Insufficient sleep is an independent risk factor for overweight and obesity [1
], and interventional studies demonstrate greater food intake after a night of curtailed sleep [2
] or total sleep deprivation (TSD) [3
]. Insufficient sleep, like obesity [4
], is a global problem. Although the recommended sleep duration for adults is 7–9 h per night [5
], approximately 1 in 3 American adults report sleeping <7 h per night [6
], almost half of Japanese adults sleep <7 h [7
], and nearly 1 in 3 Taiwanese adults sleep <6 h [8
]. Given the large-scale public health implications, the mechanisms by which insufficient sleep increases obesity risk deserve further study.
There are a number of maladaptive changes that occur under conditions of insufficient sleep that could promote an excess intake leading to weight gain. For example, feelings of hunger increase after a night of TSD in males [9
]. Increased ratings of self-reported sleepiness were associated with increased cravings for high-fat sweet foods [10
]. Food cravings reflect a strong desire to consume foods that are palatable; these cravings typically involve sugary, high-fat foods [11
]. An increased susceptibility to food reward under conditions of TSD has also been observed [12
]. Food reward is a measure of the temporary value of food as it is being consumed [13
], and an increased susceptibility to food reward has been associated with an increased intake [13
In addition to the mechanisms promoting the excess intake described above, increases in self-selected portion size merit consideration. The amount of energy consumed at an eating occasion is largely determined by both the type of food and the amount of food selected (portion size) [14
]. Importantly, when people serve themselves, over 90% of individuals consume the entire portion they self-select [14
]. Portion size is influenced by expected satiety, which represents how filling the food is expected to be [15
]. Expected satiety is dependent on the memory of previous experiences with specific foods [16
]. Because insufficient sleep has been associated with poor performance on a variety of memory-related tasks [17
], an exploration of the effects of insufficient sleep on self-selected portion size is warranted.
A considerable amount of work that has examined the physiological, psychological, and behavioral changes under conditions of insufficient sleep has relied on TSD, which does not reflect the real-world experiences of most individuals [18
]. Another popular approach is to subject all participants to a prescribed amount of time in bed, for example, 4 h. Due to natural awakenings during the night, this means that the actual total sleep time will be less than the prescribed time in bed. The time in bed approach results in an uneven curtailment; longer sleepers receive a more severe curtailment than shorter sleepers, which could obscure findings if the study population was skewed towards shorter sleepers or inflate effects if the study population was comprised predominantly of longer sleepers. One way to address the ecological validity issue of TSD and the uneven application of curtailment is to reduce the habitual sleep time by a percentage. Based on the work of others [18
], a 33% reduction was selected, which was projected to translate into a 2–3 h reduction.
The purpose of the current study was to examine the effects of a more modest sleep curtailment, taken as a percentage reduction of habitual sleep time, on factors shown to promote increased intake. These factors included: hunger sensations, food cravings, susceptibility to food reward, and self-selected portion size of foods representing a variety of sweet, savory, healthy, and unhealthy attributes. We hypothesized that all dependent variables would increase under conditions of sleep curtailment.
Insufficient sleep is associated with an increased energy intake [2
] and an increased risk of obesity [1
]. This study evaluated the effect of modest sleep curtailment on hunger, food cravings, food reward, and portion size, all of which have been shown to contribute to excess intake and possible weight gain, but previous interventional studies have used more extreme curtailments [2
] or even total deprivation [3
]. The modest curtailment in this study resulted in significantly reduced TIB, TST, SWS, and REM sleep durations as well as reduced subjective sleep quality. The percentage of REM and SWS ranged from ~21–~29% during both nights, which is consistent with healthy sleep [34
]. The reduction in sleep duration resulted in participants feeling hungrier, reporting both increased food cravings and increased susceptibility to food reward, and selecting larger portions from meal items during lunch time, even though the same amount of breakfast foods and snacks were consumed at the same time on both days.
Despite the fact that participants consumed the same amount of food at the same time each day, hunger at lunchtime was increased, as was the portion size of selected foods and the energy content of meal-associated foods after sleep curtailment. These findings are consistent with other research that reported that TSD led to higher hunger ratings after TSD compared to after a normal night’s sleep in men [3
]. These studies also observed that concentrations of the hunger hormone ghrelin increased, which could contribute to feelings of increased hunger. Others have also reported increased ghrelin as well as decreased leptin after a sleep restriction of four hours for four days [35
]. Not only is such a maladaptive change in hormones capable of influencing appetitive sensations, it can also affect food consumption as higher ghrelin and lower leptin levels are frequently associated with a higher energy intake [35
]. Unlike a previous study in nine men where hunger did not differ between a night of longer (7 h) compared to a night of shorter sleep (4.5 h) [9
], we observed higher hunger ratings in our all-female sample. Whether insufficient sleep affects appetitive sensations differently by sex warrants further investigation. Taken together, these findings suggest that insufficient sleep results in increased hunger and the selection of more energy, in part due to an appetitive hormone dysregulation; however, it should be noted that the question of changes in appetitive hormones under conditions of insufficient sleep is not settled; others have observed that after five days of five-hour sleep curtailment, ghrelin concentrations decreased while leptin concentrations increased [36
]. Clearly, further work is needed to resolve these discrepancies.
The total food craving scores on the G-FCQ-S were higher after the CN compared to after the NN, suggesting that participants had an increased urge to eat palatable foods. Higher food cravings have been associated with excess energy intake and with obesity [11
], although people do not always give in to cravings [37
]. Previous work noted that sleepiness is related to food cravings for both savory and sugary high-fat foods [10
], and individuals who reported a short sleep duration (<7 h) reported higher cravings for high-calorie foods [38
]. Thus, evidence suggests that individuals who experience modest sleep curtailment may also experience increased food cravings, which could promote increased energy intake in susceptible individuals.
Food reward was higher after the sleep curtailment for the work-for-chocolate task, but no differences in the pay-for-food task were observed. It could be the case that a more severe curtailment is necessary to observe differences for both measures. Changes in the brain activity associated with the food reward have been noted after insufficient sleep [33
]. Previous research demonstrated that TSD increased the willingness of men to pay more for food [12
]. That study also observed changes in the hypothalamus activity associated with food reward. A separate study noted that sleep deprivation increased activity in the right anterior cingulate cortex, which is associated with an increased appetite for energy-dense food [33
]. These findings suggest that insufficient sleep increases brain activity when individuals are exposed to food, which likely explains our findings of an increased willingness to engage in the work-for-chocolate task.
Larger portions are associated with increased food consumption [39
]. Previous work reported that people establish appropriate meal sizes based on experience [14
]. This experience plays an important role in determining portion size, more so than hunger [40
]. Still, self-selected portion sizes are subject to variation [3
]. Insufficient sleep is known to impair the ability to pay attention [17
], and eating while distracted can lead to increased portion sizes and an increased intake [42
]. This could be a mechanism by which sleep affects portion size selection. In terms of lunch options, participants plated 12.4% more total calories after the CN. Given the high correlation between self-selected portion sizes and intake [14
], this result suggests that even modest sleep curtailment may result in an increased energy intake. The dietary intake for the rest of the day was not measured, so it is not known if dietary compensation occurs after a larger lunch. However, previous work demonstrated that while energy expenditure increases after insufficient sleep, energy intake overwhelms that increase, resulting in a net positive energy balance [36
Macronutrient content was significantly different for total food selected (combining meals and snacks) after the CN. Calories from fat and total percent calories from fat were significantly higher after the CN, which agrees with previous work demonstrating that sleep curtailment led to higher fat consumption in adolescents [43
]. In addition to fat content, the present study also observed that participants plated more protein from meal foods and from all foods after the CN than after the NN. Protein is considered to be the most satiating macronutrient and can postpone the feeling of hunger longer when compared to carbohydrate and fat [44
]. Whether changing the macronutrient content at meals counteracts the effects of a curtailed night of sleep warrants future testing.
This study did not observe significant differences between the two sleep conditions for energy selected from snacks. This finding is inconsistent with previous work that noted that after TSD or sleep curtailment, participants selected larger portions of snacks [3
]. Since the present study asked participants to choose snack foods at lunchtime, if participants did not consider snack foods appropriate for lunch, differences might be less likely. However, the present study observed that participants took more chips after sleep curtailment, which led to increased fat selected among the snack options. Potato chips are a high-fat, savory food, and previous work suggests that the intake of these foods is higher among adolescents who slept less than 8 h per night compared to those who slept more than 8 h per night [46
]. The fact that potato chips are often consumed during meals as well as during snacks could explain the increased selection. Regardless, these results suggest that sleep curtailment led individuals to choose a high-fat, palatable food that resulted in increased energy coming from fat.
The strengths of this study included the randomized design to minimize order effects, use of objective sleep measurements rather than self-reporting, matching the time and content of breakfast to avoid confounding, centering the sleep curtailment to minimize circadian rhythm disruption, and conducting study visits at the same time two weeks apart to reduce the risk that participants would remember their responses between visits. There are several limitations to the present study. First, the actual food intake was not measured. However, self-selected portions resulted in more than 90% of participants cleaning their plates in a previous study [14
]. Habitual sleep patterns were obtained from self-reports, which can be unreliable, although others have reported moderate correlations between subjective and objective measures of sleep duration (r
= 0.45) [47
]. Only one night’s worth of sleep data for each sleep condition was obtained, which obscures the effects of a possible accumulation of sleep debt; however, we purposefully recruited “good” sleepers with habitual bedtimes in order to minimize night-to-night variability. While menstrual cycles were not measured, randomization should have minimized the effects of menstruation on the study outcomes. Our female-only sample limits generalizability as the effects of insufficient sleep may differ by sex [48
]. While breakfast was not standardized, diet recalls were used to confirm that each day’s breakfast was the same. It is possible that the additional time awake led to increased energy expenditure leading to increased hunger; however, the timing of meals did not differ between days, so this effect is likely minimal but warrants further exploration.