National Institutes of Health
A typical night for a young, healthy adult, showing 5 cycles of REM and NREM sleep. Light gray areas represent non-rapid eye movement (NREM) sleep.
Clinical research suggests that deep sleep (Stages 3 and 4) is the most important stage, in that those that are sleep deprived first make up time spent in deep sleep. During deep sleep, blood flows primarily to the muscles and not the brain. It is during deep sleep that our physical bodies are replenished with energy, children grow, and our immune systems are most capable of fighting illness. Deep sleep also enables the brain to help clear itself of toxins.
The function of REM sleep is less understood, although research suggests both REM and NREM sleep help consolidate memory, facilitate learning, and help clear brain plaques associated with Alzheimer's disease. Neurological evidence also shows the limbic system -- those brain structures most responsible for the regulation and processing of emotions -- are more active during REM sleep stages. For most people, REM sleep may also help to in effect reset the emotional state, in that a lack of REM may result in one feeling more irritable or grouchy the next day.
More clues about the purpose of REM sleep come from biological studies of primates. Of all primates, humans sleep the least but have the highest percentage of REM.
Evolution, Medicine, & Public Health, 2016(1), 227-243
Of all primates, humans sleep the least but have the highest percentage of REM dream sleep.
In a simple way then, we might look at NREM or deep sleep resetting the physical body, and REM or dream sleep resetting the mind, and especially one's mood, for a new day.
As we age, the amount of time spent in deep sleep tends to decrease. This is normal. Older individuals will often enter REM sleep faster than younger people. This is but one change in sleep over time as we age.
The time between sleep cycles is important to understand
Because sleeping is lightest between cycles, it’s common for normal sleepers to briefly awaken the five or six times between cycles per night, and then fall back asleep within seconds. Typically these very brief periods of awakening are so short they are forgotten by morning.
Moreover, even the best sleepers will awaken to some degree at least 10 to 15 times per night and forget about it by morning. Finer indications of arousal, such as brain wave activity measured in a sleep lab, show that normal sleepers actually awaken some 10 times or more per hour on average, although these arousals typically last less than 15 seconds. Transient awakenings like these are benign, and by themselves nothing to worry about.