Does sleep make it easier to exercise or does exercise make it easier to sleep. Both. And human growth hormone is tied up in both.
Human growth hormone (often abbreviated HGH, or GH for growth hormone, or hGH) is an important part of the body’s endocrine system and is especially active in the growing child’s maturation. (It is not the only physiological factor that makes kids get taller.) HGH is released by the brain into the bloodstream during sleep, and its release is part of the repair and restoration function of sleep.
The hormone is a complex protein produced by the pituitary gland in the brain, and in addition to promoting growth in childhood, it helps maintain healthy bodily tissue even during adulthood. The pituitary gland releases growth hormone non-continuously – the release looks like a pulse. Sleep and exercise induce the release of the hormone.
In normal healthy people, the major period of HGH release is in the first period of Stage 3 sleep during the night.
If a person stays up all night when he or she normally sleeps, there is no surge in growth hormone release. After a period of sleep deprivation, there is extra hormone released when sleep is resumed, and the pattern departs from the normal pulse during slow-wave sleep.
Scientists did an experiment where they hid people away from daylight and other environmental cues that act as circadian hooks. The subjects were allowed to set their own sleep schedule which became disentrained from the 24-hr cycle. This experiment has been done many times, but in this case the scientists measured the blood levels of growth hormone and followed when the pituitary gland released the hormone. They found the peak levels of hormone released during sleep decreased significantly. The first slow wave sleep (SWS) period of the night was shorter and the first REM period happened sooner.
The researchers concluded that the timing of sleep stages can change the amount of hormone released during sleep.
In middle age, the brain starts producing lower quantities of growth hormone. This has spurred interest in use of supplemental hormones to keep the body young. The effectiveness of this is not widely accepted by credible medical authorities, and is not approved by the FDA. (Supplemental HGH is approved for some pathologies, but not for general aging.)
Elite athletes sometimes use HGH supplements (maybe the recombinant form called somatropin) with an eye toward improving performance. This use breaks the rules of many or most leagues and sporting authorities. It is similar to the use of steroids. Responsible doctors always oppose steroid use for performance edge because of the side effects. The side effects of growth hormone may not be as dangerous, but the medical community still looks down on this practice. However, the connection between HGH and sleep coupled with the connection between good sleep and enhanced athletic performance are too similar to be coincidental.
ESPN recently had an article that floated the idea that sleep itself
was a "magic pill" for athletic performance. http://espn.go.com/espn/commentary/story/_/id/7765998/for-athletes-sleep-new-magic-pill
Smart trainers and world-class athletes understand the importance of regular sleep.
Elite athletes often suffer problems due to domestic or occupational schedules that do not permit normal sleep schedules and to rapid travel across multiple time zones (jet lag). Endurance athletes often have a problem with immuno-suppression and chronic reduction in sleep can contribute to this. Indeed, even in non-athletes, sleep deprivation can suppress the immune system.