Do you dream sweet dreams each night - drifting gently in and out of sleep? Or is the boundary between sleeping and waking a more frightening experience?
It is thought close to 8% of people are prone to experiencing "sleep paralysis" - a condition which can leave sufferers with a temporary inability to move or speak. It is not the same as a bad dream or unsettled sleep.
"You are in a state where you are awake, but your body is paralysed and your breathing is restricted," says Emily Sargent from Wellcome Collection.
"You are aware of your surroundings but are unable to cry out.
You may have visual or audible hallucinations. You could feel presence of an imagined malign intruder."
Sleep paralysis is one of a number of conditions on the edge of human consciousness being explored at the Wellcome Collection in London in the exhibition, States of Mind.