing their lifestyles [8], and inducing people to work at home, which becomes a stressor, considering that it does not have a vaccine and can only be treated symptomatically at present [9]. People lifestyles were changed since the COVID-19 pandemic started, especially sleep, nutrition, and physical activity habits, and during confinement, it could become difficult to shop for fresh groceries, and besides that, shortages of certain food products might happen, causing disruptions in food chains around the world [10]. Sleep restriction has been associated with negative consequences on health and mental wellbeing [11], and it has been demonstrated a relationship between biological rhythms, sleep, and the COVID-19 pandemic [12], and although during the isolation period the public life came to a standstill, some people experienced greater time flexibility regarding social schedules due to home office, leading to an improved individual sleep-wake timing, while others are experiencing sleep restriction and circadian misalignment [11]. Among the main consequences of confinement are changes in chronobiological rhythms and perceptions, feelings of anxiety and fear, and, very importantly, functional and metabolic impairments dictated by a sedentary lifestyle [9]. Changes in the neuroendocrine and immune setting produce adaptive and compensatory responses which effects could last for months, and even for generations (epigenetics changes generated through major nutritional and lifestyle challenges have been evoked as the cause behind the association with a higher incidence of chronic diseases, such as diabetes and cardiovascular diseases, through two or more generations), which require healthy habits to restore the metabolic processes that have been dysregulated [13].

Sleep duration influences the length of time of the day available for eating [14]. Importantly, environmental and behavioral factors induce sleep restriction [15]. Besides that, prolonged wakefulness may induce alterations on digestion, absorption, and nutrient metabolism [16]. These changes in eating and sleeping behaviors may occur during the COVID-19 pandemic and their effects are expected to translate into physical and psychological consequences that will last for a long time after the pandemic period [9]. In this sense, we aimed to summarize the effects of the COVID-19-induced confinement with habit modifications on sleep and nutrition. Moreover, we highlight the possible factors which could be related to sleep disturbances development during the quarantine.

The relationship between sleep, eating habits, and COVID-19 pandemic

The circadian modulation may occur by different stimuli [17]. The regularity of the sleep-wake cycle is important for optimizing health, as well as the function of the circadian system and biological clock. Our organism is governed by rhythmic oscillations that have a periodicity of approximately 24 h, for example, body temperature, cortisol, melatonin, and the sleep-wake cycle [18]. The factors that regulate and synchronize these rhythms are called “zeitgebers,” or time givers. The main synchronizer of circadian rhythms is light, but food [5] and regular sleep times also influence circadian rhythms. Also, the mood exhibits circadian rhythmicity, showing peaks in the late afternoon and early evening [10], as well as the gastrointestinal tract motility, the production of gastric acid by the stomach, and the absorption of nutrients occur during the daytime [19, 20], and may fluctuate in situations of sleep restriction, influencing the transport of amino acids [21] and carbohydrates, which favors the increased risk of metabolic syndrome. Concentrations of fatty acids in terms of lipid absorption also vary during the day, and situations that lead to disturbance of the circadian rhythm also increase the risk of dyslipidemias [22]. The suprachiasmatic nucleus is the body’s central clock [18] which controls the organism using these peripheral clocks. The time of meals is an important “zeitgeber” (particularly those in the liver and adipose tissue), and alterations in feeding time can uncouple the central and peripheral clocks, leading to circadian misalignment and consequently to a disruption in homeostasis and disturbances in many metabolic functions [23].

There are environmental and internal cues related to food intake that are important for the synchronization of feeding and the involvement of pathways that conduct information on food availability for brain structures responsible for the expression of variables synchronized to the feeding schedule. This signaling may be related to responses during the cephalic phase of digestion, or postprandial signs, occurring through neural and/or humoral signs modified by the presence of food in the digestive system; and also for information about the availability of food, which is transmitted to the central nervous system before and during ingestion, for example, through olfactory and/or taste signs [24].

There are multiple humoral signs in controlling food intake [25], which vary over the 24 h of the day, such as ghrelin and leptin hormones, which adjust to the feeding schedule during the fasting period [26, 27]. Both are humoral feedback signals from the periphery to the hypothalamic neuronal network regulating energy homeostasis [27]. Ghrelin promotes deposition of fat [28], whereas leptin stimulates energy expenditure [29]. Importantly, ghrelin and leptin are in part related to sleep because sleep deprivation alters plasma concentrations of these hormones [30], or sleep restriction, which is followed by shifts in hormone secretions [31]. The increase of body weight is related to increased food intake in response to mental stress and more time spent at home, increased opportunity to feed, and increased visual and olfactory stimulation to eat [6]. In this sense, it represents a potential risk of overfeeding nowadays [32]. Sleep restriction is considered to be predictors of weight gain [7], because the rhythmicity in brain areas is therefore a sensitive feeding schedule, increasing the time frame of daily feeding, and may promote fluctuation in body weight, metabolism, and parameters of cardiovascular diseases in humans [33].

In the current scenario caused by the pandemic of the COVID-19, there is growing evidence of a relationship between reduced sleep quality, increased anxiety and mental stress, increased weight, and reduced physical activity due to the COVID-19 and physical isolation, which may also cause physiological alterations, and even result in circadian rhythm disturbances and sleep disorders (e.g., insomnia). Considering that any misalignment in circadian rhythms can, therefore, disrupt a range of biological processes, people who sleep less than they need to feel refreshed consume more calories from fats or refined carbohydrates [34, 35] and have more irregular meal patterns characterized by a predominance of snacks over main meals [14, 36, 37], and lower fruit and vegetable intake [38,39,40]. Highlighting, this period of less healthy food intake is rising as a food habit, because short sleep duration may affect the motivation to eat a healthy diet [41] as well as decrease the desire to eat healthier foods in comparison with energy-dense foods [42] and also represent a trigger to raise the high-fat food intake [43].

As such, the food consumption at night may decrease the energy expenditure during the COVID-19 pandemic and increase energy intake as a consequence of the disruption in the patterns of secretion of the leptin and ghrelin [44]. This disruption may be more expressive among sleep restriction, increasing ghrelin, as well as decreasing leptin hormones [45, 46]. It emerges as an urgent problem of public health, considering that people are developing unhealthy eating habits, rising as a key factor for sleep and mental disorder development during the quarantine period.

Conclusion

There is a direct relationship between sleep and eating behaviors, and both are related to the impacts of physical isolation. There is an influence of eating disorders on the sleeping quality, and the pandemic can predispose to eating disorders and sleeping disturbance due to more free time, more stress, and anxiety on the individual. These factors can negatively affect sleep, which in turn impacts eating habits, inducing stress, which becomes a feedback cycle, contributing to sleep disorders.