In spite of some obvious phenomenological similarities between hypnosis and sleep, they appear to be unrelated physiologically. Hypnosis and sleep may share control mechanisms that may partly account for individual differences in the ability to experience hypnosis and in the ease of falling asleep and maintaining voluntary control of sleep processes. Relationships between hypnosis, napping sleep induced response to suggestion, absorption and treatment outcome are discussed to highlight individual differences in the ability to process and voluntarily control states of consciousness.