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

State of Consciousness

Refers to the level of awareness of oneself and the environment, including wakefulness, sleep, and altered states such as meditation or intoxication.

Physiological Research

The scientific study of the normal functions and mechanisms occurring in living organisms and their parts.

Mental Changes

Alterations in cognitive functions, emotional states, or behavior patterns, which can be due to various causes including health conditions, life experiences, or aging.