Is it normal how tired i feel depends on a sort of cycle?

*Warning*: Not for retards or people who can't do basic math.

I read somewhere a long time ago that humans sleep in cycles of 1.5 hours, an that it will be hard to wake someone up in the middle of the cycle because they will have gone into deep sleep.

I think this is actually true because every night I count how many hours I'm going to sleep and set my alarm for a multiple of 1.5. Like if I sleep at 12 and have to be up by 7 to get ready for school, I'll set the alarm for 6 because 6 is a multiple of 1.5. If I get an amount of sleep that isn't a multiple of 1.5, I feel really groggy and tired most of the day. I've been doing this since high schoool and I'm pretty sure it works.

(But it could be a placebo effect.)