Deep sleep and the glymphatic system: how the brain cleans itself at night

The glymphatic system is the network through which cerebrospinal fluid washes the brain and removes the waste produced by neurons, and it works hardest during deep sleep: in a study published in Science in 2013, the space between brain cells increased by 60% during sleep, and beta-amyloid was cleared faster than during wakefulness. As a psychologist and neuroscientist, I like to tell people that at night the brain rests and, at the same time, does its cleaning. Below I explain what happens in your brain during deep sleep, what disturbs it and how you can build an evening that supports it, including a red light therapy ritual with the compact CORE 120 panel or with the nose and ear device.

What is the glymphatic system?

The glymphatic system is the mechanism through which the brain eliminates its waste, using cerebrospinal fluid and glial cells. It was described in 2012 by Maiken Nedergaard's team at the University of Rochester, and the name combines "glia" and "lymphatic".

The rest of the body has lymphatic vessels that collect the residue between cells. The brain has very few, so it built its own system. Cerebrospinal fluid flows alongside the brain's arteries, in narrow spaces around them, then passes between the neurons with the help of water channels called aquaporin-4, located on astrocytes. Along the way, it picks up metabolic waste and carries it towards the veins and the lymphatic vessels in the meninges, from where it reaches the general circulation.

Think of a city that washes its streets at night, when the traffic stops. Your neurons consume an enormous amount of energy during the day, and every thought, every emotion and every decision leaves residue behind. Night is the moment when the city cleans itself.

How does the brain clean itself at night?

The direct answer: during deep sleep, brain cells make room, the space between them grows, and cerebrospinal fluid circulates more easily and carries the waste away with it. Studies from recent years show that this process is synchronised with the slow waves of sleep.

Xie and colleagues, 2013 (Science). In mice, natural sleep increased the interstitial space, that is, the space between brain cells, by 60%. The exchange between cerebrospinal fluid and the fluid between cells increased strongly, and beta-amyloid, the protein associated with Alzheimer's disease, was cleared faster than during wakefulness. The authors concluded that the restorative effect of sleep may come from the removal of potentially toxic waste accumulated while we are awake.

Fultz and colleagues, 2019 (Science). Researchers at Boston University and Massachusetts General Hospital followed 13 young adults who slept inside the MRI scanner, with EEG on their heads. During deep sleep they saw a precise sequence: first the slow waves of the neurons, then a drop in the brain's blood volume, then a large wave of cerebrospinal fluid, roughly once every 20 seconds. It is the first evidence in humans that brain activity during deep sleep "pumps" the washing fluid.

Hauglund and colleagues, 2025 (Cell). Nedergaard's team discovered the conductor of this rhythm. During NREM sleep, the brainstem releases waves of norepinephrine roughly once every 50 seconds, and these make the blood vessels contract and relax rhythmically, like a pump. In mice given zolpidem, a widely used sleeping pill, the norepinephrine waves were 50% smaller and fluid transport in the brain dropped by more than 30%, even though the animals fell asleep faster. The result still needs to be confirmed in humans, but it tells me something important: falling asleep and having restorative sleep are not the same thing.

What happens to the brain when you do not sleep enough?

In short, the waste stays in the brain for longer. Two human studies show this clearly, even after a single sleepless night.

Shokri-Kojori and colleagues, 2018 (PNAS). Researchers at the National Institutes of Health scanned the brains of 20 healthy adults, aged between 22 and 72, after a night of normal sleep and after about 31 hours without sleep. After the sleepless night, beta-amyloid increased by about 5% in the hippocampus, an area essential for memory, and in the thalamus.

Eide and colleagues, 2021 (Brain). In Norway, researchers used MRI to follow how a contrast agent administered into the cerebrospinal fluid is cleared from the brain. In people who stayed awake for a whole night, clearance was slower than in those who slept, and the difference was still visible after they caught up on sleep the following night.

As a psychologist, I see the real-life consequences every day: the mental fog after a bad night, the irritability, emotions that seem bigger than they are, memory that "skips" words. Part of the explanation is exactly this: the brain did not have time to tidy up. And chronic stress, which fragments sleep, adds pressure upon pressure. I wrote about how stress leaves marks all the way down to the DNA level in the article on chronic stress and telomeres.

What the newer studies say: is sleep the only time for cleaning?

Not everyone agrees with the full picture, and that is a healthy sign for science. In 2024, a team at Imperial College London (Miao and colleagues, Nature Neuroscience) used a different measurement method and found, in mice, slower clearance of a dye during sleep and under anaesthesia.

The debate is about methods and about how much of the cleaning happens at night compared with during the day. What remains solid, from the human studies, is that lack of sleep increases beta-amyloid and slows the clearance of substances from the brain, and that deep sleep is accompanied by large waves of cerebrospinal fluid. For your day-to-day life, the conclusion does not change: deep, regular sleep is one of the best long-term investments in your brain.

How do you support deep sleep? The habits that matter

Deep sleep cannot be forced, but the conditions for it can be created. The brain slips into slow waves more easily when it feels safe, when the rhythm of the day is predictable and when the evening is not full of alarms.

Habit	What you actually do	Why it helps the brain
Consistent schedule	You go to bed and wake up at the same time, including at the weekend	The internal clock aligns its slow waves and melatonin release
An evening without alarms	No work emails or news for 60-90 minutes before sleep	The amygdala calms down, and the nervous system switches from fight mode to repair mode
Warm, dim light	Warm lights at home in the evening, screens away, a dark bedroom	You protect melatonin, which opens the door to sleep
A cool room	Around 18-20 °C, a suitable blanket	The drop in body temperature is a natural sleep signal
Alcohol in the evening, in moderation	Avoid alcohol close to bedtime	It may put you to sleep quickly, but it fragments sleep in the second half of the night
Evening ritual	10-15 minutes of red light therapy and slow breathing, in the same place	The brain learns the association between the ritual and sleep, like any habit

If you snore loudly, wake up often or feel tired even though you sleep enough, talk to a doctor. Sleep apnoea fragments deep sleep and deserves to be assessed.

An interesting detail: a study in rats (Lee and colleagues, 2015, Journal of Neuroscience) showed that glymphatic transport was more efficient in the side-lying position than on the back or on the stomach. It is an animal result, so I do not turn it into a rule, but if you already sleep on your side, you have one more reason to stay that way.

What does red light therapy have to do with sleep and brain clearance?

The honest answer: red light therapy does not "wash" the brain in place of sleep. What the studies show is that red light therapy can support sleep quality, and animal research suggests that red and near-infrared light may stimulate the brain's lymphatic vessels, especially when applied at night.

Sleep and melatonin. In the study by Zhao and colleagues (2012, Journal of Athletic Training), 14 evenings of whole-body red light therapy, 30 minutes each, improved sleep quality and melatonin levels in female basketball players, compared with the group without red light. It is a small study, but it shows that red light in the evening does not block melatonin, the way the blue light of screens does.

The neurons' mitochondria. Red light (around 660 nm) and near-infrared light (810-850 nm) are absorbed by cytochrome c oxidase in the mitochondria, which supports energy production and an anti-inflammatory response, as Michael Hamblin describes in his 2016 review on photobiomodulation of the brain.

The brain's lymphatic vessels. The team of Oxana Semyachkina-Glushkovskaya (Humboldt University of Berlin and Saratov State University) published two reviews on this topic in 2023 in the International Journal of Molecular Sciences. In mouse models, transcranial photobiomodulation stimulated the lymphatic vessels in the meninges and the clearance of beta-amyloid, and stimulation performed at night was more effective than daytime stimulation for memory and for reducing beta-amyloid accumulation. The authors propose photobiomodulation during sleep as a research direction. In humans, these data are not yet proven, but the direction is one of the most interesting in the neuroscience of sleep.

From my experience with people, the greatest value of red light therapy in the evening comes from the combination of biology and psychology: 10-15 minutes of red light, lying down, without a phone, with the breath slowing down. For the brain, it is a clear signal that the day has ended. I wrote in detail about the intranasal route and what the studies show about anxiety in the article Red light for the brain: intranasal photobiomodulation, anxiety and sleep.

What does my evening ritual for deep sleep look like?

The best ritual is the one you repeat every evening, at almost the same time. The brain loves predictability: when it knows what comes next, it lets its guard down more easily.

About an hour before bed, I turn off the bright lights and leave my phone in another room. I do 10-15 minutes of red light therapy, lying down or in the armchair, with the nose and ear device or with the CORE 120 panel. During the session, I let the exhale become longer than the inhale, with the help of the CALM One pendant. A long exhale activates the vagus nerve and lowers the heart rate, and the body tells the mind that it is safe. After the session, I no longer open anything that might set off my alarm: no emails, no news.

For durations and frequencies for each type of device, see the guide how many minutes of red light therapy per day. If you notice that an evening session makes you more alert, move it to the afternoon. Every brain reacts a little differently.

Frequently asked questions

What is the glymphatic system?

The glymphatic system is the brain's cleaning system. Cerebrospinal fluid flows alongside the blood vessels, passes between the neurons with the help of astrocytes and carries away waste such as beta-amyloid and tau protein. It was described in 2012 by Maiken Nedergaard's team and works hardest during deep sleep.

When does the brain clean itself?

Mainly during deep, slow-wave sleep. In mice, the space between brain cells increased by 60% during sleep, and in humans large waves of cerebrospinal fluid were observed entering the brain roughly once every 20 seconds during deep sleep.

What happens to the brain if you do not sleep for a night?

Human studies show that waste clearance slows down. After about 31 hours without sleep, beta-amyloid increased by about 5% in the hippocampus and thalamus, and in a Norwegian study the clearance of a substance from the brain was slower even after the recovery night.

How do I increase my deep sleep?

Through a consistent sleep schedule, evenings without stressful stimuli, warm and dim light, a cool and dark bedroom and avoiding alcohol close to bedtime. A repeated evening ritual, with red light therapy and slow breathing, helps the brain associate that moment with sleep.

Does red light therapy help with sleep?

There are good signals. In a 2012 study, 14 evenings of red light therapy improved sleep quality and melatonin levels. Animal studies also suggest that red light applied at night may stimulate the brain's lymphatic vessels, but in humans these effects still need to be confirmed.

Read also

• The complete guide to red light therapy
• Red light for the brain: intranasal photobiomodulation, anxiety and sleep
• Chronic stress and ageing: what happens to telomeres
• Red light therapy: benefits and what the studies show

Sources

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• Fultz NE, Bonmassar G, Setsompop K et al. Coupled electrophysiological, hemodynamic, and cerebrospinal fluid oscillations in human sleep. Science. 2019;366(6465):628-631. PubMed
• Hauglund NL, Nedergaard M et al. Norepinephrine-mediated slow vasomotion drives glymphatic clearance during sleep. Cell. 2025. DOI
• Shokri-Kojori E, Wang GJ, Wiers CE et al. β-Amyloid accumulation in the human brain after one night of sleep deprivation. Proc Natl Acad Sci USA. 2018;115(17):4483-4488. PubMed
• Eide PK, Vinje V, Pripp AH, Mardal KA, Ringstad G. Sleep deprivation impairs molecular clearance from the human brain. Brain. 2021;144(3):863-874. Brain
• Miao A et al. Brain clearance is reduced during sleep and anesthesia. Nat Neurosci. 2024;27(6):1046-1050. UK DRI
• Lee H, Xie L, Yu M et al. The Effect of Body Posture on Brain Glymphatic Transport. J Neurosci. 2015;35(31):11034-11044. PubMed
• Zhao J, Tian Y, Nie J, Xu J, Liu D. Red light and the sleep quality and endurance performance of Chinese female basketball players. J Athl Train. 2012;47(6):673-678. PubMed
• Hamblin MR. Shining light on the head: Photobiomodulation for brain disorders. BBA Clin. 2016;6:113-124. PubMed
• Semyachkina-Glushkovskaya O, Fedosov I, Penzel T et al. Brain Waste Removal System and Sleep: Photobiomodulation as an Innovative Strategy for Night Therapy of Brain Diseases. Int J Mol Sci. 2023;24(4):3221. DOI
• Semyachkina-Glushkovskaya O et al. Phototherapy of Alzheimer's Disease: Photostimulation of Brain Lymphatics during Sleep: A Systematic Review. Int J Mol Sci. 2023;24(13):10946. DOI

This information is for educational purposes and does not replace medical advice. NeuroGenesis DNA red light therapy devices are intended for wellbeing and are not intended to diagnose or treat diseases. If you are dealing with insomnia, sleep apnoea or memory problems, seek the support of a doctor.