Reddit reviews Dreamland: Adventures in the Strange Science of Sleep

Disclaimer: Layman's understanding.

The brain inhibits motor neuron activity during the REM phase of the sleep (the phase in which dreams occur). As a result, your body will be paralysed (almost). This is why you don't physically walk, run or destroy stuff in real world although you might be doing it in your dream.

If this inhibition doesn't work correctly you'll start sleep walking and do other crazy shit while still asleep.

Dreamland is a short, naïve but nice intro to sleep science.

edit: link formatting.