Adapted from "The Science of Sleep: REM Sleep, Sleep Stages, and More" from HelpGuide.org & Harvard Health here

--

There are two main types of sleep throughout the night quiet (Non-REM) and REM sleep.

• Quiet sleep is important because it helps restore the body
• REM sleep restores the mind and is important for both learning and memory

Sleep Architecture refers to the predictable patterns of alternating cycles between NREM and REM sleep throughout the night.

Quiet (NREM) Sleep

The three stages of quiet sleep (non-REM sleep) are:

• Stage N1: lasts about 5 minutes, characterized by slower brain waves and a drop in body temperature. During this stage, the predominant brain waves slow down to four to seven cycles per second, a pattern called theta waves. People in stage N1 sleep lose awareness of their surroundings but can be easily awakened.
• Stage N2: lasts 10-25 minutes, characterized by slower heart rate and breathing, irregular brain activity, and sleep spindles. During this stage, your eyes are still, and your brain’s electrical activity is irregular. Large, slow waves intermingle with brief bursts of activity called sleep spindles, when brain waves speed up for roughly half a second or longer. You spend about half the night in stage N2 sleep.
• Stage N3: deep sleep, characterized by large, slow brain waves and difficult-to-awaken sleepers, important for physical restoration and repair. Eventually, large, slow brain waves called delta waves become a major feature on the EEG, and you enter deep sleep. Breathing becomes more regular. Blood pressure falls, and the pulse slows to about 20% to 30% below the waking rate. The brain is less responsive to external stimuli, making it difficult to wake the sleeper. Deep sleep seems to be a time for your body to renew and repair itself.

REM Sleep

REM sleep, or dreaming sleep, is a state in which your brain is active and your eyes move rapidly behind closed lids.

• During this stage, your body temperature rises, your blood pressure increases, and your heart rate and breathing speed up to daytime levels.
• Despite all the activity, your body hardly moves, except for intermittent twitches.
• REM sleep is important for restoring the mind, perhaps in part by helping clear out irrelevant information. Studies show that REM sleep facilitates learning and memory, and people who are deprived of REM sleep tend to perform worse on cognitive tasks.
• REM sleep occurs about 3-5 times a night, or about every 90 minutes, and the first episode usually lasts only a few minutes, but REM time increases progressively over the course of the night. The final period of REM sleep may last up to a half-hour.

Sleep Architecture

Sleep architecture refers to the predictable pattern of alternating between non-REM and REM sleep stages throughout the night.

• During a normal night's sleep, a person alternates between different sleep stages, including both REM and non-REM sleep. This pattern is called "sleep architecture" and can be charted on a hypnogram.
• In a young adult, this architecture typically consists of four or five alternating non-REM and REM periods, with most deep sleep occurring in the first half of the night. As the night progresses, periods of REM sleep get longer and alternate with stage N2 sleep.
• Later in life, the sleep skyline changes, with older adults getting less N3 sleep, more N1 sleep, and more awakenings throughout the night

Control of sleep architecture is thought to reside in the brainstem, which also controls other bodily functions like breathing and heartbeat.