Why do you think you are not dreaming at all, rather than just not remembering your dreams?
It is very unlikely that you are not dreaming at all.
REM sleep is one of the
5 stages of sleep that take place in every sleep cycle. It is the time in which you are most likely to dream.
The whole sleep cycle doesn't usually take much more than an hour and a half so assuming you are sleeping between 2 and 4 - you fall asleep as soon as your head hits the pillow and don't lie awake - you should have some REM sleep, and some time for dreams, in there.
It's possible, however, that you are just not getting enough sleep. Most people need to sleep more than two hours a night.