There are two natural remedies you could try: melatonin (which is also a powerful antioxidant) or valerian. A cup of camomile tea (but not too much, so you have to get up in the middle of the night!) can also calm you down, as well as taking calcium before bedtime.
Actually, all these remedies work to help people get to sleep. I also have problems with waking up at some ungodly hour (4:00 usually) and not being able to get back to sleep until much later, then having to wake up again shortly thereafter. I notice it happening more when I'm under stress.
What time do you exercise? Maybe doing it later in the day would help?