I say quality, because quantity is not always a good predictor to justify if you are getting the benefit of sleep. Some people sleep six hours a night and feel great, while others feel the need to sleep ten hours and still wake up tired. The quality of sleep has a lot to do with the adrenal glands. The major stress hormone secreted by the adrenal gland is cortisol. Fluctuations in this cycle can understandably reap havoc on sleep cycles.
Overtrained athletes have a tendency to output high levels of cortisol throughout the entire day.
They usually feel jittery and always on the go. Depending on how this is handled, the roof can cave in and their highs will soon drop to lows. Other individuals may suffer from too much or prolonged stress, resulting in low cortisol levels throughout the day. They go to bed exhausted and wake up tired. Exercise usually gives them a jump-start to get them through the day.