Reviewing the past studies of exercise effects on sleep suggests several pathways through which exercise could produce effects on sleep quality (Figure ). Those pathways could be classified into three categories based on acute effects, acute, but with subsequent chronic effects, and effects arising after patterns of chronic exercise.
Possible effects of exercise on sleep. These mechanisms should be explored in the future works.
Conceivable acute effects would include CNS fatigue, BT elevation, or HR/HRV changes. Whether fatiguing physical exercise causes a directly analogous CNS fatigue is an ongoing question. Even so the effect should not be significant, given the inconsistent results obtained when the effects of one bout of acute exercise on SWS are examined. The hypothesis that BT elevation itself enhances SWS is theoretically interesting, but should be studied in more detail, particularly in humans, especially to compare the effects of passive body heating with those after late-night exercise. Most studies have confirmed HR elevation during subsequent sleep, when HR is elevated during exercise. In addition to HR elevations, sympathetic enhancement was also reported. However, whether these changes have disturbing effects on sleep also needs further study.
Some acute endocrine and metabolic effects of exercise may produce chronic effects when exercise becomes habitual. Habitual repetitions of acute elevations in glucose metabolism, GH, and BDNF releases are conceivable mechanisms that could lead to alterations in sleep. Late-night exercise could lower the glucose level during sleep, although we still do not know its effects on sleep. Even without long-term adaptation, exercise related glucose reductions could have a favorable effect for Type II diabetes, as glucose levels are typically relatively high during sleep (Van Cauter et al., 1991
). Similarly, exercise increases daytime GH release, which might directly affect sleep quality. However, since GH secretion is closely related to SWS (Takahashi et al., 1968
), the long-term effect of repeatedly increased GH releases over 24
h could alter sleep quality.
It is also known that exercise acutely enhances BDNF production. It has not yet been established whether BDNF directly alters sleep quality or not. However, the anti-depressant effects of exercise have recently been well studied, and improved mood attributable to a BDNF elevation attributable to exercise could indirectly improve sleep quality. Such pathway relationships between acute and chronic effects of exercise and alterations in sleep should be examined in the future studies.
One certain beneficial effect of habitual exercise is improved health. Besides, some psychological benefits (which should not be ignored) as well as their interactions with acute and chronic physiological alterations over time, could themselves improve sleep. The addition of non-invasive long-term physiological measurements that do not disturb sleep, such as measures of sympathetic predominance measured by HRV, can reveal such sleep promoting alterations. Studies have shown that exercise definitely changes body composition (Garrow and Summerbell, 1995
) and cardiovascular fitness levels (Warburton et al., 2006
). Meta-analyses (Figure ) indicate that chronic exercise yields more stable and persistent improvements in sleep. These benefits likely reflect the summed and interacting benefits of numerous physiological alterations by chronic exercise, which directly and indirectly affect sleep quality (Figure ).
The lifelong, apparently passive, automatic homeostatic cycling between wakefulness and sleep is influenced by several factors including physical activity, which also involves the prior activity in a large network of neurons of the CNS. These activities influence the global tendency of the CNS for sleep–wake oscillation (Kumar, 2010
). Exercise could be a robust stimulus for the auto-regulatory global phenomenon which affects the entirety of the sleeper’s physiological mechanisms. Appropriate amounts of exercise could alter those mechanisms in a preferable direction, toward restoration of the levels humans originally needed and adapted to for living, offsetting some important health consequences attributable to conveniences in modern society.