According to your text, older adults sleep less because

A) their sleep cycles become more efficient.
B) of physical discomfort.
C) they no longer require REM sleep.
D) they no longer require NREM sleep.

REM Sleep

A stage of sleep characterized by rapid eye movements, more dreaming and bodily movement, and faster pulse and breathing.

NREM Sleep

A phase of sleep characterized by non-rapid eye movements, comprising several stages of increasing depth of sleep.

Sleep Cycles

The patterns of alternating REM (rapid eye movement) and NREM (non-rapid eye movement) sleep stages that occur throughout a typical night.