Sleep is the body’s chance to reset for the coming day, so while you’re sleeping, a lot happens. Over the course of the seven to nine hours recommended for adults each night, your body goes through several sleep cycles, each comprising four unique stages.
The four stages of sleep are broken into two phases: the three stages of non-rapid eye movement (Non-REM or NREM) and the single stage of rapid eye movement (REM) sleep. Each is different and provides benefits for the brain and body.
Sleep is necessary for our overall health. Not only are we resting for the next day, but our bodies are also recovering and rebuilding from our day, strengthening our immune abilities, repairing muscles, and supporting our memory. Sleep also aids in adolescence.
Hormones like melatonin, growth hormone, and cortisol are produced and controlled while we sleep. Too much or too little sleep alters the level of hormones in the body, which can lead to poor health.
The body’s systems work differently during sleep than waking time, but they continue to work. The body temperature decreases to use less energy so the brain and heart can work more efficiently. The brain uses glucose differently during REM and NREM sleep, as it works to clear and store memories.
Ensuring enough proper sleep can help us recover from injuries and reduce our risk for many health issues and illnesses. But what does this look like? The recommended seven to nine hours each night breaks down to four to six cycles of NREM and REM. Each cycle takes an average of 90 to 110 minutes, and each needs to be fully complete for a restful night’s sleep.
Of all of the phases, REM gets the most attention. REM sleep is the last phase when the most vivid dreaming occurs. This phase is less restful than the NREM stages but imperative for memory consolidation. If you do not complete your ideal sleep cycles or are awakened unexpectedly in the middle of one, it could affect how rested you feel the next day.
But the NREM stages of sleep are where we spend the majority of our time. We also dream in these phases, though these dreams are not as memorable or wild as the dreams during REM.
NREM sleep stages
The patterns of our sleep change over the course of the night. “Sleep can be divided into three or four stages, plus REM, depending on the model followed,” Jillene Grover Seiver, senior lecturer in psychology at Eastern Washington University, explains. A cycle moves from NREM1, NREM2, NREM3, (NREM4), NREM2, REM, then repeats. Seiver continues, “After completing the first Stage 4 of the night, the sleeper will experience Stage 3, Stage 2, and then their first REM sleep of the night. Then they cycle through repeatedly — Stage 2, 3, 4, 3, 2, REM — in roughly 90-minute cycles throughout the night. As the night progresses, the Stage 4 sessions become shorter, and the REM sessions become longer.”
NREM1 (Stage 1)
NREM1 is the lightest and shortest stage, lasting for approximately 5% of the sleep cycle, or roughly 5 minutes. During NREM1, breathing remains regular, muscles are still active, and the brain begins moving between consciousness and unconsciousness. It is still easy to be awakened in this phase.
Seiver describes this stage as shallow, where dream-like hallucinations can take place. People may report “dreaming of falling” in this stage, which can cause hypnic jerks. Here, the brain displays alpha waves, a state of relaxation without concentration or focus. People are still easily awakened and may “jolt” awake, not realizing they’d been asleep.
NREM2 (Stage 2, also called light sleep)
In NREM2, sleep becomes somewhat deeper as the heart rate slows and body temperature decreases. This stage is the longest, initially lasting around 25 minutes in the first cycle and getting longer with each cycle. NREM2 is a total of 45% of the sleep cycle. During this stage, sleep spindles are noted on electroencephalographic (EEG) testing. Sleep spindles are bursts of activity, or types of brain waves, that are believed to occur as part of memory formation and plasticity (the ability of the brain or nerves to change based on input).
Seiver describes this stage as light sleep, where people may sleep-talk and reply or wake to stimuli around them.
NREM3 (Stage 3)
NREM3 is the deepest stage and accounts for about 25% of the sleep cycle. In NREM3, delta brain waves, also known as slow waves, appear. These brain waves indicate deep relaxation and beneficial sleep. This is the hardest stage to awaken from — some people won’t wake in this stage regardless of what goes on around them. People who are awakened during this stage may struggle to function for 30-60 minutes due to confusion or disorientation. Stage 3 is vital for the body’s growth and repair.
Some sleep models include a fourth stage, though Seiver states, “Stages 3 and 4 are often combined in some models of sleep because nothing very noticeable occurs during Stage 3 sleep, which is referred to as transitional sleep, displaying periodic sleep spindles.”
NREM 4 (Stage 4)
“Stage 4 is a very important sleep stage where human growth hormone is released, and deep rest in the brain occurs. Of course, the brain never really rests, but the cerebral cortex is minimally active during this stage, revealing slow delta waves. During this stage, a person is very difficult to wake up. Early in the sleep period, the person will spend more time in Stage 4 sleep than any other stage,” explains Seiver.
As the night progresses, you spend different amounts of time in each phase, with more REM sleep occurring just prior to waking up.
“The front-loading of Stage 4 sleep early in the sleep period highlights how important Stage 4 sleep is for overall health,” Seiver says. “People who do not get enough Stage 4 sleep will feel physically fatigued, will be more prone to infections of all sorts, and will have difficulty regulating their emotions.”
Those who wake up early may be cutting short their REM sleep. “Missing REM sleep tends to have cognitive effects, such as memory impairment, since memories are thought to be transferred to long-term memory during this stage,” Seiver explains.
Improve your NREM quality
Improving your sleep hygiene can help decrease your risk of illness. Proper nutrition, exercise, and regular sleep scheduling can help improve your sleep.
To do this, Seiver recommends, “Standard sleep hygiene should be practiced. Go to bed at roughly the same time every night and wake at roughly the same time every morning. This trains the brain to prepare for sleep and waking so that all sleep stages can be experienced. Do not use blue screens within two hours of bedtime. Try to make your sleep area as dark and quiet as possible, or mask noise with a white noise machine.”
“Do not consume caffeine within six hours of bedtime, as it takes four to six hours to clear the body,” she continues. “Get regular exercise, as that will convince the body that it needs the human growth hormone that is released during Stage 4 sleep. New research is suggesting that exercise at any point in the day is equally beneficial, but each person should determine what time works best for them and their sleep habits.”
NREM sleep is several levels or phases of sleep that progressively get deeper until REM occurs. Several of these phases are necessary throughout the night for the body to rest and reset for the coming day. When these cycles are broken or not enough are reached, you feel tired and run down. This can also lead to physical and mental health issues.
