GH is regulated over very short periods. GH pulses occur about every 4 hrs, although GH itself has an inhibitory effect on its own release, it's IGF-1 that is the main negative feedback molecule. IGF-1 increases in response to GH, and it's really the levels of IGF-1 that will determine when the next GH pulse occurs. IGF-1 increases expression of somatostatin, but as IGF-1 levels decrease, expression of GHRH becomes dominatn over somatostatin, and the result is another GH pulse (release of GH from the anterior pituitary). Again, this occurs roughly every 4 hrs, with things like fasting, resistance training and sleep patterns altering it slightly