Energy content of the evening meal alters nocturnal body temperature but not sleep

Abstract

Meals of varying energy content and episodes of sleep influence body temperature. We compared the effect of an evening meal, varying from high-energy (11.91 ± 0.86 MJ) to average (5.74 ± 0.88 MJ) and a 10-h fast (no evening meal), on nocturnal body temperature and sleep. Seven healthy men (20–24 years, mean body mass index of 23.4 ± 2.6 kg/m²) reported to the laboratory for an evening meal at 2000 h having consumed similar amounts of food before 1300 h. After completing the meal, subjective hunger ratings were assessed, and a venous blood sample taken. The subjects spent 4 nonconsecutive nights (an adaptation night, followed by either of the two meal conditions or the fast in random order) in the sleep laboratory when polysomnographic recordings were made from 2300 to 0700 h. Meal energy content and serum concentrations of insulin, triglyceride, and low-density lipoproteins (LDL) varied significantly. Lower rectal temperatures were measured during the fast than following the meals. Over the 8-h recording period, thermal response indices (TRI) varied with higher body temperatures following the higher energy meal. Similar rectal temperatures were attained by the end of the sleep periods. There were no significant differences in any of the subjective or objective sleep measures. The physiological responses associated with the transient dietary changes of an evening meal or a 10-h fast altered nocturnal body temperature but did not significantly affect sleep of good sleepers when sleep was initiated 2 to 3 h after finishing the meal.

Introduction

Pronounced temporal differences in eating patterns commonly occur. Daily food intake on weekends or holidays greatly exceeds that of a “working weekday” [1]. This difference is further exacerbated on celebratory occasions when the macronutrient content of meals tends to shift in favour of those with a higher fat content [2]. In addition, there are times when people go to sleep either soon after eating a large meal or without consuming an evening meal. The type and timing of a meal influences the concentrations of fuel metabolites, substrate utilization, and the associated hormones 3, 4, 5, 6. Furthermore, these physiological responses to a meal affect the thermic effect of food (TEF), and the concomitant changes in body temperature 7, 8, 9 may, in turn, be influenced by postprandial sleep [10].

Postprandial sleep studies have focused on daytime naps shortly after a meal, but food intake is also relevant for nighttime sleep, particularly when a large meal is eaten in the evening, close to bedtime. In mammals, sleep is a common postmeal behavior [11], with humans frequently reporting increased subjective ratings of sleepiness after a large meal 12, 13. Ratings of sleepiness also show a close temporal relationship with heat loss [14].

The energy and macronutrient content of a meal with their concomitant effects on insulin, other metabolic and gastrointestinal hormones, and fuel substrates may influence metabolic rate, the TEF, body temperature, and heat loss 7, 14, 15 as well as sleep. Factors influencing individual variations in the TEF include body fat content [16] and insulin status 7, 9. To a large extent the TEF depends on meal size [16], spice content [17], and macronutrient composition [18], with a clear hierarchy of macronutrient effects. Diets high in protein induce the highest TEF, and those high in fat, the lowest [18]. A lower TEF is also associated with lower energy meals or a behavioral change such as an episode of postprandial sleep rather than wakefulness [13]. Body temperature varies in association with the metabolic effect of food, being highest when TEF is high 7, 15.

In contrast to the body temperature raising effect of food, the onset of sleep is associated with a decrase in metabolic rate and in body temperature by about 0.4°C (for review, see [19]). Metabolic heat production in humans decreases during sleep by between 5 and 17%, being lower in rapid-eye movement (REM) sleep than during wakefulness, and lowest during nonREM sleep [19]. Within a normal physiological range, altered nocturnal temperatures may not significantly fragment and disrupt sleep, whereas extremes of temperature do [19]. Thus, the intake of a meal may induce sleepiness, which may be enhanced by the concomitant TEF and subsequent heat loss. Indeed, under constant routine conditions the induction of heat loss appears to be a good predictor of a shorter sleep onset [14]. Should sleep occur soon after a meal, variable dietary intake and the subsequent effects may alter sleep architecture [20].

To date, acute alterations in daily energy intake on nocturnal sleep and body temperature have not been investigated. Midday postprandial sleep studies showed no change in sleep with altered energy content [13]. However, the meals in that study were given in a liquid form [13], and more recent evidence has shown a shorter sleep onset following a solid meal [21].

Because sleep is associated with a reduction in metabolic rate, heat loss, and body cooling, acute perturbations in energy intake that would be expected to affect metabolism and lead to changes in body temperature, possibly lasting for more than 4 h, may also influence sleep. Eating an evening meal with a high-energy content may increase sleepiness [22], but should the higher body temperatures be sustained, sleep may be disturbed. In comparison, by going without food for 1 evening and imposing a need to conserve energy, heat loss and body temperature would be reduced while non-REM sleep may be increased. We, therefore, examined nocturnal body temperature and sleep in healthy young men in response to extremes of energy intake: after a high-energy meal, a 10 h fast when no meal was consumed in the evening, as well as after a control meal of average energy value.