I came across the concept of 90 minute sleep cycle on LJ few years back. If I remember correctly, it was either bluesmoon 's post or comment somewhere.
Couple of weeks back I was talking to a friend on how I wake up fresh when I sleep for time periods which are multiple of 90 minutes. He tried the same formula and confirmed, it probably works true for him also. Yesterday I was supposed to wake up another friend at 6.30 am, but I gave a wake up call 15 minutes earlier. My friend suggested I stick to the 6.30 am time sharp, otherwise, it disturbs his sleep pattern. I was reminded today about this, wondering if 6.30 am fell into the end of his 90 minute sleep cycle slot and probably thats the reason he feels fresh at 6.30 am and not anything before it. Probably it is not exactly 90 minutes for everybody, but I think there is some reason and equation for everybody atleast which works out approximately around 90 minutes.
I often have to remind myself that sleep, which is mostly an involuntary activity could trigger so much of research behind. Wiki says it all.
I have to shut down googling on sleep, before I lose my sleep over it. Otherwise, i would have to wait for the next 90 minutes to time myself to wake up at my usual time :)