What are the 4 Stages of Sleep?

Non-REM stands for non-rapid eye movement and the first three sleep cycles fall into this phase. During Non-REM, the body moves from falling asleep to a light sleep before settling into a deep, restful sleep. Much of the body’s activities such as breathing, heart rate and brain waves slow down during this phase. Body temperatures fall, and relaxation sets in across the muscles while eye movement slows considerably. This is a restorative and rejuvenating phase because during this time tissue is repaired, while muscles, bones, and the immune system are strengthened¹.

REM, aka ‘rapid eye movement’ happens during the fourth stage (or cycle) of sleep. During this stage, brain activity pumps up, twitching can take place in the limbs and face, and breathing speeds up. This is also when dreams occur. Most people spend around 25% of their total sleeping time in this phase². A lack of REM sleep can have serious consequences on your overall physical and mental health.

REM sleep plays a significant role in helping your brain consolidate and process new information. This information is then retained in your long-term memory. REM sleep also helps ensure better mental concentration and mood regulation — two things that are critical to both your daily work performance and overall quality of life. If you don’t experience the amount of deep slumber that your body needs, you may have a weaker immune system, more substantial pain, and a lack of new healthy cell and tissue growth³.

The two phases of sleep are broken down into four cycles. The first three cycles take place in non-REM, and the last stage happens during REM sleep. During these stages, a person’s body goes through multiple physiological changes that are vital for physical and cognitive health⁴.

Stage 1 (N1) is the first of the sleep cycles stages. Stage 1 occurs during the Non-REM sleep phase.

During Stage 1, you alternate between being awake and asleep. Therefore, Stage 1 is considered a light sleep, and you can awaken easily. This stage is very short, lasting roughly 10 minutes.

Your heart rate, breathing, eye movements, and brain waves start slowing down at this time. The muscles also begin to relax, but you may experience some twitching as well.

Stage 2 (N2) is the second stage of sleep that occurs during the Non-REM sleep phase.

You’re still in a light sleep at this stage. However, Stage 2 is much longer, lasting about 30-60 minutes. During Stage 2, your muscles relax further, and your heart rate, breathing, eye movements, and brain waves continue to slow down.

The brain also starts to create sleep spindles, which are bursts of fast, brain wave activity. Sleep spindles are normally tracked on an electroencephalogram (EEG), which measures brain activity when you're at rest or asleep.

Spindles can occur in different regions of the brain. Furthermore, researchers believe spindles may play an important part in sensory processing and memory consolidation.

Stage 3 (N3) is the third stage of sleep and the final stage of the Non-REM sleep phase, also known as the deepest stage of sleep.

Stage 3 is also known as ‘Slow Wave’ or ‘Deep Sleep’ because the brain waves are slowest during this time. The slow brain waves cause you to enter a deep sleep, and it can be harder to wake you up during Stage 3.

Stage 3 lasts for roughly 20-40 minutes. During Stage 3, your muscles are relaxed, and your heart rate, breathing, eye movements, and brain activity hit their lowest levels.

During Stage 3, your body also releases vital hormones for functions like development and appetite. Stage 3 is also known as the restorative stage because it is when the body repairs tissues and replenishes its energy.

Stage 4, also known as REM sleep, is the final part of the 4 stages of sleep and the end of the full sleep cycle.

REM usually occurs around 90 minutes into a sleep cycle. During Stage 4, your brain waves speed up, and your eyes quickly move from side to side. Brain activity is at its highest during the REM stage.

You should aim to get about 90 minutes of REM sleep a night, or spend about 20-25% of your total rest in REM sleep.

Stage 4 is also when you dream, a byproduct of heightened brain activity. However, your muscles are in a state of paralysis to prevent you from physically acting out those dreams.

The reality is, a variety of factors can affect your stages of sleep. These include age, gender, light, stress and anxiety, sleep disorders, pain, jet lag and shift work, and sleep environment⁵. We go into detail about how these factors affect your sleep below:

• Age – Sleep changes as we age. Infants and children typically have longer deep sleep stages because their bodies are experiencing a greater amount of development. However, the amount of deep sleep declines as a person reaches adulthood. Elderly individuals usually have shorter and fewer episodes of deep sleep.
• Gender – Gender can affect the sleep stages. For example, women undergoing puberty, menstruation, pregnancy, and menopause may experience changes in their sleeping patterns.
• Light – Light can affect sleep stages because it can prevent the onset of Stage 1. The body has a natural internal clock that regulates sleep, and one of its most significant influences is light. More specifically, when it’s daylight, that tells the body to wake up, and when it’s dark, the body prepares itself for bed by increasing melatonin production. Melatonin is a sleep-inducing hormone. However, exposure to light – either natural or blue light from tech devices – can delay the natural sleep onset process.
• Stress and Anxiety – Stress and anxiety can also prevent the onset of Stage 1 because your body cannot relax enough to doze off. Additionally, anxiety may affect REM sleep. People dream during REM, but those with anxiety may be more likely to experience disturbing dreams or nightmares that cause interruptions to their sleep cycle.
• Sleep Disorders – Sleep disorders can also affect the sleep stages. Some disorders, like Restless Legs Syndrome or Insomnia, may delay you from entering Stage 1. However, other disorders, like Sleep Apnea or REM Sleep Behavior Disorder, can cause you to wake up during the later stages of sleep, such as N3 or REM.
• Pain – Chronic pain and discomfort may make it difficult for you to fall asleep or stay asleep during the night. Even though the first two sleep stages are lighter, most people sleep through them. However, uncomfortable pain could cause someone to wake up at the beginning of a cycle, preventing them from resting continuously through the night.
• Jet Lag and Shift Work – Jet lag is a sleep disorder that occurs when you travel across time zones quickly. Jet lag can throw off your internal clock, and as a result, you may feel overly tired or have insomnia. Shift work is a sleep issue that results from working overnight or on an inconsistent schedule. When someone works overnight, they go against the body’s natural circadian rhythm that responds to light and dark. As a result, many shift workers cannot get enough rest during the day and often become sleep-deprived.

We included tips for improving your sleep cycle below, though the main ones we mention are practicing healthy habits, resting in a comfortable environment, and addressing sleep disorders⁵.

• Stick to the same sleep schedule – Keeping a consistent bedtime and wake up time every day, including weekends, to help train your body to feel tired and awake at the appropriate times.
• Avoid daytime naps – Snoozing during the day may make it harder for you to doze off at night.
• Power down electronics – Devices like smartphones, TVs, and computers emit a blue light that can delay sleep onset by reducing melatonin production. Melatonin is an important hormone that the brain releases to promote sleep, so it may be wise to power down electronics an hour or two before bed.
• Establish a relaxing nightly routine – Participating in stress-reducing activities such as taking a warm bath, meditating, or reading a book can help you relax and fall asleep easier.
• Exercise regularly and refrain from late-night snacking – Exercise helps release endorphins to manage stress, which can help you rest better. However, if you eat too much food before bed, this can cause you to feel uncomfortable. Instead, reach for a light snack to curb any late-night hunger.

• Invest in a quality mattress – Well-made mattresses can help alleviate pain and discomfort and allow you to sleep more comfortably. Quality mattresses provide pressure relief and support.
• Keep the bedroom cool – Set the thermostat low may help you sleep better. Body temperature changes according to the stages of sleep by decreasing before your bedtime and continuing to drop while you sleep. However, if a person’s core temperature is too high, this can hinder sleep onset and decrease time spent in deep sleep and REM sleep.
• Block out light – Making your room as dark as possible by using room darkening curtains and shutting down electronics or by sleeping in a sleep mask may help you achieve better sleep quality.
• Reduce noise – Silence cell phones if possible, and especially light sleepers may want to invest in a pair of earplugs.

• Talk to a physician – If you have implemented healthy sleep habits and are still struggling, talk to your physician who can help you address the issues from a medical standpoint.
• Participate in a sleep study – Sleep studies can evaluate the issues that keep people awake by studying how often participants are waking during the night.
• Try a CPAP machine – Those experiencing sleep apnea will often be prescribed a CPAP to treat it. It is advisable to always use the CPAP machine when sleeping.

Another helpful tip to achieve optimal sleep is to use a sleep cycle calculator. The Sleep Advisor Sleep Calculator is designed to show you when to wake up during Stage 1, which is when people are at their lightest stage of rest.

You can then enter the time that you need to wake up and the calculator will provide you with the optimal time to go to bed. Or, you can enter the time you’re going to bed and the sleep cycle calculator will tell you when to set your alarm.

Although deep sleep and rapid eye movement (REM) sleep are both essential, they are not the same. In fact, they not only have different brain wave patterns and are characterized by different physiological changes, but they also play different roles in our overall health and happen at different times.

Deep sleep makes us feel restored and rejuvenated. It helps support the central nervous system and when we are experiencing deep sleep, we do not move or shift a lot. However, when we are in REM sleep, our brain actually shows the kind of activity that it does when we are awake, but our body stays immobile. REM sleep helps us retain information.

A variety of factors can affect the stages of sleep. However, the main ones are age, light exposure, stress, pain, and sleep environment.

References

[1] “Definition of Non-REM sleep”. National Institute of Health/National Cancer Institute. Last modified 2023​​.
[2] “Sleep Basics” Cleveland Clinic. Last modified, Dec 7, 2020.
[3] “The Science of Sleep: Understanding What Happens When You Sleep”. Johns Hopkins Medicine. Last modified 2023.
[4] “Physiology, Sleep Stages”. National Library of Medicine. Last modified September 7, 2022.
[5] “External Factors that Influence Sleep”. Harvard Medical School. Last modified 2023.
[6]”Sleep tips: 6 steps to better sleep”. Mayo Clinic. Last modified May 7, 2022.
[7] “Creating a Good Sleep Environment”. Centers for Disease Control and Prevention. Last modified April 1, 2020.