I have occasional problems with sleep disruption in the winter months (October to March, 49 degrees latitude). The problem is that natural sunlight simply isn't strong enough to reset your body clock.

When it happens I take a melatonin at night, and it seems to reset my sleep cycle and gives me a really good night's sleep. I don't normally take it more than once a week at most, just in case there are any negative effects. The delayed release tablets seem to work best (Jamieson, I believe it is), as they slowly release the melatonin over a period of 8 hours.