In studying the phenomenon usually referred to as sleep, we are actually dealing with more than one phenomenon. In point of fact, we spend the night alternating between two different types of sleep, each with different brain mechanism and different purposes.
As a person falls asleep, his brain waves develop a slower and less regular pattern than in a waking state. This is called orthodox sleep. In this state the brain is apparently resting. Its blood supply is reduced, and its temperature falls slightly. Breathing and heart rate are regular. The muscles remain slightly tensed. After about an hour in this state, however, the brain waves begin to show a more active pattern again, even though the person is apparently asleep very deeply. This is called paradoxical sleep because it has much in common with being awake.
Paradoxical (active) sleep is marked by irregular breathing and heart rate, increased blood supply to the brain, and increased brain temperature. Most of the muscles are relaxed. There are various jerky movements of the body and face, including short burst of rapid eye movement, which indicate that we are dreaming. Thus, we spend the night alternating between these two vital ‘restoration jobs’: working on the brain and working on the body.
From the text we may conclude that while we are sleeping _______.
(A) our brain is restoring our physical and mental condition
(B) we can really be as active as when we are awake
(C) the tensed muscles are caused by the changing phase of sleeps
(D) orthodox sleep is more important than paradoxical sleep
(E) irregular heart rate while sleeping is due to bad dreams