Why am I having these crazy dreams?

I often nap between my 8 AM class and 11 AM class for about an hour and a half. During this hour and a half period, I usually have really vivid, believable dreams. A frequent recurring one is me sleeping through my 11 AM class. They’re often layered-consciousness dreams. For instance, I’ll think I’m waking up, but then realize I’m really not conscious, and this will happen many times until I finally DO wake up.
These are usually accompanied by weird, vivid dreams, like I said. But I find it odd because REM sleep, when most clear dreaming occurs, only happens every 90 minutes. That’s usually about the duration of my entire sleep, though. So then why am I having such bizarre dreams?