The record holder reported hallucinations, paranoia, blurred vision, slurred speech and memory and concentration lapses.
People can take cat naps with their eyes open without even being aware of it.
The ideal is between 10 and 15 minutes, meaning you're still tired enough to sleep deeply, but not so exhausted you feel sleepy by day. It's possible there may not be a single moment of our sleep when we are actually dreamless. No-one knows for sure if other species dream but some do have sleep cycles similar to humans. Others reckon we dream about things worth forgetting - to eliminate overlapping memories that would otherwise clog up our brains. The system was first used on US pilots during the bombing of Kosovo.