My first thought would be that: you're body may be releasing endorphins (depending on how far you push yourself, i.e. if the intensity of your running is high), which will give you a sort of high that is easy to get addicted to. However the addiction is not really physiological (but mental) and has no negative effects; I think it's simply an evolutionary function to help us like hard work.
Though, also, sleeping-in can make you tired, so don't do that; as others have said already.
I'm not really sure why you sleep-in in the first place if you end up feeling more tired. Is it simply the feeling at the time? (you don't feel like you want to get up?) |