Intermittent Fasting increases Growth Hormone
Melatonin promotes GH release
Here is another study that I have come across. Basically it appears that shorter rests between sets increase the secretion of GH in response to exercise. Strictly the experiment was only concerning trained women, so perhaps we shouldn't extrapolate too far, but it is certainly another reason why more intense circuits - for example like the metabolic conditioning workouts from Crossfit or some of Bryce's routines - might have some additional attraction.