REM is a rapid eye movement which, during the sleep phase, is characterized by rapid eye movements and increased dreaming. Name-REM corresponds to non-rapid eye movement sleep phases that alternate with REM phases during sleep cycles. To fall asleep means to lose consciousness and fall to respond to a stimulus that would produce a response in the waking state. At least 50 million Americans suffer from chronic, long-term sleep disorders and another 20 million Americans experience sleep problems on a regular basis. The scientific study of typical sleep habits has provided insight into several sleep disorders such as: sleepwalking, somnambulism, nightmares, night terrors, insomnia, apnea and narcolepsy. Nightmares are frightening dreams that occur during the REM sleep phase and are usually remembered. Night terrors are frightening and terrifying dreams that occur during a NREM sleep state in which a person has difficulty waking up and cannot remember the contents. Insomnia is a sleep disorder characterized by staying asleep throughout the night. Apnea is a sleep disorder characterized by difficulty breathing during the night and feeling exhausted during the day. Narcolepsy is an inherited disorder characterized by sudden daytime sleepiness and sudden loss of toe muscles following emotional movements.