Final answer:
NREM and REM sleep proportions change throughout the night, with less slow-wave NREM sleep and more REM as the night progresses. Over the lifespan, total sleep and REM sleep decrease, with older individuals typically sleeping less than 7 hours.
Step-by-step explanation:
The proportion of time spent in NREM and REM sleep changes both during a single night's sleep and across the lifespan. During a single night, a person typically progresses through the stages of NREM sleep before entering REM sleep. This cycle repeats multiple times with gradually less NREM sleep, specifically stage 3 and 4 (slow-wave sleep), and more REM as the night progresses. As for changes over the lifespan, sleep patterns alter significantly. Infants may sleep up to 16 hours per day, much of which is REM sleep. As we age, total sleep time decreases and we experience less REM sleep. By old age, average sleep can be fewer than 7 hours per day with a significant reduction in slow-wave sleep.