Over-tired is usually the culprit.
When you go long past tired enough to sleep, your body ends up filled with stress hormones. Those hormones don't clear out of your body right away, and leave you ragged and unable to relax.
Strangely, the solution is not to go to bed earlier (although that may help, too), but to get up earlier and take naps. At least twice a day. Not for very long, about 40 minutes is enough. If you're working in the daytime, nap at lunch for as long as you can, and plan to nap again after you get home. If you plan ahead, you can have dinner cooking while you sleep and eat when you wake up.
After a few days of a much better balance between activity and rest, you'll find it easier to relax at night.
You may like to get your iron levels checked, and possibly your thyroid... but the agitation when you attempt to sleep sounds more like too much doing and not enough resting, to me.
at 12:01 AM on Aug. 8, 2010