IMPORTANCE OF SLEEP

an excerpt from Rippetoe and Kilgore's "Practical Programming for Strength Training"

A spectrum of physiological changes occur during sleep.
Among them, hormonal secretion is perhaps the most
important for recovery from physical exertion. An increase in
anabolic (muscle-building) hormone concentrations and a
decrease in catabolic (muscle-wasting) hormone concentrations
and activity take place during the sleep cycle. Levels of the
anabolic hormone testosterone begin to rise on falling asleep,
peak at about the time of first REM, and remain at that level
until awakening. This means that sleeping shorter durations
limits the recovery contributions possible from testosterone.
Another anabolic hormone, somatotropin, or human growth
hormone, also has a characteristic secretion pattern during
sleep. Shortly after deep sleep begins, growth hormone
concentrations begin to rise, leading to a sustained peak lasting
1.5 to 3.5 hours. A major function of growth hormone is the
mitigation of the negative effects of the catabolic hormone
Cortisol. A disruption or shortening of the sleep period will
reduce the beneficial effects of these important anabolic
hormones.