Relationships between sleep, physical activity and human health

Abstract

Although sleep and exercise may seem to be mediated by completely different physiological mechanisms, there is growing evidence for clinically important relationships between these two behaviors. It is known that passive body heating facilitates the nocturnal sleep of healthy elderly people with insomnia. This finding supports the hypothesis that changes in body temperature trigger somnogenic brain areas to initiate sleep. Nevertheless, little is known about how the core and distal thermoregulatory responses to exercise fit into this hypothesis. Such knowledge could also help in reducing sleep problems associated with nocturnal shiftwork. It is difficult to incorporate physical activity into a shiftworker's lifestyle, since it is already disrupted in terms of family commitments and eating habits. A multi-research strategy is needed to identify what the optimal amounts and timing of physical activity are for reducing shiftwork-related sleep problems. The relationships between sleep, exercise and diet are also important, given the recently reported associations between short sleep length and obesity. The cardiovascular safety of exercise timing should also be considered, since recent data suggest that the reactivity of blood pressure to a change in general physical activity is highest during the morning. This time is associated with an increased risk in general of a sudden cardiac event, but more research work is needed to separate the influences of light, posture and exercise per se on the haemodynamic responses to sleep and physical activity following sleep taken at night and during the day as a nap.

Introduction

Intuitively, one might consider that sleep and physical activity are separate and distinct behaviors. It would seem sensible to suggest that the physiologic mechanisms at work to ensure a relatively inactive and restful sleep are completely different from those that maintain homeostasis during intense exercise. Nevertheless, there are some important relationships between sleep and exercise that have, in fact, been appreciated throughout antiquity.

It was thought in biblical times that the physical work of a labourer promotes good sleep, and that this favorable effect is present irrespective of whether the stomach is full or not; ‘The sleep of a labouring man is sweet, whether he eats little or much’. Nevertheless, there was also a warning for the well-fed, yet idle, rich; ‘…but the fullness of the rich will not suffer him to sleep.’ (Ecclesiastes 5:12). Shakespeare suggested that the above relationship between exercise and good sleep was not just unidirectional. In his play Macbeth, Shakespeare stressed the importance of obtaining a good nights sleep in order to recuperate properly from prior physical work; ‘Sleep that knits up the ravell'd sleave of care, The death of each day's life, sore labour's bath’ (Macbeth 2.2.46–51).

As more scientific knowledge has been gained about sleep, more links between sleep and physical activity have been postulated over and above the general and much-discussed notions that exercise is good for sleep [1], and vice versa [2]. For example, it is interesting that narcoleptic individuals have an increased propensity for sudden bouts of daytime sleep, sometimes in response to strong emotions or even exercise [3]. Sleep apnea is apparently present in a large proportion of American professional footballers [4]. One risk factor for sleep apnea is a neck that is large in circumference relative to its length [5]; this anthropometric characteristic is prevalent amongst many American footballers. There is also a growing body of evidence that the links between sleep, physical activity and the prevalence of obesity are more complicated than were previously believed. For example, a short, not a long, sleep length has been found to be associated with an increased risk of obesity [6]. Given the negative impact of a general sedentary daily lifestyle on obesity levels, such findings are surprising and are, therefore, discussed in more detail later in this review.

The aims of the present review are to summarise the various relationships that have recently been highlighted between the human behaviors of sleep and physical activity. The basic physiologic responses to these behaviors are first outlined, with particular emphasis on the thermoregulatory responses to exercise. The particular problems of disrupted sleep and physical activity patterns of the shiftworker are then discussed, which is followed by a summary of the recent research on short sleep length and obesity. Lastly, some new findings about the impact of an increase in physical activity after waking from sleep on haemodynamic responses and cardiac safety are analysed.