User blog comment:PlantyThePottedPlant/Random Topic Blogging II: Electric Blogaloo/@comment-3006892-20120109164338/@comment-1821137-20120109222239

Actually, an hour and a half is the first time the REM cycle occurs. Exactly 90 minutes after you fall asleep (it varies a little for different people). It's just that we mostly remember the dreams that occur in our later REM cycles, like, 6 hours after we've fallen asleep.