Which of the Following Is True of Sleep

Which of the following is true of sleep?

A) Most of the night is spent in the deepest level of sleep (stage 4) .
B) One spends an hour or two at each stage in turn, spending the last few hours at stage 4 and then at REM.
C) One passes through all stages of sleep, including REM, several times in a normal night.
D) One cycles in and out of various stages throughout the night instead of staying in one stage all night.