In essence, the sympathetic system is about action, whereas the parasympathetic system is about rest and restoration. The sympathetic system is activated when we are moving through the world (including when we are exercising). This system requires that we expend energy—we burn calories. The sympathetic system is particularly important when we are seeking to escape something (or someone) that would like to hurt or even kill us. I will have much more to say about these external threats a little later. Conversely, the parasympathetic system is engaged when we are not threatened and can therefore attend to other important physiological functions such as digestion of food, elimination of bodily waste, procreation (the production of sperms for males)—and preparing for and falling asleep. The parasympathetic system is replenishing the calories we burned off when the sympathetic system was in charge.
The important role played by exercise in the preparation for sleep becomes evident when we combine the fundamental principle of homeostasis with the shifting between sympathetic/parasympathetic. The key to optimal health resides in the balance between sympathetic and parasympathetic operations. The human body (and the body of all “higher-order” animals) requires time spent every day in both systems. If the sympathetic system is always in operation then we become chronically stressed, find it hard to eat – and can’t readily fall sleep.
This is the very unhealthy status to be found among many citizens of our post-industrial world. If the parasympathetic system is dominant (a rare state for most of us), then there might be bliss, but also the danger of prolonged inactivity—leading eventually to death (or at least over-reliance on the “kindness of strangers”). The condition of prolonged inactivity can be misleading. We are often in a state of “freeze” (to be discussed shortly), which is when the sympathetic system is fully activated but there is no action (think of simultaneously pushing on the gas pedal and the brake – not very healthy for the car).
We of the 21st Century are challenged in achieving the balance between sympathetic and parasympathetic, given that most of us, living in a world of technological convenience do not have to hand wash three tubs of dirty laundry, broom-sweep three rooms, hunt or forage for food, cultivate twenty acres, extract minerals from the earth, extract fish from the sea, or cut down trees in the forest. The temptation is to sit back, watch TV or surf the Internet, and munch on some junk food (or perhaps a healthier substitute).
Exercises becomes an important substitute for physical labor. After a one-mile jog, 40 minutes on the treadmill or 45 minutes of handball, we are “tired” and want to rest: our parasympathetic system kicks in to balance off the sympathetic functions inherent in our exercise. This is all good and very healthy. We can fall asleep right away or wait until the evening to draw the blankets over our weary body and fall asleep. The latter condition leads us to the second impact and the challenge of explaining how exercise can have a delayed impact.
Delayed Impact: From morning to evening
Here is the question: why is there a delayed impact of early morning exercise on our ability to sleep in the evening? It is like what occurs in the bouncing of a billiard ball off several walls and into the pocket. First (and most importantly), exercise in the morning strengthens the circadian cycle. The rhythm of day activity and night sleep is reinforced, leading to a more strongly enforced command for sleep in the evening. This strengthening is particularly strong if we have the chance to exercise outdoors (wouldn’t it be nice if we could all exercise on the Waikiki beach as portrayed in many of the old exercise videos! Outdoor exercise enables natural stimulants to be absorbed which reinforced the circadian rhythms. With exercise, the circadian drum is beating loud and clear. The band is marching on. You need to start marching in order to keep in time with the circadian rhythm. To be a bit less poetic, exercise during the day promotes alertness and raises the body temperature slightly, so that in the evening, homeostasis kicks in to reduce the body temperature a bit (which induces sleep).
