I am the same after a few days rest. It is always the hardest thing getting a good nights sleep before an early and due to a long commute a 5 am start for me means getting up at 2:45 am.
Do the obvious and avoid caffeine as much as possible throughout the day before. Try not to eat too late also. I always find setting my alarm on my last day off to get up early can help so that it's a bit easier to get to sleep that night. Try something strenuous also as you will feel warn out enough to sleep.
I don't like to take any medication to help sleep and even herbal remedies have made me feel worse the next morning if I have taken them. Just try not to put too much pressure on yourself to get to sleep, relax with a good book for half an hour before you turn the lights off and whatever you do, don't clock watch!
