Mark

Levels of testosterone begin to rise upon falling asleep, peak at about the time of first REM, and remain at that level until awakening

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.

All of these disciplines, and many more, make epistemological assumptions.