You want it. You crave it. You can’t wait to get more. But somehow, it eludes you. It’s sleep, and most people aren’t getting enough. A poor diet, alcohol, caffeine, anxiety, and stress can all interfere with a good night’s sleep, but what you might not know is the price you pay when the deep sleep stage is compromised. The price is your brain health.

How Sleep Works
When a healthy person sleeps, they cycle through 5 stages. There is REM (rapid eye movement) sleep, during which dreaming occurs, and then there are four stages of non-REM sleep. The first stage is light sleep. Your muscles, heart rate, and breathing start to slow down. During stage 2, your brain waves begin to slow down. In stage 3, very slow “delta” brain waves begin, with intermittent periods of faster waves. In stage 4, the deepest stage, you are profoundly asleep and it is very hard to wake you. Finally, REM sleep kicks in and you begin dreaming.

The complete cycle is important for various reasons. REM sleep, for instance, is tied to a healthful circadian rhythm of sleeping and waking. This stage can be compromised by certain medications, alcohol, or caffeine, keeping you in light sleep without the mental health benefits of dreaming: REM sleep helps with general cognition, creativity, and emotional stability.

But what I really want to talk about is deep sleep because it is the source of some new and exciting research.

The Brain’s Own Waste Removal System
Many important things happened during deep sleep—your body works on repair, your brain releases growth hormone for regeneration, and your brain consolidates memories. We have also recently discovered something else that happens during deep sleep, and it has to do with taking out the trash.

You may have heard of the lymphatic system—the network of lymph nodes and vessels in your body that help to flush out waste. Until recently, scientists believed that the lymphatic system did not extend past the blood-brain barrier, and technically it doesn’t. However, we now know that the brain has its own similar system, called the glymphatic system, that appears to activate only during deep sleep.

When your brain shifts into the slow-wave stage, it triggers a sort of pulse or oscillation of neuronal activity that actively supports the consolidation of experiences from the day into memories. At the same time, the flow of blood into your brain slows significantly, lowering your cerebral blood volume by about 10%. New research demonstrates that this allows cerebrospinal fluid to flow into the brain, then be flushed back out, like a washing machine filling with water, then emptying out again when the clothes are clean. The reason for this is likely the cleaning out of waste products that accumulate in the brain from normal brain metabolism.

Brain Waste and Dementia
Why is this so significant? We know that people with chronic sleep problems are more vulnerable to Alzheimer’s disease, and this may be the link: When waste stays in the brain and isn’t properly flushed out through this process of oscillating blood volume and the flushing in and out of cerebrospinal fluid, it may accumulate and form the plaques and tangles characteristic of Alzheimer’s disease. These plaques and tangles can irreversibly degrade the brain, with symptoms that start as mild cognitive impairment and gradually progress to full-blown dementia.

If you are concerned about your brain health, and especially if you have a family history of dementia, getting enough deep sleep in particular is critical. How do you know if you are getting enough? There are some objective and subjective ways to tell.

Sleep Trackers
Health trackers are trendy right now, and in addition to those that count your steps, exercise minutes, calories, and heartbeat, there are some that track sleep. There are apps you can use on your phone, but I like those you can wear, such as rings, wrist bands, ankle bands, and clips, because they are likely more accurate in detecting your movement, breath rate, and heart rate—all indicators of which sleep stage you are in. These can tell you, in the morning, not only how long you slept but how well you slept, and the best ones will show you a graph demonstrating how long you were in light sleep, deep sleep, and REM sleep, as well as when you were awake.

Just one more step in the evolution of personalized medicine, these trackers will help you understand your sleep so you can intervene to improve it. If you see that you are not getting enough deep sleep, even when you are sleeping for the recommended 7 to 9 hours, then there are some things you can do to work on improving your deep sleep time.

Sleep Interventions
Improving your sleep hygiene, or pre-sleep habits, can help improve the quality of all the stages of your sleep, and some of these are also beneficial for more REM sleep, but they are also all likely to improve your time in deep sleep:

• Don’t have any caffeine after noon.

• Don’t drink alcohol after dinner (or have more than 1 drink with dinner). This is very important for both REM and deep sleep because alcohol may make you fall asleep faster, but you will tend to stay in light sleep longer, with less REM and deep sleep.

• Keep your carbs light at dinner. Emphasizing protein, healthful fats, and vegetables instead of carbs can help you get more deep sleep (think a big salad with some salmon and a lot of veggies, dressed with olive oil and lemon juice, or a warming bowl of veggie-packed soup).

• There are some who find that a low-carb dinner, or a dinner too low in calories, interferes with sleep. If that’s you, I recommend a small amount of carbohydrate-rich, fiber-rich food before bed, such as a small sweet potato or a small bowl of oatmeal—not more dessert! Keep your portion small, or it could have the opposite effect.

• Take a warm bath or a sauna before bed, but keep your bedroom cool.

• Meditate or breathe deeply for 5 to 15 minutes right before going to sleep.

• Keep lights low and turn off all screens at least an hour before going to sleep, to help you brain release melatonin, which helps you feel tired. You could also take a melatonin supplement.

• Listen to ambient noises like ocean waves, forest noises, or rain sounds. Called “pink noise,” these sounds can help to induce deep sleep.

• Do what works for you. We are all different, so experiment to see which interventions impact your deep sleep, and which ones don’t seem to make much difference.

Deep sleep matters…a lot. But how you get there isn’t necessarily what works for everyone. That’s the beauty of sleep trackers. They give you the feedback you need to know what works. And if you don’t have a sleep tracker? The best way to tell if you are getting deep sleep is how you feel in the morning. If you feel groggy, you probably didn’t get enough. If you awake refreshed and ready to face the day, you can be pretty sure your deep sleep is on point, and your brain is self-cleaning, exactly the way it should.



Coupled electrophysiological, hemodynamic, and cerebrospinal fluid oscillations in human sleep, Nina E. Fultz, et al., Science, 366, issue 6465, pp. 623-31, 01 Nov 2019.

Deep sleep drives brain fluid oscillations, Soren Grubb and Martin Lauritzen, Science, 336, issue 6465, pp. 572-3, 01 Nov 2019.

Fluid dynamics during sleep, Peter Stern, Science, 366, issue 6465, pp. 583-5, 01 Nov 2019.

Brain consolidates memory with three-step brainwave, by Radboud University, Medical Xpress, September 22, 2015:

‘Waves’ of neural activity give new clues about Alzheimer’s, Laura Chaparro, Spanish Foundation for Science and Technology, Medical Xpress, September 6, 2017:

Pink Noise: effect on complexity synchronization of brain activity and sleep consolidation, Junhong Zhou et al., Journal of Theoretical Biology, 306: pp. 68-72, August 7, 2012.