Which of these statements is most accurate about REM sleep?

a. These periods are the longest at the beginning of the sleep period, then get shorter through the night.

b. These periods are the shortest at the beginning of the sleep period, then get longer through the night.

c. During these periods, brain activity is as reduced as muscle activity.

d. The brainstem continues to pass on motor activity signals to the rest of the body during REM sleep.

e. The dreams during this stage are frequently interrupted by sleep spindles.