The body has two cycles: A generative cycle which is operative whenever we are active, and a re-generative cycle which is operative when we are sleeping and resting.
First I should remind everyone that people cultivate Qi in four ways: Eating, Moderate Exercise, Sleeping, and Resting.
The generative cycle uses up qi, if we stay in the generative cycle we will slowly get more and more tired until, if we are still healthy, the re-generative cycle grabs us and throws us on the couch. If for some reason we don't have an opportunity to rest our reserves of Qi called Yuan Qi, or original Qi, start getting used. This is what people sometimes call "getting their second wind." If we habitually tap into our reserves of original Qi, it often leads to insomnia, and then slow degeneration; the body's ability to store and distribute fluids becomes impaired leading to weakening of the teeth and spine, loss of flexibility, and eventually death (a final return to the re-generative cycle).
OK, that was grim. But remember sleeping and rest allow our bodies to re-generate so that we don't expend Yuan Qi. Eating, Exercising, sleeping, and resting are all essential. A change in one of the four will produce noticeable changes in the other three.
So where do internal martial arts fit in? If practiced correctly they fall in between exercise and rest. It is possible to practice in such a way that you move between the generative and re-generative cycles. The long term effect of daily practice is that you can easily start up the re-generative cycle while you are in slow motion, doing simple tasks, like making tea, or taking a stroll.
More on this soon: Internal Arts and Death
I got the Art work above from Jamie J. Rice, check it out