Here's the output from that Sleep Cycle app for last night:
Basically it uses the accelerometer in the phone to sense how much you move around, and then watches for your most active periods to determine how awake you are. It tried to time its alarm for the peaks of your awake-ness.
This seems to have timed it properly at the top of a cycle, so that's good. The jaggies at the end of the graph are me smacking the phone for snooze purposes.
(Yes, I slept just five hours last night! Ugh. I had a bit of a crisis to deal with in my online business last night. I'm going to bed early tonight to try and balance that out...)
Apparently these 1.5 hour cycles are very typical, which makes sense since my body is in the habit of expecting about 7 hours of sleep. (divide by four = 1.5 hours each). Of course what my body REALLY wants is FIVE cycles of sleep for around 8.5 hours total. I can usually sleep even longer than that given the opportunity, but that's due to sleep deficit. I think if I could arrange for 8.5 hours of sleep a night, I'd be much better off.