I just can't go more than three hours without a sleep killing nightmare.
A "full" sleep cycle is typically 2-3 hours long. So, a good healthy night of sleep will consist of (generally) at least 2 sleep cycles, and as many as four - depending on the person, the length of their cycle, etc.
It takes longer to travel 'down' into the part of sleep where the most vivid/present dreams or nightmares will occur; coming back 'up' to the restart of the cycle is markedly faster. That's just how our sleep patterns are designed to work.
We are more likely to remember our dreams when they are vivid enough to short-cut the last stages of sleep; sometimes, causing us to miss them altogether.
(I'm really over-simplifying this, but don't have the brainpower to go pull up more detailed info right now).
This is all just to say - waking up every 2-3 hours is something that the human body will do naturally as a part of the sleep cycle. We are not meant to rise to full wakefulness, but pretty darn close. Most of the time, if we are sleeping well, we don't remember this. But if we are dealing with agitation in our dreams, the odds are good that we will wake up prematurely - not always immediately, but it's often perceived as being immediate. And even when the dreams aren't bad - if they are active, there's a good chance that your body will fully wake up at the end of the sleep cycle instead of staying just beneath wakefulness.
Shallow sleepers also have this problem. They generally don't make it into the deeper phases of REM sleep more than one cycle per night, if that. Dreams do seem to occur at another, mid-sleep cycle point, though they aren't as well understood. I believe people can wake up from those as well, though.
I have been dealing with waking up every 2.5-3 hours whether I want to or not, for awhile. Learning about the arc of the sleep cycle helped me to de-mystify some of it, which made it a little less stressful. Hoping that can help you as well.
Also, knowing that we dream whether we remember it or not - what really affects the remembering has to do with how we cycle back up out of the dreaming phase. That helped me not stress out so much about worrying if I would have dreams. Now, I just accept that dreaming will happen; what I don't know is whether it will be disturbing enough to wake me up.
Not sure why that's comforting to me, but it is.
