If you're lying awake more than half an hour then get up and do something or you'll tie yourself in knots thinking about falling asleep. Avoid any caffeine after 4pm, there are some great apps with soothing noise to tune you out from the humdrum of the day too. Your body goes through 90minute sleep cycles which is why you can wake up feeling refreshed after 6 hours but terrible either side of that.

Pharmacies sell sleep aid tablets (Nytol) which is a sedating antihistamine, they can help get you back into rhythm which is useful for varying shift work. Avoid taking every night but 1 in 3 nights to guarantee a good night's kip can really help.