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 reading our blogs for a while but still. Scientists believe that deep sleep and REM sleep are particularly influential when it comes to brain health and the risk of dementia.
A study published last month on people deficient in deep sleep and REM sleep showed that the subjects’ brains showed signs of atrophy on M.R.I. scans 13 to 17 years after the deficiencies were identified; the atrophy resembled what you would find in the early stages of Alzheimer’s disease.
With atrophy, the cells in your brain are damaged in several places. Those damaged cells eventually die. This causes all kinds of symptoms. For example, you get difficulty talking, moving and breathing.
What is deep sleep anyway?
Deep sleep is that stage when 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 NREM) is the collective name of sleep stages 1 through 3 in which a gradual slowing of brain waves takes place. The first 2 stages transition between wakefulness to slumber (stage 1) and light sleep (stage 2) before finally transitioning to the third stage or deep sleep.
Each sleep stage, the stages repeat cyclically several times a night, you have a proportion of light sleep, deep sleep and REM sleep. The proportion of light sleep is the largest by the way. 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 and the immune system is strengthened. Learning processes and the consolidation of memories also take place during this phase. Another important element during this sleep phase takes place when your brain flushes out waste products, as it were, 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 accumulate. Scientists say this can contribute to long-term brain diseases such as Alzheimer’s.
Why is deep sleep especially important now?
According to the New York Times article, there is growing evidence that deep sleep is essential not only for clearing waste products, but also for storing memories properly 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 experiences of the day.
In short, without enough deep sleep, we not only become physically exhausted, but also compromise our long-term brain health.
What about REM sleep?
You may have heard that REM sleep or dream sleep – that phase when you dream – is also very important. That’s right! REM sleep especially helps with emotional processing and creativity. But these new insights show that if you need to prioritize, deep sleep is the foundation on which everything rests. Without deep sleep, the rest of your sleep pattern also gets messed up.
Sleep scientist Merijn van de Laar explains it comprehensively 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 stages affect the risk of dementia in different ways.
As part of the flushing process in deep sleep, your brain flushes out amyloid proteins that are a hallmark of Alzheimer’s disease. Years of interrupted deep sleep and incomplete flushing – known as glymphatic failure – could hasten 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 is linked to dementia risk, says Dr. Roneil Malkani, associate professor of sleep medicine at Northwestern University’s Feinberg School of Medicine.
A 2017 study of more than 300 people over the age of 60 found that a shorter amount of nighttime REM sleep and longer latency to the REM phase in each sleep cycle were both predictors of dementia later in life. That could be because REM 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 used, said Dr. Pase, who co-authored the study.
Direct impact – Science, dementia and aging
It is also difficult to figure out the “chicken and egg” relationship between sleep and dementia, and whether poor sleep is the definitive cause, Dr. Pase says in the article in question. Adults (especially women) naturally spend less time in deep 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,” he said.
Finally, scientists point out thatboth stress and too little sleep can betriggers. So good sleep starts during the day is a truth. Stress is healthy; it makes you alert and improves your performance. Too much stress, on the other hand. So also take time to relax, have fun and exercise.
The amount of sleep time 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 balance with your natural biorhythms, you can learn how much sleep you really need.
Conclusion: more deep sleep and REM sleep?
Sounds great right? All about deep sleep and REM sleep…. However, know that once you sleep you can’t choose. The process happens naturally and thankfully! When you sleep one night less, your body will automatically spend more time in deep sleep.
We also think it’s important to remind you of our view that sleep is not engineerable. In a time of smartwatches, all kinds of technological gadgets and a world of all kinds of (nutritional) supplements, it seems as if you can model sleep. A big mistake we think. Approaching sleep as a choice or manipulable process can be dangerous and contrary to your wishes, more likely to lead to sleep problems.
Sleep is not a luxury, it is pure necessity – especially considering the role of deep sleep and REM sleep in our brain health. Thanks to this new research confirmed 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 for it.
Source:
Sneed, Annie. “How Deep Sleep Helps Your Brain Stay Healthy.” The New York Times, April 25, 2025. Link to article
