Would you be this upset if she were staying overnight at a convention or with relatives? Or is it just that she's with a lover? If it's the first, it could be that you just need to get used to sleeping alone and finding things you like to do that you don't have a chance to when she's around. Also ask yourself if you're being a bit too needy of her company to feel complete. I know that some couples just never spend much time apart and can't fall asleep easily without the other. My husband and I were never like that, as each of us would take trips away fro various reasons (never lovers - we were mono) and while we missed each other to a degree, we also enjoyed spending time apart. I have always liked a little alone time.
But if it's the second thing, and it really is jealousy and not knowing what shes doing, then I would try to dig down deep under the insecurities you feel and keep asking yourself "what if" questions to find out why it bothers you so much. It sounds like you are okay with her having lovers, but it's just the overnight stays that irk you - so maybe there is a belief system operating or some possessiveness underneath it all that you haven't totally dealt with.