Relationships between sleep, physical activity and human health
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.
Section snippets
Physical activity, thermoregulation and sleep quality
The sleep–wake cycle is one of the more obvious circadian rhythms in the human. What is not apparent, until physiological measurements are obtained, is that sleep is usually initiated when body temperature is falling and individuals tend to wake up after body temperature has begun to rise [7]. These observations, taken together with other findings–that thermoregulation differs between sleep stages and that sleep is disrupted in non-thermoneutral environments [8]–suggest that there is an
Shiftwork, sleep disruption and physical activity
About 20% of the European workforce is employed in a shiftwork schedule that involves nightwork [35]. Shiftworkers are needed in order to meet society's needs for 24-h services and emergency cover, for maintaining continuing process industries and to offset the economic effects of industrial plant shutdown. Shiftwork is no longer restricted to heavy industry but is now more common in “E-commerce” and call-centre occupations in order to meet the demands for round-the-clock retail, service and
Sleep disruption and eating habits
The results of previous research work by our group indicate that the timing and type of food eaten by shiftworkers are determined more by the opportunity afforded by the work schedule than by hunger [47], [48] (Fig. 2). Altered eating habits are a source of concern in nightworkers, who tend to “nibble” their way through crisps and chocolate bars during the night shift rather than eat a healthy and substantial meal in the middle of it [49], [50], [51]. Indigestion is common in this group of
Sleep, exercise and cardiovascular safety
Myocardial infarction, sudden cardiac death and stroke show circadian variation with peak incidences observed between 06:00 and 12:00 h and the lowest incidences during the nighttime hours [60]. A mechanism that has been postulated for this circadian variation in sudden cardiac events is the rupture of an atherosclerotic plaque leading to thrombosis [60]. Possible triggers of plaque rupture in the morning include increases in platelet aggregation, catecholamine and cortisol levels as well as
Summary
In this review, we have postulated a number of physiologic relationships between the behaviors of sleep and exercise. The transition period between physical activity and sleep onset might be important for ensuring good sleep quality. We have suggested that thermoregulatory and pineal function mechanisms are important in this relationship. Besides the lack of specific knowledge on exercise, thermoregulation and sleep, the prescription of exercise interventions to people, such as shiftworkers,
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