Sleep cycles and age
Age seems to be the most powerful determinant of a person's sleep.
The average time a person is asleep per 24-hour period drops from about 16 hours per day for neonates, to about 8 hours at the age of 12 before leveling out at about 7 hours between the ages of 25 and 45. From then on, their total sleep time continues to decline gradually, until it reaches about 6 ½ hours in old age.
Infants spend most of their sleep time in REM and stage 2 sleep. Sleep cycles last only 50 to 60 minutes in newborns, and NREM stages do not exhibit fully developed EEG patterns for at least two to six months. Once infants develop EEG patterns for delta sleep, these patterns become quite prominent. Young children have the most delta sleep of any age group, and they have a higher arousal threshold, especially in the first cycle of sleep when they can often sleep through loud noises.
In adolescence, delta sleep begins diminishing rapidly, by 40% within a decade. At age 60, sleepers may no longer experience delta sleep. Men are more likely to lose all delta sleep, but women may lose it as well.
Most people maintain REM sleep into old age; however, organic brain dysfunction correlates with diminished amounts of REM sleep, so people with Alzheimer's or Parkinson's disease tend to exhibit decreasing amounts of REM sleep as their illness progresses.
Elderly sleepers exhibit more variation in the duration and quality of their sleep than do young adults: However, many of the variations that occur between elderly sleepers of the same age group are probably the result of differences in physiological age despite a correlation in chronological age . Most elderly sleepers experience less delta sleep and more fragmented sleep than younger sleepers. Sleep fragmentation results from sleep arousals that occur as "transient" and extended awakenings.