What's the best way to reset body clock?

I appear to of become nocturnal – not an insomniac (yet) but rarely able to get to sleep before 4am (currently 04:28am) I get up same time every morning (09:30) but it’s starting to catch up with me. I’m eating ok, I don’t have music, tv or distractions when I go to bed (approx midnight) – just lie there – usually getting anxious as I can’t sleep. Doesn’t seem to matter whether I’ve been sitting in sofa all day or playing sports all day…
Any tips greatly welcomed.