Because your body starts working against it making it less and less effective over time, just like it does to GH, so you need that off time to let your body return to homeostasis before hoping back on. So when u hop back on your body won't be expecting it and you'll it'll work again, mind you it is more complicated then what i stated, i just simplified it because i don't feel like goin into the science geek stuff right now i'm tired, lol.