Why deep sleep and REM sleep are so important for a healthy brain
We all know that sleep is good for us, but a new article from The New York Times (April 25, 2025) reveals that a good night’s sleep is not just about quantity or the number of hours you sleep. Sleep quality – the kind of sleep that makes you feel refreshed and ready for the day – is crucial to a healthy brain.
Nothing new under the sun if you’ve been following our blogs for a while, but still. Scientists believe that deep sleep and REM sleep are particularly influential for brain health and the risk of dementia.
Very recently, a study was published on people with a deficiency in deep sleep and REM sleep. MRI scans showed that the subjects’ brains showed signs of atrophy as long as 13 to 17 years after the deficiencies were identified. The atrophy was similar to the early stages of Alzheimer’s disease.
With atrophy, the cells in your brain are damaged in various places. Those damaged cells eventually die off. This causes all kinds of symptoms. For example, you may have difficulty speaking, moving, and breathing.
What exactly is deep sleep?
Deep sleep is the phase in which your brain waves slow down (slow-wave sleep) and your body relaxes completely. This sleep is also called nREM stage 3. Non-REM sleep (non-rapid eye movement sleep, abbreviated as NREM) is the collective name for sleep stages 1 through 3, during which a gradual slowing of brain waves occurs. The first 2 stages facilitate the transition from being awake to slumber (stage 1) and light sleep (stage 2), finally moving into the third stage, or deep sleep.
In every sleep phase – the phases repeat cyclically several times a night – you have a portion of light sleep, deep sleep, and REM sleep. The proportion of light sleep is actually the largest. And don’t underestimate its power! At the beginning of the night, you spend more time in deep sleep than at the end of your sleep cycles.
During deep sleep, bones and muscles are built up, and the immune system is strengthened. Learning processes and the consolidation of memories also take place in this phase. Also important in this sleep phase is that your brain, as it were, flushes away waste products—substances that have accumulated during the day.
Researchers call this the “glymphatic system,” a kind of cleaning system specifically for your brain. When you don’t get enough deep sleep, these waste products build up. According to scientists, this can contribute to long-term brain diseases such as Alzheimer’s.
Why is deep sleep especially important now?
According to the article in The New York Times, there is growing evidence that deep sleep is not only essential for clearing waste products but also for properly storing memories and keeping us emotionally balanced. During deep sleep, important recovery processes take place, both physically and mentally. Your immune system gets a boost, your heart rate drops, your muscles recover, and your brain processes the day’s experiences.
In short: without enough deep sleep, we not only become physically exhausted but also jeopardize our long-term brain health.
What about REM sleep?
You may have heard that REM sleep or dream sleep – the phase in which you dream – is also very important. That’s right! REM sleep primarily helps with emotional processing and creativity. But these new insights show that if you have to set priorities, deep sleep is the foundation on which everything rests. Without deep sleep, the rest of your sleep pattern also becomes disrupted.
Sleep scientist Merijn van de Laar explains it clearly in a YouTube video he recorded in collaboration with the Dutch Brain Foundation.
Why are deep sleep and REM sleep so important for your brain according to scientists?
Scientists believe the two phases affect the risk of dementia in different ways.
During the flushing process in your deep sleep, your brain flushes away amyloid proteins that are a hallmark of Alzheimer’s. Years of interrupted deep sleep and incomplete flushing – known as glymphatic failure – could accelerate the onset of dementia, says Dr. Maiken Nedergaard, a professor of neurology at the University of Rochester Medical Center who researches the glymphatic system.
Scientists understand less about how REM relates to the risk of dementia, says Dr. Roneil Malkani, associate professor of sleep medicine at Northwestern University’s Feinberg School of Medicine.
A 2017 study of over 300 people over the age of 60 showed that a shorter amount of nightly REM sleep and a longer wait time until the REM phase in each sleep cycle were both predictors of dementia later in life. This could be because REM sleep is “vital” for storing and processing memories.
The loss of that capacity weakens the brain’s defenses against cognitive decline. And atrophy (damage to brain parts) can accelerate in parts of the brain that are not being used, says Dr. Pase, who co-authored the study.
Direct link between dementia, old age, and sleep
It is also difficult to untangle the chicken-and-egg relationship between sleep and dementia, and whether poor sleep is the definitive cause, Dr. Pase says in the article. Adults (especially women) naturally spend less time in deep sleep and REM sleep as they age. Scientists already know that aging itself increases the risk of dementia, but dementia also worsens sleep. It is possible that the two processes reinforce each other.
Finally, scientists point out that both stress and lack of sleep can be triggers. “A good night’s sleep starts during the day” is therefore a truth. Stress is healthy; it makes you alert and helps you perform better. Too much stress, on the other hand… So also take time to relax, have fun, and move.
The amount of sleep you need is personal. Not everyone needs the same amount of sleep. On average, people sleep between 6 and 8 hours per night. Especially during vacations, when it is easier to get in balance with your natural biorhythm, you can learn how much sleep you really need.
Conclusion: more deep sleep and REM sleep?
Sounds fantastic, right? Everyone to deep sleep and REM sleep… But unfortunately, once you are asleep, you cannot choose. The process happens automatically, and fortunately so! When you sleep a little less one night, your body will automatically spend more time in deep sleep.
We also find it important to point out our vision that sleep is not engineerable. In an age of smartwatches, all kinds of technological gadgets, and a world of various (nutritional) supplements, it seems as if you can sculpt your sleep. A big mistake, in our opinion. Approaching sleep as a choice or a manipulable process can be dangerous and is more likely to lead to sleep problems. And that is exactly what you don’t want.
Sleep is not a luxury; it is a pure necessity. Especially when we look at the role of deep sleep and REM sleep for our brain health. This new research confirms why a good night’s sleep is so valuable. So give yourself permission to sleep really well. Your brain (and your future self) will thank you.
Good to know: this article provides general information and is not a substitute for medical advice. Have you been sleeping poorly for a while or are you concerned about your health? Talk to your doctor. Read our disclaimer
Source:
Sneed, Annie. “How Deep Sleep Helps Your Brain Stay Healthy.” The New York Times, April 25, 2025. Link to article