This has only happened a couple times. I hardly function on less than 5 hours of sleep, and not very well on less than 6-7. So in this case I would get up at 5 to maintain the habit, do some reading for an hour, then go back to sleep for a couple hours. As I said, there are tradeoffs. I'm pretty much fried by 9-10PM, which is having an impact on my social life.