I'm thinking you're having high melatonin-levels which could indeed be a result of execessive seratonin-levels. Once you're in a cyle of sleeping a lot your body will produce consequent high levels of melatonin. Try to brake the cyle, clock your sleep on 8h/night and get enough daylight as well.
Obviously you shouldn't be taking melatonin or 5-htp supplements, so are you? And who doesn't love sleeping anyways
it's healthy but to mutch of it can make one lazy and overall tired/unmotivated. Maybe pickup some sports if you haven't already.
It also just might be the case that you need allot of sleep, some people just require more (or less) sleep than other, approx. ranging from 5 - 11 hours a night.