The Four Stages of Sleep

By Karen Kilpatrick

Each night as you drift off to sleep, your brain begins a series of sleep stages. These stages combine to form a sleep cycle, but it doesn’t end there. You go through several sleep cycles each night. What happens during those stages, and do we really need them all? We spoke with Dr. Kin Yuen, a sleep medicine specialist at the University of California San Francisco, to learn more.

You may have heard of rapid eye movement (REM), which takes up 20% to 25% of each night, but that’s only one stage. There are three other stages of non-REM sleep: stage one, when you feel drowsy and start falling asleep; stage two, often referred to as light sleep; and stage three, called deep sleep.

The sleep stages and cycle change throughout the night, and each stage is significant for different reasons.

Let’s take a look at each of the four stages.

1 2 3 4

Stage 1: Falling Asleep

The first stage of sleep happens as you close your eyes and move from being awake to sleeping. According to Dr. Yuen, this stage usually only lasts a few minutes, but most people go through it several times each night. It takes about 5% to 10% of total sleep time.

During this stage, both brain and body activity slow down, but it’s easy to wake up. This is also when “hypnic jerks” happen – those involuntary contractions of muscles that sometimes feel like you’re falling.

Stage 2: Light Sleep

As you move into stage two, your breathing and heart rate slow down. This stage is referred to as “light sleep,” but that doesn’t mean it’s not needed. “Light sleep has an important role,” says Dr. Yuen. “This stage is necessary for memory formation and consolidation.”

Dr. Yuen explains that stage two is characterized by certain spikes in the electrical activity of the brain – called "sleep spindles" and "K complexes" by sleep scientists – which relate to how we encode and store information learned during the earlier day. This stage lasts between 10 to 25 minutes of each cycle, but it accounts for about half of the total sleep time.

Stage 3: Deep Sleep

Stage three is called “deep sleep” or “slow wave” sleep. In this stage, brainwaves slow down. Muscle tone, heart rate, and breathing rate all decrease. It’s hard to wake up from this stage, and when you do, you’re likely to feel pretty groggy.

This stage helps to restore the body and boosts the immune system, creativity, and memory. About 15% to 20% of the night is spent in deep sleep. Dr. Yuen says this stage mostly takes place in the first part of the night.

Stage 4: REM Sleep

In REM sleep, brain activity increases to levels that appear similar to wakefulness. Your eyes move rapidly even though they are closed. Dreams can happen in any stage, but they are especially vivid in REM sleep.

While the breathing muscles and the eye muscles stay active, the rest of your body’s muscles enter a state called “atonia,” which is like a temporary paralysis. Your breathing and heart rate become slightly irregular. Dr. Yuen notes that our bodies do not have temperature regulation in REM sleep. “In REM, we take on our ambient temperature. If the room temperature is set to 68 degrees, you are going to cool down substantially. A lot of people wake up because they feel chilly,” Dr. Yuen adds.

The REM sleep stage gets longer as the night goes on. The first sleep cycle of the night might only have a few minutes of REM, but by the end of the night, it’s likely to be closer to an hour.

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Putting It All Together

Over the course of a night, most people will have 4 to 6 sleep cycles. The first cycle of the night typically lasts about 70 to 100 minutes, and the cycles tend to get longer throughout the night. Dr. Yuen says that stage three (deep sleep) is usually the longest stage in the beginning of the night. As the night goes on, REM sleep increases in length while deep sleep decreases.

Not only are all the stages needed, but they also work together to accomplish different tasks during the night. “We know that for the first two sleep cycles, we actually need alternating stage two, slow wave sleep, and REM sleep for most of our memories to be consolidated,” says Dr. Yuen. “Declarative memory is more likely to be consolidated in stage two, then other parts like procedural skills are more likely to be consolidated during REM sleep. We need the alternation of the stages.” This means if you don’t get two full cycles of sleep in a night, any learning from that day might not be cemented in your memory.

According to Dr. Yuen, the stages don’t always go in the same order. It’s possible, for example, to go from stage two (light sleep) directly into REM sleep, skipping stage three (deep sleep). “It’s more likely to happen in the latter part of the night, going from stage two to REM,” says Dr. Yuen. “You might not have any slow wave sleep left by then.”

Keep In Mind

There are a lot of factors that can influence your sleep cycles. Sleep stages and cycles change as we age; the sleep cycles for an infant are very different from those of an adult, for example. “If you’re sick with a cold, that might influence sleep stages,” says Dr. Yuen. “If you’re in an environment you’re not familiar with, these stages might shift.” Other factors include exercise, alcohol, prescription medications, jet lag, and shift working.

Our innate biological rhythms, or circadian rhythms, also influence the sleep cycles. As Dr. Yuen said, “If you’re a night owl and you have to get to a meeting at 8 in the morning, you’re not going to get all of these sleep stages.”

If you’re using a sleep-tracking device, it’s important to remember that while current technology tries to capture sleep stages, the device may be less accurate than inpatient sleep studies. Dr. Yuen notes that these devices tend to overestimate the times you wake up during the night.

Everyone is different and has unique sleep patterns that change from night to night. It’s normal to have a night or two when things feel a little off and you don’t feel refreshed, but if it seems like you’re consistently missing out on that important, restorative sleep, it’s a good idea to talk with your doctor

Dr. Kin Yuen is an assistant professor at the University of California San Francisco and an adjunct assistant professor at Stanford University. Her research interests include cardiac arrhythmia, medical devices, and health economic evaluations in sleep disorders.