If REMs show we can dream for an extended period, why can dreams in micro-sleep be so incredibly detailed?

There's the famous case of the man who dreamt he was a victim in the French Revolution, mounting the scaffold and being guilotined. He woke up and found a picture rail had fallen on his neck. The whole dream must have come into existence to account for the sensation on his neck. Similarly, I sometimes start nodding off and have these dreams in intense detail with a feeling that whatever it is has been going on for ages - then I come ot and find it just occupied a period of seconds. Why is this, and how does it relate to the discovery that rapid eye movements or REMs show that we can dream for a long time?