pretty cut and dry reason why
when you are fasting your insulin stays low and your growth hormone stays high. they run inverse
when you eat insulin rises of course, while hgh drops.
as far as how long, the literature says once you hit 20 hours of a fast is when GH starts to rise and continues to rise for 5 days. think about why this happens, imagine if our ancestors couldn't conserve muscle during a famine, we would have went exctinct