Sleep and muscle recovery – Current concepts and empirical evidence
Generally, sleep is thought of as a state of rest that helps to recover the body and brain from the physical and cognitive activities during wakefulness. The main characteristics of sleep in contrast to wakefulness support this idea: a barely present motor activity and a very low reactivity to both internal and external stimuli. The long-term atonic state of skeletal muscles is taken as an indication that important restorative processes on the musculoskeletal system take place during sleep – and the state of unconsciousness as well as inadequate responsiveness suggests that the central nervous system pauses to engage in important synaptic plasticity processes. About 40 years ago, therefore, in sleep research, the idea was that especially NREM sleep fosters synthetic processes of growth and repair in the body, and REM sleep was believed to be directed towards increases in synthetic processes within the brain, including the consolidation of memory (Horne, 1979).
The opinion that SWS is mainly for body restitution was encouraged by the observation that after falling asleep during the early night period, human growth hormone (hGH) is present in relatively large amounts in the plasma (Born et al., 1988). Additionally, it was shown that after high loads of physical activity during the day, in the following night the percentage of SWS increased (Shapiro et al., 1981). Moreover, sleep deprivation seems to weaken muscle recovery by increasing protein breakdown, which adversely affects protein synthesis and promotes muscle atrophy (Dattilo et al., 2011, 2012). Muscle recovery would potentially be impaired because this process is strongly regulated by the anabolic and catabolic hormones mentioned above, which are strongly influenced by sleep. On the other hand, the heart muscle and the diaphragm (the main muscle for respiratory activity) prove that the vital contractile elements of a muscle maintain their function without rest over the lifespan. And if they did, it seems more likely to be the motor unit of the supporting and target muscles as a whole (including the reflex pathways) that needs rest (Fitts, 2008). Furthermore, the muscle protein synthetic response following resistance exercise seems rather time-dependent than state-dependent, e.g. amino acids are taken up in skeletal muscle tissue within hours independent of being awake or asleep (McGlory et al., 2017). However, the aforementioned release of hGH during the early hours of sleep and the results from sleep deprivation studies point in the direction that some regeneration processes are dependent on sleep, and most likely on SWS (Dattilo et al., 2011).
The proposed hypothesis that bodily recovery and regeneration are sleep-dependent is widely used, but often lacks extensive supportive experimental evidence. In this presentation we will discuss the current concepts of muscle recovery during sleep and review the studies in that field.
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