REM vs core vs deep sleep: Unpacking the different sleep cycles

Sleep is a key part of our daily lives, but most people are unaware of the complex processes our bodies go through on a nightly basis. While it may seem like you simply fall asleep at night and wake up in the morning, dreaming through the night, we actually go through several different phases of sleep cycles while we rest. The different stages are clear, but their purposes are still not fully understood. While the physiological need for sleep has yet to be discovered, it’s hypothesized that sleep is needed for a variety of important functions, such as neural development, facilitation of learning and memory, the clearance of metabolic waste, and the conservation of energy. So, what exactly do people mean when they discuss REM vs core vs deep sleep?

Understanding sleep stages

Although we may not know the exact function different sleep stages serve, we can identify them. Sleep is grouped into two distinct phases: REM sleep (rapid eye movement) and non-REM sleep. Non-REM sleep is further broken down into three stages, for a total of four key sleep stages in a normal night of sleep. The different sleep stages last on average between just a few minutes to over an hour, and are each necessary for a good night’s sleep.

N1 sleep

The N1 sleep stage is known as light sleep. This is the first step in the cycle after you fall asleep. During N1, your brain activity slows, your muscles begin to relax, and brief involuntary twitches known as hypnic jerks occur. You might be familiar with this common sensation as the feeling of tripping or falling as you begin to sleep. Think of this phase as your body “cooling down” and preparing to enter deeper sleep.

N2 sleep

While N2 is also considered light sleep, important physiological processes begin in this stage. N2 is the longest sleep stage, and as your muscles continue to relax, your heart rate and body temperature decrease. It’s believed that this stage is the most important for memory consolidation as well, where your brain processes and stores information from the day before.

N3 sleep

Deep sleep is finally reached during the third stage, N3. In this stage, your brain activity decreases as you reach slow-wave sleep, where delta brain waves decrease your awareness of the physical world and improve crucial cognitive processes. This is also the time when 90% of growth hormone is secreted, which is one theory as to why children and adolescents need more sleep than adults. 

REM sleep

Finally, your brain enters REM (rapid eye movement) sleep. You may be familiar with the term REM as a synonym for deep sleep, but what is it, really? During REM, your brain is nearly as active as it is when you’re awake, and this is the stage where dreaming happens. While our brain may be active, our bodies are not. According to sleep expert Dr. Guy Leschziner, “in REM sleep, the stage of sleep that we most associate with dreaming, our muscles lose significant tone. In fact, most of the muscles in our bodies are paralyzed.” This relaxation of our bodies during REM is one common factor in obstructive sleep apnea (OSA), as the muscles in the chest tongue, and throat relax, they can obstruct the airway.

What is core sleep?

Core sleep is not one of the categorized stages of sleep, but rather comprises the earlier and most restful cycles of the night. The non-REM/REM sleep cycle doesn’t happen just once per night, but repeats several times. Core sleep is considered to be the first three sleep cycles your body goes through throughout the night, which are essential, and it mostly occurs during non-REM sleep. Any more sleep beyond that is deemed optional, but always helpful!

Wondering if sleep apnea is affecting you? Take a few seconds to check out the Dumbo Health Sleep Quiz, and with a few simple questions, you can begin your treatment journey today.

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Frequently Asked Questions

Do naps include all sleep stages?
No. Short naps typically only reach light sleep (N1 and N2). Longer naps that exceed about 60–90 minutes may include deep sleep and sometimes REM, depending on how long you sleep and at what time of day.

Can alcohol or caffeine change my sleep stages?
Yes. Alcohol tends to reduce REM sleep in the first half of the night and fragment deep sleep later. Caffeine delays sleep onset and reduces total deep sleep by blocking adenosine, a sleep-promoting chemical in the brain.

Why do sleep stages matter for memory?
Different stages contribute to different forms of memory. Deep sleep (slow-wave) strengthens factual recall, while REM supports emotional and procedural memory. Missing either stage can impair learning and performance.

Do children have the same sleep stage distribution as adults?
No. Children and adolescents spend a larger share of the night in deep slow-wave sleep compared to adults. With age, the proportion of deep sleep declines, which is one reason older adults may experience lighter, more fragmented sleep.

Can disorders affect specific stages more than others?
Yes. Sleep apnea often disrupts REM because airway muscles relax more in that stage. Insomnia tends to fragment light/core sleep, while conditions like depression may shorten REM latency, causing REM to occur earlier in the night.

Wondering if sleep apnea is affecting you? Take a few seconds to check out the Dumbo Health Sleep Quiz, and with a few simple questions, you can begin your treatment journey today.