Intensive research is ongoing to understand more fully why we sleep. Historically the focus of research has been on the brain, but one study found evidence sleep is essential to prevent damage to the gut's ability to effectively function. That may not be as surprising as it first seems, because the intestines contain millions of neurons and are closely connected to the brain via the vagus nerve.
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 helps reduce anxiety, and 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 to 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 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.
Sleep and the EEG
When the EEG, or electroencephalogram, was invented, a whole new world opened for exploration — the previously unknown world we all enter when we fall asleep.
Before the EEG, sleep was often thought of as a death-like experience, wherein one loses consciousness each night, hopefully to return in the morning. Ancient philosophers grappled with it, lacking any way to measure sleep objectively. Aristotle described sleep as a "siezure of the primary sense organ". Our remembered dreams gave us glimpses of this other world, often comprised of bizarre fantasies and sometimes disturbing nightmares, but no one knew for sure what was going on.
By measuring electrical activity in the brain, the EEG gave us a much better understanding of this unknown world. The brain -- powered by millions of neurons, or brain cells -- produces measurable waves of electricity the EEG can record. The first recording EEG machine was invented by German physiologist and psychiatrist Hans Berger in 1924.
Almost 30 years would pass before before scientists got the bright idea of attaching one of those devices to someone while asleep. In 1953, two researchers at the University of Chicago, Eugene Aserinsky and Nathaniel Kleitman, first measured the alternating periods of REM and NREM during all-night EEG recordings, launching the era of modern sleep research.
Now, many decades later, the EEG still remains a primary tool for laboratory sleep evaluation, and what researchers find looks something like this:
EEG recordings for various stages of sleep. Notice the similarity between REM sleep (bottom) and an awakened state (top), and the striking difference between those and the slow rhythmic waves of Stages 3 and 4, the deepest forms of sleep.
Stage 1, or drowsiness, begins to show theta waves indicative of sleep. True sleep starts in Stage 2, with the appearance of sleep spindles and K complexes, and the deepest form of NREM shows regular rhythmic waves of electrical activity slowly sweeping across the brain.
Research has shown during these deepest slow wave sleep stages -- when the brain both clears itself of toxins and replenishes its energy supply for a new day -- these regular cycles of electricity literally draw cerebrospinal fluid into and through the brain to facilitate this cleansing and renewal process.
If awakened during a deep NREM stage, we usually remember little about our dreams and are typically very groggy. In complete contrast, the REM dream stage is much closer to a fully awakened state. In REM our dreams are vivid, and by completing the final REM stage of the night one likely feels better rested and more emotionally restored for a new day.