Yujiro Yamanaka and colleagues found that the time of day we exercise influences the autonomic nervous system. Their study involved 22 healthy young males; seven acted as the control group, seven randomized into a morning exercise group, and eight were in the evening exercise group. All participants lived in facility with dim light conditions and followed a strict sleep, eat, and exercise schedule.

Beginning on day three and through day six, participants in the exercise groups performed interval exercise with a bicycle ergometer for two hours. The morning group began to exercise three hours after waking, and the evening group began exercising 10 hours after waking.

Heart rate, which is stimulated by the sympathetic nervous system, significantly increased during sleep in the evening exercise group. The sympathetic nervous system stimulates the flight-fight response. The authors say, “…the evening exercise may continuously stimulate the sympathetic nervous system several hours after physical exercise.” In other words, it looks like exercise late in the day kept the fight-flight response active even during sleep in this group.

In contrast, morning exercise significantly increased parasympathetic activity during sleep. The parasympathetic nervous system slows heart rate and activates digestion, rest, and healing responses. The study indicates that morning exercise improves the quality of nighttime sleep by increasing parasympathetic nerve activity.

The researchers are not sure why or how time of exercise affect the autonomic nervous system during sleep. One hypothesis is that physical activity affects “clock genes” in muscle tissue. Although the primary circadian pacemaker resides in the hypothalamus, organs throughout the body—including skeletal muscle—have clock and clock-controlled genes that display circadian rhythms.

In their 2018 review article, Mirizio et al cite animal studies that show clock-gene experssion increases with physical activity. Not only does clock-gene expression affect circadian rhythm, it also affects muscle function and metabolism.

It may be worth your effort to consider your body, and whether you notice a connection between time of exercise and sleep quality or muscle function.

REFERENCES

Mirizio GG, et al. The Impact of Physical Exercise on the Skeletal Muscle Clock Genes. Kinesiology. 2018;50 Suppl. 1:5-18.

Yamanaka Y, et al. Morning and evening physical exercise differentially regulate the autonomic nervous system during nocturnal sleep in humans. Am J Physiol Regul Integr Comp Physiol.  2015;309:R1112-R1121.