As people get older, the pattern of sleep also changes—especially the
amount of time spent in deep sleep. This explains why children can
sleep through loud noises and why they might not wake up when
moved. Across the lifespan, the sleep period tends to advance,
namely relative to teenagers; older adults tend to go to bed earlier
and wake earlier. The quality—but not necessarily the quantity—of
deep, non-REM sleep also changes, with a trend toward lighter
sleep. The relative percentages of stages of sleep appear to stay
mostly constant after infancy. From midlife through late life, people
awaken more throughout the night. These sleep disruptions cause
older people to lose more and more of stages 1 and 2 non-REM
sleep as well as REM sleep.