haha, i had experienced that. Since you can't go back to sleep, you might as well have fun during that time. Like play a game, read a good book, mylot(i suppose you are already doing this now) and etc.
If you wanna go back to sleep you shouldn't use your computer because it will make you even more awake and the time you have will go by fast and you wouldn't have gone back to sleep:P
Maybe it is stress or you are used to getting up at this time.