[saalweachter]

I don't usually wake up in the middle of the night, but I have noted the 4 hour (well, more like 3.5-4 hour in my case) block of sleep in my own sleep patterns. It's much easier to wake up after a block of sleep than in the middle of one, and I usually sleep 2 or 3 blocks rather than just a little bit more than 2. On the rare occasions when I was very well rested for a long period of time, I would occasionally wake up after 1 block of sleep, but I never turned that into a two-sleep pattern. I just thought, "Huh, guess I must not be tired any more." and started my day early.

[yummypaint]

This was exactly how it started for me. Even if you don't switch your sleep cycle it can still be useful to keep in mind. For example if I have to get up in the middle of the night to take an early-morning flight and have 5.5 hours to sleep, I'll plan to wake up after 4 hours to avoid being wrecked the next day.