1. Introduction
Shiftwork, characterized by work that occurs outside of regular daytime hours, is increasingly common in today’s society [
1]. Many industries, including health care, aviation, transport, and mining, require workers to be available 24/7 [
2]. However, shiftwork, particularly nightshift, requires workers to be awake and working at a time that conflicts with their internal circadian clock [
3]. Consequently, shiftwork can be associated with a high risk of long-term consequences for the workers, including metabolic disorders, insulin resistance, type 2 diabetes and obesity [
4,
5], in addition to a higher risk of mood disorders [
6,
7]. One reason for the health consequences of shiftwork may be that, in a 24 h society, eating becomes a 24 h behaviour [
8]. While those on a more traditional day work schedule are most likely to eat three meals per 24 h, with food consumed during the daytime hours [
9,
10], shiftwork usually leads to altered eating patterns with food consumed across the 24 h period, including at night [
8,
10,
11,
12]. Shiftworkers in multiple industries frequently report altered meal timing, with a sample of nurses reporting an increase in altered temporal eating patterns and unbalanced diets compared to day workers [
13]. Similarly, miners on rotating shifts report difficulty with following traditional daytime meal patterns [
14]. The size of the meal eaten on-shift is also influenced by shiftwork, with an increase in snacking behaviors reported during the nightshift compared to the dayshift in samples of nurses [
15], airline crew [
16], oil refinery workers [
17], and transport workers [
18].
Eating at night may be of concern given that, during the night, our bodies are primed for sleep [
1]. During the night, many digestive processes under complex autonomic, hormonal and circadian control, function differently compared to during the day, including slower rates of gastric emptying [
19], gut motility and gastric acid secretion [
20], and reduced glucose tolerance [
21]. This may play a role in the high rate of long-term gastrointestinal disease that is seen in shiftworker populations, including increased rates of irritable bowel syndrome and peptic ulcers [
22], and a high risk of weight-change and obesity [
23]. In the short-term, gastric symptoms are commonly reported on-shift by workers [
8,
24], including disturbed appetite, gastric upset, epigastric pain, gas, and bloating [
20]. In numerous studies, these gastric symptoms have been reported during a non-standard shift and linked with eating behaviours. In flight attendants, eating during the nightshift has been associated with increased reporting of stomach aches, bloating, disturbed appetite, diarrhea, and constipation [
25,
26]. Similarly, in a sample of nurses, certain foods, including core foods, were preferred during the nightshift to reduce digestion issues [
27]. This indicates that food choice on-shift may be influenced by the experience of gastric upset at night [
11].
Reducing the experience of these gastric symptoms on-shift and the long-term burden of metabolic disease on shiftworkers is a priority. A recent laboratory study has shown that not eating during the nightshift and redistributing meals to the daytime hours may limit the metabolic consequences of shiftwork [
28]. Further studies have found that consuming a smaller snack compared to a larger snack during the nightshift leads to reduced glucose impairment following breakfast [
29], and that eating in the morning to avoid meals during the nightshift reduces insulin levels compared to eating during the night [
30]. Similarly, cognitive performance and mood during the nightshift has been shown to be impaired by a large meal compared to not eating during the nightshift and consuming food during the daytime hours [
31,
32]. Taken together, these findings suggest that altering meal timing leads to improved health and performance outcomes for workers.
It is also important to consider how endogenous hunger and appetite sensations influence when shiftworkers may eat. A previous laboratory study used 13 days of forced desynchrony consisting of twelve 20 h days with 13 h 20 minutes of wakefulness and 6 h 40 minutes of scheduled sleep, to help separate endogenous and behavioural effects on daily circadian rhythms, with careful control of meals, sleep, activity, posture, temperature, and light [
33]. This showed endogenous circadian rhythms in hunger, desire to eat, and appetite, all of which decreased during the night compared to during the day [
33]. However, there is very little evidence from controlled studies to help inform whether experiences of hunger, and outcomes of eating, are altered after eating during the nightshift. Only one study has found that participants who did not consume food during the nightshift period reported increased hunger and mild stomach upset compared to those who ate a large meal during the nightshift [
32].
The lack of specific recommendations about different meal size and meal timing leads to confusion for shiftworkers about which behaviors to adopt. For example, flight attendants have reported choosing to avoid eating while on-nightshift to improve health outcomes [
26] whereas paramedics have expressed concern that not eating during the shift could be detrimental to their health [
34]. Further, in nurses, reducing meal size during the nightshift has been reported as a strategy for minimizing gastric symptoms during the night [
35] and avoiding food altogether has been reported as a strategy to maintain alertness [
35]. To extend these results and the recommendations that can be made to workers, alternative meal timing and meal size, including snacking behaviour, should be investigated. The aim of this study was to investigate the impact of eating a large meal, a snack or not eating during simulated nightshifts on hunger, gut reaction, subjective sleepiness, and mood throughout the night.
4. Discussion
This is the first study to investigate the impact of consuming either a meal, a snack, or not eating, during four simulated nightshifts, on hunger, gut reaction, and sleepiness. Ratings of hunger, fullness and thoughts of food followed a dose-dependent response to the eating conditions, with the greatest hunger and least amount of fullness and thoughts of food in the meal at night condition. When considering the increases in sleepiness, headaches and dizziness across the night, those who ate the snack reported the smallest increases in all of these measures.
Participants who consumed a meal during the nightshift reported feeling less hungry, less desire to eat, less thoughts of food, and increased fullness compared to those who consumed the snack or did not eat during the nightshift. This is as expected, given that the food intake pattern of the meal condition provided 20–30% more of their daily EER to be distributed during the night compared to the other conditions. Those who did not eat during the nightshift were significantly hungrier and had greater desire to eat and thoughts of food compared to those who ate the large meal, however they did not report extreme levels of hunger. Whilst this may predominantly reflect a longer inter-meal interval, hunger displays a circadian rhythm and is decreased during the night [
33] and could also play a role. One potential mechanism for this effect could be levels of leptin and ghrelin during the night. Leptin has been shown to exhibit a nocturnal rise and ghrelin a nocturnal decrease [
47,
48]; this corresponds with our results showing no extreme levels of hunger during the nightshift. However, shiftworkers have reported increasing hunger across nightshifts and describe hunger as a factor influencing their decision to eat during the nightshift [
8,
35]. Leptin and ghrelin may potentially be influenced by sleep restriction [
49,
50] and sleep deprivation [
47]. This has also been shown to influence meal choice. In a laboratory study in which sleep was restricted to a 4 h sleep opportunity per 24 h, the odds of choosing to eat a snack were greater than in the moderate or severe sleep restriction conditions [
51]. While leptin and ghrelin levels were not measured in the current study, participants had a 7 h sleep opportunity after each nightshift and were therefore not sleep restricted across the nightshift protocol. Many shiftworkers are chronically sleep restricted and have sleep that is shortened by 2–4 h compared to traditional day workers [
52]. Further, this sleep is often disrupted by external factors such as light and environmental noise [
1]. This may result in increased hunger during the nightshift for shiftworkers in the real world who are working multiple shifts and not receiving adequate sleep, particularly across consecutive shifts when cumulative sleep loss may occur [
1].
Participants who consumed a snack during the nightshift were less hungry and had less thoughts of food than those who did not eat during the night, while also reporting less fullness than the meal at night group. This was expected given that they consumed less food during the time the ratings were captured. Interestingly, there was no difference in desire to eat between those who had a snack and those who had a meal. This suggests that although the snack did not lead to feelings of fullness, participants did not want to eat more after eating the snack. Given the recent research highlighting the potential metabolic consequences of eating a large meal during the night [
28,
30], a small snack, representing only 10% of daily EER, may be a more appropriate and realistic option for shiftworkers. Indeed, snacking is frequently reported by shiftworkers, with many shiftworkers reporting consuming small portions of food during the nightshift, such as muesli bars, nuts, chips, and cookies [
15,
35,
53].
Studies in shiftworkers, such as paramedics, suggest a level of concern that not eating during the nightshift may lead to poor health [
34]. However, that notion is challenged by the current findings suggesting reductions in physical symptom reporting with reduced food consumption during the night. There was an impact of meal timing on ratings of headaches and dizziness, with greater headaches and dizziness reported after eating the meal compared to after the snack or not eating. Studies suggest that shiftworkers report headaches and dizziness on shift when they work overnight [
22]. The current findings suggest that altering meal timing to avoid large meals during the nightshift and consuming a snack or not eating could potentially reduce these symptoms overnight. However, it is important to note that ratings of headaches and dizziness symptoms remained quite low. Future research should further investigate the relationship between meal timing during the nightshift and physical symptoms, such as headaches and dizziness. Interestingly, we found no difference in stomach upset or bloating based on meal timing. Participants who consumed a snack or a meal during the night consumed food when gut function is typically at its lowest [
19,
54]. Given that shiftworkers report an increase in gut related symptoms in response to eating during the nightshift [
26,
27], we expected to find an increase in reports of gut-related symptoms in the group who ate a large meal during the night. In the current study, food was only consumed on one occasion, at 00:30, and perhaps in the real world shiftworkers who report gastric upset are consuming greater amounts of food and eat later in the shift, nearer to the trough of gut function [
19]. Further, one study found that nurses split meals into several snacks consumed across the shift [
35]. This may influence gut reaction differently to one snack or meal consumed at the beginning of the shift. Shiftworkers also often report consuming foods at night that are high in fat and sugar, including cookies, chocolate, cake, and biscuits [
10,
17,
54], whereas in the current study foods were lower in fat and higher in carbohydrate content. Perhaps the foods that are higher in fat have a greater impact on gastric upset during the nightshift. Additionally, while pre-study food diaries were used to ensure that the study diet was not substantially different from the participants’ typical food intake, there may have been some changes for all participants. This could have masked some differences between conditions for gastric upset. Future research should be mindful of using pre-study diets.
Regardless of eating condition, all participants reported increased sleepiness and fatigue, and decreased vigour, across the nightshift. This is expected due to increasing circadian sleep pressure and decreasing alertness across the night [
1]. We found that consuming a snack during the nightshift attenuates the increase in sleepiness and decreases in vigour across the shift relative to consuming a meal or not eating during the night. Consistent with our previous research, we found that sleepiness increased across the nightshift but did not differ between those that had a large meal and those that did not eat during the night [
31]. Given that many shiftworkers eat during the nightshift [
15,
35] the current research has furthered this previous result and showed that shiftworkers may be able to eat during the night without feeling increased sleepiness, however this should be a small snack and not a large meal. During the day, post-prandial increases in sleepiness are commonly reported, with greater sleepiness after a lunch meal [
55,
56,
57]. It is important to note that post-lunch, individuals typically experience an increase in sleep pressure [
58], and therefore a lunch meal potentially exacerbates sleepiness during the day. Consuming the large meal may similarly be exacerbating subjective sleepiness during the night. Of note, in the daytime studies showing post-prandial sleepiness after lunch, the meals were high in carbohydrate [
55,
56]. However, in the current study the snack consumed during the night was higher in carbohydrate than the meal consumed at night (for example, on day 3, the meal consumed during the nightshift was 38% carbohydrate and the snack at night was 56%;
Figure 2). This suggests that the post-prandial effects of different macronutrients can differ based on time of day.
Shiftworkers often report that one of the main factors influencing why they eat on shift is to increase alertness [
11]. In a sample of nurses, dietary strategies and altering eating behaviours were reported as a strategy to maintain alertness during the nightshift [
59]. Similarly, a sample of oil refinery workers reported snacking on savory and sweet snacks while on-shift to improve alertness [
17] and, in the aviation industry, pilots and air stewards report snacking for the same reasons [
60]. Given that participants in the current study received a 7 h day sleep opportunity after each nightshift, sleep restriction was likely minimal. However, in the real world, shiftworkers must cope with factors including environmental disruptions and family commitments, all of which may impact their sleep quantity and quantity [
1], and consequently may lead to increased sleepiness in response to food consumed at night. Given that sleepiness and fatigue are frequently reported by shiftworkers as influencing their eating behaviours, more research is needed to understand the impact of altered meal timing on sleepiness.
This study represents an important first step to investigate the specific impact of altered meal timing during the nightshift. There are two deliberate design elements of this study that preserve experimental control, but limit real-world generalizability. First, this study was conducted in a controlled laboratory environment, with controlled sound, lighting, temperature, and no environmental timing cues, and was conducted with a sample of young and healthy individuals. The sleep laboratory was an ideal sleep environment and participants (screened to ensure that they were ‘good’ sleepers) were allowed a 7 h sleep opportunity after each nightshift. All participants utilized this sleep opportunity, with no differences between eating condition in sleep quality or quantity. This limits direct generalizability to shiftworkers, who may be experiencing chronic sleep restriction [
1], which could lead to altered responses to food intake during the night [
61]. Further studies where daytime sleep is restricted would be of benefit. Second, shiftworkers were not recruited in this study, as this is a group that will commonly experience circadian misalignment and disrupted sleep/wake patters [
1], which may further exacerbate subjective symptoms explored in this study. Further, the generalizability of the results will be strengthened when samples reflect shiftworker populations more closely. This includes older participants and those with habits such as smoking [
62], in addition to the psychological and physical health issues that are especially common among shiftworkers, including obesity [
63], metabolic disorders [
4,
64], sleep disorders [
65], and mood disorders [
66,
67]. Further, shiftworkers report frequent caffeine use whilst on shift [
68,
69] and this is known to influence gastric upset [
70]. These factors were controlled in the current study, however these health factors may be influencing both eating habits and the biological response to food that shiftworkers experience [
71], potentially worsening the impact of food on subjective reactions during the night.
To overcome these limitations and build on these findings, there are several recommendations for future research. While altering meal timing to avoid consuming a meal during the nightshift should be investigated in real shiftwork environments with shiftworkers, there are also a number of laboratory studies that will allow for greater investigation of these variables. In the current study, participants consumed one meal or snack during the nightshift, however we know that shiftworkers often snack multiple times throughout the shift [
9,
53]. Investigating the impact of multiple snacks throughout the nightshift, especially later in the night is important, particularly given hunger reported by those who did not eat during the nightshift peaked at 5:00. Further, investigating different macronutrient balances is important for understanding the impact of eating at night on gut reaction, as shiftworkers typically report consuming foods high in sugar and fat, including chips, chocolate, and fast food [
11]. To recommend eating patterns most applicable to shiftworkers, it is important to understand the factors that influence eating behaviour [
11]. To understand whether altering food timing to consume no food or a small snack during the nightshift is viable in various shiftworker contexts, qualitative methods, such as interviews, will be necessary.