What Is the State of Consciousness Known as Sleep,according to Behavioural

What is the state of consciousness known as sleep,according to behavioural and physiological research

A)  active in terms of mental changes, but inactive in terms of physiological changes 
B)  several distinct stages, each with its own characteristic level of mental and physical activity 
C)  a relatively uniform experience, with the exception of periods of dreaming 
D)  a uniform state in which the brain is "turned off" temporarily