During experiments to monitor the brain activity of animals and humans while asleep, it was noted that the brain seemed to move through a series of levels of activity. In deep sleep there are slow, rhythmic brain waves. These at times would give way to faster rhythms of a more dynamic nature. This was at first called ‘desynchronised’ sleep because during it the muscular system relaxed deeply even though the brain was active. It was also known as paradoxical sleep, but more recently has become internationally known as ‘active sleep’. During active sleep the rapid eye movements – REM – characteristic of dreaming occur. The brain’s activity was found to be a better indicator of dreaming in animals than REM because some creatures, such as owls, do not move their eyes. In this way, all mammals were seen to exhibit active sleep or dreaming. Birds also dream, and, measured in this way, so do many types of fish, reptiles and some amphibians.
In humans all the voluntary muscles are outside of conscious control during this phase of sleep. This is often called sleep paralysis. This is not really an accurate definition though, as the voluntary muscles can be activated by the process creating the dream. This clearly shows the two levels of will that we have. The one is conscious, enabling us to walk and speak, but not to alter the beating of our heart. The other will deals with all the autonomous functions and is active while dreaming, causing emotional, sexual and muscular activity. See We lose Control During Dreaming; science sleep and dreams; movements during sleep; self-regulation in dreams and fantasy