A good night’s sleep is typically measured by how refreshed you feel, how often you wake up during the night, or how many hours it lasts. However, there is little room in these metrics for the quality of sleep and how long you spent in each of the four stages of sleep, which are equally essential to achieve the full rejuvenating impact of sleeping.
Four key sleep stages make up the sleep cycle. One is rapid eye movement (REM), where the brain is active, and we do most of our dreaming, with the other three classed as non-REM (NREM) sleep stages. Each stage is defined by brain activity, physiological changes, and ease of arousal. All four are crucial to the restorative nature of sleep, and most adults experience 4-5 sleep cycles per night.
Sleep is an individual experience, but everyone should experience each of the four stages at least once per night. When you first fall asleep, the stages are typically experienced in order, creating a sleep cycle lasting around 90 minutes. Subsequent cycles are unstructured, generally broken up by a period of REM sleep. The number of cycles and time spent in each stage will change as we age, and the numbers below are for a typical healthy adult.
Stage One: NREM1, N1
NREM1 is a short phase that may only occur once per night, commonly lasting between 1-5 minutes accounts for up to 5% of your total sleep. N1 is the lightest sleep you’ll experience, and it’s often cited as the “dozing off” phase. Your body has relaxed fully, and reflex twitching and shifting as your subconscious seeks comfort is normal. These twitches, termed hypnic myoclonia, can be more pronounced if you are stressed, caffeinated, or enter this phase in an unusual environment (i.e., not your bed or couch).
The awake brain exhibits alpha waves, and during stage one, these waves eventually slow to about 50% of normal wakeful levels. Near the end of the phase, the waves shift toward theta waves. While it is easy to arouse someone in this state – a subtle noise or a tap on the shoulder will do it – if left in a quiet environment, they will progress to stage 2.
Stage Two: NREM2, N2
Once you’ve progressed to stage two, several physiological changes occur. Your heart and breathing rate will drop, as will your core temperature (by around 1-2 °F). As your muscles relax, eye movement ceases, and your brain waves start to change entirely to a theta wave pattern. Notably, the theta waves are broken up by short bursts of activity, recognized by their dramatic rise in amplitude and frequency. These bursts are called spindles and originate in the hippocampus. While a definitive cause of the spindles is unknown, it’s postulated that they prevent the brain from being disturbed to keep you asleep during this stage of sleep.
Stage two is relatively short during your first sleep cycle of the night, typically lasting between 10-25 minutes. However, this stage increases in length as the night wears on. Overall, half your sleep is spent in NREM2.
Stage Three: NREM3, N3
Deep sleep is stage three, and it’s difficult to wake someone from this stage. Studies have shown that for up to 30 minutes after being awakened from NREM3, you can experience greater sleep inertia – which means cognitively impaired – than waking from any of the other stages. If you’ve ever been forced awake by an alarm or crying child, then forgotten how you got out of bed, you’ve likely experienced this.
Compared to stage 2, your body temperature, breathing, and heart rate reduce further. A significant change is the relaxation of your muscles, including your trachea, which is why snoring is typical in this phase of sleep. Your brain activity changes significantly, too; delta waves occur, which are slow and spaced-out brain waves.
Studies have shown this stage is essential to the restorative nature of sleep, helping not only your energy but your immune and digestive systems too. Despite deep sleep occurring when the brain is least active, data supports that NREM3 is critical to memory, creativity, and problem-solving. Crucially, NREM3 is when the brain “cleans” itself, removing beta-amyloid plaque, a buildup of which is seen in Alzheimer’s and dementia patients.
The essential nature of this phase in our functioning is why we experience most of our deep sleep early in the night. NREM3 can last up to 40 minutes during your first sleep cycle, whereas deep sleep can last as little as five minutes or be skipped altogether by the end of the night. In essence, the sleep cycle has evolved, so you get the rest you need to stay healthy as quickly as possible during sleep. Expect to spend 25% of your sleep in NREM3 during a typical night.
Stage Four: REM
The fourth stage of sleep is unique and is classed by its distinctive physiological characteristic – rapid eye movement. Unlike NREM sleep, the brain is highly active during this phase, almost at similar levels as wakefulness. Other significant physiological changes separate this phase from the others too. Your heart and breathing rates increase, and your core temperature climbs back up. Significantly, your body experiences atonia, the temporary paralysis of the muscles, except for those that control breathing and the eyes. Without this evolutional quirk, you would move a lot during REM, increasing the risk of injury and waking yourself, ruining your sleep.
During REM, your brain experiences similar alpha brain waves to those seen while awake. Most of our dreaming occurs during REM, and it’s the dreams we experience during this phase that we recall most as they are vivid, intense, and last longer as the night progresses. While you dream, sleep works on most cognitive functions, such as memory, learning, and creativity. It is thought this is when the brain processes the day’s information, creating long-term memories.
Due to the importance of NREM sleep in the early part of the night, you won’t typically experience REM until 90 minutes into your sleep. At this point, REM only lasts a few minutes, but during your last cycle, it can last up to an hour. Overall, 25% of your night is spent in REM.
Waking isn’t an officially recognized part of the sleep cycle, but it is when it ends. If you wake naturally, it will typically follow a short phase of NREM2 after a long REM period. Ideally, you will wake naturally, as this means your sleep cycle is complete, and your brain is ready for another day. If you have trouble waking without an alarm, start by setting a strict sleep schedule of 7-9 hours and sticking to it (even at the weekends). Once you have a healthy plan established, your body will adjust and complete its sleep cycle in the allotted time.
Human sleep cycles consist of four key stages, defined by physiological changes. We experience 3 phases of non-REM sleep before progressing to REM as the last phase of a single sleep cycle. On average, you will experience 4-5 cycles per night during a 6-8 hour sleep. Most of the early hours are spent in NREM3 (deep sleep) when our bodies restore and repair themselves. NREM2 and REM take over as you progress through cycles, improving memory and cognitive functions. You do not move through sleep in this precise order after the first cycle. For example, following REM, you might go to NREM3, then back to REM before returning to NREM2.
Each stage is critical to leading a healthy life, and you should do everything you can to help your body progress through these stages seamlessly. To do so, make the best environment you can – mattress, pillows, sheets, noise, etc., and stick to a daily nighttime routine and wake at the same time each day.