Why do some dreams become nightmares?

Why do some dreams become nightmares?

The thing about rapid eye movement sleep is our physiology is hugely uncontrolled at that time. So big changes in heart rate and respiration and so on. How one interprets the motion depends on the context, the context of that emotion. So if I go back to work I used to do on parachuting, I used to say you can research either fear or euphoria with parachutists in early jumps but when you're looking at the physiology of the emotion it was very similar for both and a run of experiments showed that depend-- [SOUND GOES OUT ON VIDEO AFTER THIS]