3.1. Nature of Shift Work

There are different working hours according to each country, e.g., approximately 35 h per week in France [12], 37 h per week in Denmark [13], and more than 40 h per week in United States (US). In accord with the US, the Association of Southeast Asian Nations (ASEAN) countries, which include Brunei, Cambodia, Indonesia, Laos, Malaysia, Myanmar, Philippines, Singapore, Thailand, and Vietnam, also have more than 40 h of working hours per week [12,14]. Shift work is characterized by working during daytime hours, which comprises fluctuating or rotating patterns among night, early morning, and evening schedules [15].

The magnitude of shift work is regularly growing in the modernized environment as a result of an ascending financial system and enhanced global transportation [16]. Night shift is characterized by the time of the work interval between midnight and five o’clock in the morning, and the shift lasts for at least seven consecutive hours. Shift work allows continuous operation of certain industries by establishing a rotation work schedule among employees [17]. Shift work schedules vary greatly with regard to timing and duration for every shift, and the speed of shift rotation. However, it is widely accepted that prolonged exposure to a shift work schedule is related to increased health complications compared with those working at normal daytime hours (Table 1) [18]. According to World Health Organization (WHO), the definition of “health” is a state of complete physical, emotional, as well as social wellness, and not only mere absence of illness or ailment [19].

An earlier study by Atkinson et al. (2008) reported numerous adverse effects on health outcomes related to shift work, including increased risk of sleep alteration and fatigue, cardiovascular diseases, and obesity [20]. Physiologically, sleep disturbance among shift workers is a result of desynchronization between light–dark phase, sleepiness, and intake of food [8]. This, in turn, gives impact to their physical and psychological wellness and also negatively affects the performance at the work place [21]. Figure 1 illustrates the effect of circadian rhythm disruption on different body systems that include cardiovascular, gastrointestinal, and central nervous system [22,23,24,25,26,27,28].

Open in a separate window

Figure 1

Schematic diagram on the effect of circadian rhythm disruption on different body systems.

3.2. Sleep/Wake Cycle (24-h Circadian)

Humans are mainly active or awake during the day and inactive or asleep at night. The cycle is in synchrony with the light and dark cycle during day and night. Nevertheless, this has evolved remarkably since the 19th century with the artificial light discovery that directed to the opening of the first power plant, which consequently led to the availability of a continuous and reliable point of supply for the electrical power through day and night. The advancement of modern technology provides the artificial light to brighten the night. Hence, this condition is capable to alter the humans to be active at night during the normal sleeping time. When the normal synchronization of the sleep/wake and light/dark cycles are disrupted, humans override the circadian control of the sleep/wake cycle, and that, in turn, promotes internal desynchronization between circadian rhythm and sleep [25,29].

The circadian system coordinates physiology and behaviors towards the environment in a way that the body works as a finely harmonized clock. The suprachiasmatic nucleus (SCN) of the hypothalamus acts as the master clock, synchronizing 24 h rhythms in physiological behavior in the body, inclusive of other brain regions and peripheral tissues. When aligned accordingly to the environment, the clock stimulates sleep and related anabolic functions at night, such as immune function and hormone release, and wakefulness and its associated catabolic functions during the day, i.e., food intake and metabolism, physical activity [30,31]. In addition, the endogenous melatonin rhythm that is controlled by the SCN clock often demarcates the internal biological day and night. For instance, in humans, there is a high level of melatonin secretion during the biological night, and low melatonin level occurs during the biological day [23,32]. Moreover, similar clock oscillators to the SCN clock were identified in peripheral tissues such as gastrointestinal tract liver, muscle, or adipose tissue [33]. It has been shown that consuming food which sometimes contradicts our circadian rhythms can entrain rhythms in peripheral tissues, like liver [34], resulting in long-term health issues [35]. Desynchronization caused by feeding or alternative mechanisms of entrainment in a population of shift workers can lead to defective use of substrates, resulting in the disruption of metabolic pathways, thereby leading to intramyocellular accumulation of lipids and insulin resistance [8].

The natural periodic condition of rest for the mind and body is by having adequate sleep, and the average duration of sleep for an adult is 7 to 9 h per day, as per recommendation by The National Sleep Foundation [36,37]. The individuals may act and think slow, and have tendencies to create more errors if they do not get sufficient sleep. Consequently, sleep deprivation can lead to susceptibility of having typical viral infection, diabetes, obesity, heart disorder, as well as depression [38]. This is demonstrated in Figure 1, i.e., the effect of circadian disruption on important systems in the body.

Reduced sleep quality influences significant changes in the eating behavior among shift workers by boosting their appetite late night, thereby leading to obesity [39,40,41,42,43]. While there is evidence that leptin and ghrelin are altered with sleep constraint, thereby raising appetite at night, there is also evidence that factors other than hunger drive shift workers to eat throughout the night shift. For instance, due to the time available and break availability, there is social pressure to eat with colleagues, as a strategy to stay alert, avoiding gastric upset as well as stress eating [44]. It was also emphasized by Waterhouse et al. (2003) that some of the significant factors of shift workers eating at night are due to habits, time pressure, and social factors [6]. Besides, colleagues also act as significant role models, where they copy positive and negative attitudes from one another [45]. The timing and meal quality are limited by job requirements in the workplace and the social, domestic, and rest requirements outside the workplace [5].

For humans, early morning exposure to bright light helps earlier circadian rhythm advancement progress. Conversely, early night exposure to bright light delays these rhythms [46]. The light exposure pattern can be designed to reset the central circadian pacemaker quickly to earlier or later stages [47]. The shift workers’ sleep schedule is erratic and always abruptly displaced at abnormal circadian phases [46]. The majority of the night shift workers have different degrees of circadian adaptation to their schedules of work [48]. Individuals vary widely in their degree of tolerance towards shifting work because of the intensity of circadian and sleep–wake disruptions [49]. Furthermore, it was reported that the circadian pacemaker’s full adaptation to night shift work would only take place in a minority of workers, even if they work on a fixed night shift schedule [50]. Hence, it is difficult for the adaptation to circadian rhythm to happen among shift workers in view of the rotating shift schedule which is not fixed.

In a shift work simulation study, subjects that develop sleep deprivation tend to choose low nutritional value snacks together with sweetened beverages during odd times of the day [39]. Morris et al. (2015, 2016) confirmed that circadian rhythm misalignment stimulates metabolic dysfunction that eventually leads to weight gain [51,52]. The biological clock performs the function to fix to an energy restrain mode after late night. Therefore, intake of energy-dense food at night promotes rapid fat deposition. In relation to this, meal consumption at night also demonstrates impairment of lipid and glucose tolerance in view of hormonal disturbance in the metabolism [28,53,54]. The health impact with shift work is represented in the form of a table (Table 1).

3.3. Chrononutrition

Chrononutrition is a term that describes food consumption in consideration of the meal timing. Normally, three elements of time are acknowledged, i.e., (1) inconsistency (irregular routine of eating), (2) frequency (the number of daily meals), and (3) clock time (definite time of intake) [58]. The most obvious timings of intake in discussing eating behavior are the breakfast, lunch, tea, dinner, and supper or late-night eating. Previous studies highlighted that changes in the intake for breakfast and supper have a significant impact on the body weight and health [59].

The gastrointestinal tract is lined with enteroendocrine cells that provide endless supply of different hormones based on cues from ingested foods [60]. Subsequently, signals are sent via gut–brain axis to the brain. Energy balance and homeostasis are achieved only by symphonious response of peripheral metabolism in the body. The favorable process of metabolism demands well-regulated eating habits as hormonal regulation occurs at a specific period of time to ensure the optimal process to take place, not too rapid and not too slow. Interestingly, the entire timely process works efficiently based on specific types of macronutrients [61].

The first meal of the day regulates the determination of peripheral clocks’ circadian rhythm, meanwhile, the last meal of the day brings about the process of lipogenesis and accumulation of adipose tissues [62]. Previous studies pointed out that activation of lipolysis was prolonged and lipogenesis process increased among the breakfast skippers [63,64]. There was evidence that night shift workers skipped their breakfast due to the nature of shift that ended early in the morning. It was also reported that the workers preferred to sleep after the night shift [65]. On the other hand, another study showed that post night shift workers often consume a heavy breakfast, followed by sleep [66]. Another example is that of the shift workers who might eat breakfast at the end of a night shift before driving home, while few might eat at home before their daytime sleep, and also others who eat with their family when they get home after a night shift.

It has been understood that since breakfast usually follows the longest fasting period during the 24 h daily cycle, the omission of breakfast could significantly alter metabolism and disrupt the function of gastrointestinal tract. This subsequently leads to reduced availability of nutrients to the brain and likely adverse behavioral outcomes [67]. The effect is similar in those who work normal working hours, but study has shown that shift workers are more susceptible to breakfast skipping. For instance, a study by Axelsson et al. (2004) found that the shift workers might be inclined to skip breakfast so as to allow more sleeping time [49].

However, another important issue that needs to be emphasized when targeting the time of the first meal of the day is the setting of insulin secretion. As insulin fluctuation is closely dependent on meal timing, eating the first meal of the day too early with an insufficient restraining food intake period may result in a weak resetting effect of insulin secretion. All these mechanisms contribute to significant implications on weight gain, appetite, as well as glucose and lipid metabolism [35,62,68]. Morris et al. (2016) also proposed that irrespective of the behavioral cycle consequences, glucose tolerance was lower in the biological evening than in the morning. Thus, it indicated that one of the important factors to consider in shift workers is the internal circadian time of food intake [52].

3.4. Daytime Meal

Eating breakfast is considered to have good effect on the quality of diet throughout the day [69]. Specifically, breakfast is being described as a fundamental aspect in daily nutritional necessity, which is also partly responsible for nutritional quality and energy intake [70]. The definition of breakfast is the earliest eating event that takes place in a day, within the timing of two hours of waking up from sleep, and approximately before 10:00 am [71]. Based on previous studies, it is reported that diseases of dyslipidemia [72], hypertension [73], diabetes mellitus [72], coronary heart disease [74], and weight gain [75] are associated with skipping of breakfast. However, the timing of breakfast among shift workers is subjected to the timing of last energy intake on the previous day, in order to ensure a sufficient fasting period for the body to reset its metabolism.

Barr et al. (2013) examined the association between breakfast, nutritional intake, and nutrient adequacy among Canadian adults in order to specify the type of nutrients to be consumed. Breakfast consumers, especially ready-to-eat cereal breakfast consumers, had positively higher fiber intake as well as several vitamins and minerals compared with breakfast skippers [76]. Besides, a survey of the Bogalusa Heart Study (n = 504; 58% women; 70% white; age of 19 to 28 years old) reported 74% of breakfast skippers did not achieve two-thirds of the Recommended Dietary Allowance for vitamins and minerals, compared with 41% of the breakfast consumers [77]. Furthermore, consumption of protein during breakfast causes better initial and sustaining feeling of fullness, greater satiety, and low concentration of the appetite-regulating hormone ghrelin [78,79,80]. Almoosawi et al. (2013) recommended that a long-term protective effect against metabolic syndrome can be obtained by increasing the intake of carbohydrate in the morning [81].

3.5. Late-Night Meal

Shift workers were found to have nibbling behavior during their night shifts, mainly high-carbohydrate food. The nocturnal feeding imitates night eating syndrome (NES) effects that were first described by Stunkard et al. in 1955 [82]. Multiple studies by St-Onge et al. (2017), Berg et al. (2009), and Cleator et al. (2012) reported that late-night meal was associated with obesity and cardio-metabolic health [83,84,85]. There were no reports on associated similar psychological outcome among patients suffering from NES and shift workers who had late-night eating habits.

Compared with high-fat diets, consumption of foods rich in carbohydrate at night creates a greater increment of sleepiness level and a decrement in mental performance, contrary to the physical act [86,87]. Meanwhile, there was an association between protein intake and diminished feeling of hunger, increased satiety, and reduced caloric intake, in comparison with other macronutrient intakes [88]. Protein-rich meals promote higher satiety and alertness due to the thermogenic effect [89]. Thus, it is recommended for the shift workers to consume high-protein diet at night and reduce the intake of food that is rich in carbohydrate in order to have a productive night shift work.

3.6. Dietary Habit and Meal Changes due to Shift Works

According to the systematic review and meta-analyses by Bonham et al. (2016), it was reported that the energy intake of shift workers lasting 24 h did not vary from that of fixed day workers [90]. It was suggested that eating meals specifically at the wrong time of the 24 h cycle was a key contributor to the increased risk of metabolic disturbances in shift workers [91]. Hence, it was obviously shown that meal timing and chrononutrition was important to consider.

A study conducted by Gifkins et al. (2018) among nurses working in shifts found that there was increased food craving, caffeine consumption, and snacking behaviors throughout the night shifts as well as the inability to consume enough fluids at work. The experienced nurses described more about meal skipping at work and relation with a tremendous workload as well as consumption of alcohol as the approach to rest from shift work [27]. Likewise, it was reported among the mine workers who worked in rotating shift duties that they had difficulty in following typical patterns to consume meals [92]. Meanwhile, for other jobs like flight attendants and long-distance truck drivers, the choices about the meal timing and availability of food were restricted in view of fluctuating work schedules [93,94].

Shift work was also strongly related to abnormal eating habits among nurses according to a study done by Wong et al. (2010) in a major acute hospital in Hong Kong. The Dutch Eating Behavior Questionnaire (DEBQ) scores from the participants for components of abnormal emotional, external, and restraint were 66.4%, 61.4%, and 64.0%, respectively. The results also showed that the nurses having four or more shift works in a month were more likely to demonstrate DEBQ scores of abnormal emotional (adjusted odds ratio (aOR) 2.91%, 95% C.I 1.57–5.42, p = 0.001) and restraint (aOR 3.35, 95% C.I 1.76–6.38, p < 0.001) [95]. Another study on chrononutrition emphasized that those who were having more erratic meal routine had higher risk of developing obesity and metabolic syndrome, despite using less energy than those who had regular meal patterns [58]. Other studies showed lower quality of diet in shift workers, such as greater consumption of sugar [96], saturated fat [97,98], and sweetened beverages [96,99], and poorer intake of vegetables [98,100]. Moreover, hyperphagia, obesity, and insulin resistance were seen in view of circadian rhythm misalignment [101,102,103]. This was because of the regulation of circadian expression and activity of enzymes and hormones engaged in metabolism, which was altered due to the disruption in the clock regulation in the brain and peripheral tissues [26,104,105].

Strickland et al. (2015) reported that the duration of time breaks during working late shifts has a significant effect on the healthy eating among shift workers. Time restriction impedes consumption of healthy food, reduces satiety, and increases binge eating [106]. The prevalence of eating disorder was also found to be higher among shift workers with extreme job stress, reflecting that there was inter-related relationship between psychosocial well-being and nutritional intake [107]. Similarly, an earlier study demonstrated an unusual pattern of restrained or binge eating among shift workers who struggled with high stress [108].

Researchers also suggested that working in shift schedules alters not only the usual meal timing, but also the accessibility of healthy meals, hence, giving rise to multiple health-related problems [96,109]. Shift workers who work during night shifts are likely to consume fast food and processed food with high content of sugar or salt from prepackaged meals or vending machines, due to less availability of food services [6]. Night shift workers frequently experience decreased stamina and sleepiness at work, and while this may be linked to the high content of sugar and the availability of food, it is largely due to the circadian influence of sleep pressure that is greatest at night [110].

Furthermore, there was a considerably larger proportion of energy from sugar consumed over a 24 h period with night shift workers compared with day shift workers [111]. De Assis et al. (2003) found that there was no significant difference in total food consumption between morning, afternoon, and night shift workers. Nevertheless, it was reported that morning shift workers consumed more energy and macronutrients in the morning than the other shift workers. In comparison to the morning shift workers, the afternoon and night shift workers had a higher energy consumption at noon and at dawn, respectively. Meanwhile, throughout their work shifts, night shift workers and morning shift workers ingested high-energy foods and drinks, compared with afternoon shift workers. It can be established that while total energy consumption and composition are less influenced by changes in shift work over a 24 h span, the frequency of meals was reduced and the prevalence of high-energy snacking during the specific shift worked was increased [96].

Figure 2 illustrates the regulation of hormones that synchronically regulate metabolism based on the biological clock [112].

Open in a separate window

Figure 2

Schematic diagram showing effect of different hormones based on circadian rhythm. Adapted from Bass et al. (2010) [112].

Chronobiologically, late-night meals and sleeping during the day disrupt the regulation system and affect the individual’s appetite as well as metabolism. Furthermore, during daytime, humans are predisposed to the promotion of glucose metabolism and fat storage when they normally eat, meanwhile, at night, they are predisposed to glucose sparing and fat metabolism when they normally fast. Due to this predisposition, shift workers present a lowered glucose and lipid tolerance consecutively because of the change from day to night duties [113].

A study by Bandin et al. (2015) examined the influence of meal timing changes on the energy expenditure, glucose tolerance, and circadian-related variables, in which the participants were provided with standardized meals during the two meal intervention weeks and were observed under two lunch eating conditions: early eating (lunch at 13:00) and late eating (lunch at 16:30). It was found that eating late was associated with decreased resting energy expenditure, decreased fasting carbohydrate oxidation, decreased glucose tolerance, blunted daily profile in free cortisol concentrations, and decreased thermal effect on food. It also emphasized the implications on the differential effects of meal timing on metabolic health in this study [54].

3.7. Altered Sleep/Wake Cycle and Nutritional Intake

Baron et al. (2011) and Hsieh et al. (2011) identified that there was an association between short sleep duration of less than 5 h or late sleepers (midpoint of sleep > 5:30 am) and eating late-night meals or consuming more calories late in the evening with significantly greater risk for having obesity and diabetes [53,114]. In addition to that, a study by Colles et al. (2007) also demonstrated that night eating syndrome characterized by a time-delayed eating pattern was strongly related to increased body mass index (BMI) [115].

Meal timing has an important impact on circadian rhythm in peripheral organs, causing feeding time and circadian clocks to be tightly intertwined [116]. Feeding–fasting cycles are generated by time-restricted feeding which consolidate circadian rhythmicity in gene expression and numerous metabolic pathways’ circadian activation. This is because the clock in a majority of the peripheral organs readily responds to feeding cycles, and feeding time can shift their period. Upon a few days of time-restricted feeding, the availability of food and the endogenous clocks are coordinated, irrespectively of whether the meal is served in the light or dark phase. Thus, diet which contains high fat alters circadian rhythmicity through dampening of feeding–fasting cycles [117].

3.8. Psychosocial Well-Being and Nature of Work

Working conditions play an increasingly authoritative role in psychological and mental well-being among changing social circumstances. Due to its nature, psychosocial work conditions could not be characterized by direct measurements in comparison with physical or chemical hazards [118]. Psychosocial factors include aspects like social support in the workplace, job satisfaction, or physical workload [119]. Another study by Goetz et al. (2015) found that there were a variety of psychosocial factors which correlated with burnout, such as symptoms of higher cognitive stress, conflicts between job and privacy, emotional demands and role-conflict, lower general health, satisfaction with life, demands to conceal emotions, and younger practical assistants [120].

Shift workers involved in night shifts reported more psychological and mental health issues than day workers. The problems comprise irritability, somatization, obsessive-compulsive disorder, interpersonal sensitivity, anxiety, altered mood, and paranoid disorders that had more prevalence predominantly [121]. Besides, other studies also showed the average sleep quality among night shift workers to be relatively lower compared with day and no night rotating workers [122,123]. These data highlight that working in shifts especially in night schedule has negative effects on psychological and social well-being [124].

We wish to acknowledge the authors in this review and the collaborating universities.