Why deep sleep and brake 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 think deep sleep and REM sleep are particularly influential for brain health and dementia risk.

Very recently, a study was published on people with deep sleep and brake sleep deficiencies. MRI scans showed that the subjects’ brains showed signs of atrophy as much as 13 to 17 years after the deficiencies were identified. The atrophy was similar to the early stages of Alzheimer’s disease.

In 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 the 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 for sleep stages 1 through 3 in which there is a gradual slowing down of brain waves. 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 phase – the phases repeat cyclically several times a night – you have a proportion of light sleep, deep sleep and brake sleep. By the way, the proportion of light sleep is 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 and the immune system is strengthened. Learning processes and the consolidation of memories also take place during this phase. Also important in this sleep phase is that 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. 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 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 sufficient deep sleep, we not only become physically exhausted, but also put our long-term brain health at risk.

What about brake sleep?

You may have heard that brake sleep or dream sleep – the phase when you dream – is also very important. That’s right! Brake 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 understandably in a YouTube video he recorded in collaboration with the Dutch Brain Foundation.

Why are deep sleep and brake sleep so important for your brain according to scientists?

Scientists believe the two stages affect the risk of dementia in different ways.

During the flushing process in your 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, according to Dr. Maiken Nedergaard, 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 a longer wait 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 used, said Dr. Pase, who co-authored the study.

Direct link between dementia, old age and sleep

It’s 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 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, the scientists point out that both stress and too little sleep can be triggers. “Good sleep begins during the day” is thus 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 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 brake sleep?

Sounds great right? All to deep sleep and brake sleep … But alas, once you sleep, you cannot choose. The process happens automatically, 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 in our opinion. Approaching sleep as a choice or manipulable process can be dangerous and more likely to lead to sleep problems. And that’s exactly what you don’t want.

Sleep is not a luxury, it is pure necessity. Especially considering 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 get really good sleep. Your brain (and your future self) will thank you.

Source:
Sneed, Annie. “How Deep Sleep Helps Your Brain Stay Healthy.” The New York Times, April 25, 2025. Link to article