How do you get to sleep when you have insomnia?

I work some very late shifts, almost night shifts (as late as 4am finish) and so my sleep pattern is disrupted . cant go to bed same time each day. I go through stages where i sleep great, but other times get very tired but cant sleep or wake up after only a couple of hours still tired :(
Tried hot milk, reading, tv etc.
dont want heavy duty sleeping pills but is there anything over the counter that helps? some of the herbal ones made me feel weird when I took them, someone told me tylenol pm – but would this be ok every day before bed??