Take a pad of paper and a pen with you. Get in bed, then write down everything you're worried about, even if it's just all you have to do the next day. If it is stress (which can absolutely cause this) that ought to let your brain rest, knowing it can pick the paper - and the worries - back up the next day.

Have you tried going for a long walk before bedtime, or working out until you're too exhausted to do anything BUT sleep?