Originally Posted by rowan

I've noticed that if I half wake up, check the time, go back to sleep, and check the time after more sleep, the gap is usually about 40 to 42 minutes.

If I wake up with less than 40 minutes to go before my alarm sounds, I usually don't bother going to back to sleep, because I'll end up feeling more shitty. So I reckon there is something to the claim. Better to wake up by yourself when your mind wants to rather than have an alarm pull you out of sleep.