Lots of different ideas, from a fellow midnight wanderer:
1. Have a scotch - will not be a permanent solution and you could get dependent (not good news if you require a little snifter to sleep until you are 83 years old)
2. short course of sleeping tablets from your gp - will only be a short course as these are very addictive, but you will get some kip and restart your body into a sleeping routine
3. do not kip during the day, even if you are dog tired as this will promote a dodgy sleeping pattern
4. about 1 hour before bed, have a long hot bath, read for a bit and have a milky dring - anything relaxing you think will promote sleep
5. do not exercise in late afternoon/evening, but continue usual routines earlier in the day
6. if you have a partner, wear them out in the evening (ahem!) in bed - it always works for me!
7. cut down on coffee and other similar crap, particularly none after 6pm
8. improve your diet - feeling bloated and stodgy will not induce sleep (unless you drink lots of alcohol, too!)
9. deal with any stress you are experiencing - this is usually the biggest cause of no sleep on the planet. it should be banned!
There are some ideas - they may or may not work for you. But the biggest factor has to be stress - it makes me wander in the early hours, even if i am pooped. it drives my partner mad, too as he always comes to find me to see if i am ok - sweet eh! but don't suffer for too long - lack of sleep can send you round the twist and you'll wonder why you didn't sort it sooner once you've got the knack - good luck!!!