Brain and sleep: how the glymphatic system clears the brain

What is the glymphatic system?

The rest of the body has lymph vessels that remove waste and excess fluid. The brain has almost none. Instead it uses cerebrospinal fluid, which is pumped in along the blood vessels, flushes through the tissue and carries waste products out. The name comes from glial cells – the brain's support cells – and lymph. PMID 24136970 PMID 31672896

Support cells called astrocytes wrap tightly around the blood vessels and have water channels (aquaporin-4) that help the fluid along. Among the substances flushed out are beta-amyloid and tau – the proteins that clump together in Alzheimer's disease. That is why the discovery attracted so much attention. PMID 24136970 PMID 31672896

Age plays a part too. Deep sleep becomes shorter and more fragmented from midlife, and in animal studies fluid flow slows in old animals. That is one reason researchers are investigating whether poor sleep after 50 could be one of many pieces in the development of dementia. It is not yet possible to measure the glymphatic system in an individual patient, however. PMID 24136970 PMID 31672896

Why sleep seems to be crucial

The famous study came in 2013. In mice that were asleep or anaesthetised, the space between brain cells grew by 60%. Fluid flowed more easily, and beta-amyloid was cleared faster than in the awake state. The idea that the brain cleans itself at night quickly became a staple of sleep advice. PMID 24136970 PMID 39788123 PMID 31672896

In 2025 a team from the University of Copenhagen and Rigshospitalet showed what drives the flow in mice: slow oscillations in the stress hormone noradrenaline during deep sleep make blood vessels contract and dilate. This acts as a pump that pushes brain fluid into the brain. In humans, MRI scans have shown brain fluid moving in large waves during deep sleep, in step with the brain's slow waves. PMID 24136970 PMID 39788123 PMID 31672896

The controversy: does the brain really clear more at night?

In 2024 researchers at Imperial College London measured how quickly a dye disappeared from the brains of mice. They found clearance was markedly lower – not higher – during sleep and anaesthesia. This has sparked a heated debate about methods, and the question is not settled. PMID 24136970 PMID 38741022 PMID 39788123

That does not mean sleep is unimportant for the brain. There is solid evidence that too little sleep harms memory, mood and concentration. But the explanation that sleep washes out Alzheimer's proteins is a hypothesis resting on animal experiments. It has not been proven in humans. PMID 24136970 PMID 38741022 PMID 39788123

Sleep loss, amyloid and dementia risk

In 20 healthy adults, one night without sleep produced a measurable rise in beta-amyloid in the hippocampus and thalamus – areas important for memory. In another experiment, deep sleep was disrupted with sounds without fully waking participants. The next morning amyloid in spinal fluid had risen, and poor sleep in the preceding days was linked to more tau. PMID 29632177 PMID 28899014 PMID 33879784

Whether this causes dementia in the long run is another question. In the British Whitehall II study of nearly 8,000 people, those who slept 6 hours or less in their 50s and 60s had a 22–37% higher risk of dementia over 25 years. Studies like this show association, not cause: early brain changes can themselves disturb sleep. Read more about prevention in our article on dementia and Alzheimer's. PMID 29632177 PMID 28899014 PMID 33879784

So the overall picture is this: sleep loss affects brain proteins in the short term, and short sleep in midlife is a warning sign. Sleep is, however, only one of several risk factors for dementia, alongside high blood pressure, smoking, physical inactivity and hearing loss. PMID 29632177 PMID 28899014 PMID 33879784

How to protect your brain's night-time clearance

There is no method that specifically boosts the glymphatic system in humans. The advice is therefore the same that gives good sleep in general. The upside is that it works however the debate about brain washing ends. PMID 39788123 PMID 28899014 PMID 33879784 Danish Health Authority (Sundhedsstyrelsen) sundhed.dk – Danish public health portal

A rat study from 2015 found faster clearance when lying on the side than on the back. This has not been studied in humans, so sleep in the position you sleep best in. Side sleeping can, however, reduce snoring. PMID 39788123 PMID 28899014 PMID 33879784 Danish Health Authority (Sundhedsstyrelsen) sundhed.dk – Danish public health portal

An 8-week plan for better sleep and a rested brain

Small changes you stick with beat big projects you abandon. This plan builds on the non-drug methods that sundhed.dk describes as the best treatment for insomnia. Danish Health Authority (Sundhedsstyrelsen) sundhed.dk – Danish public health portal

A sleep tracker or sleep diary can help, but look at weekly averages rather than single nights. The goal is regularity, not a perfect score. Danish Health Authority (Sundhedsstyrelsen) sundhed.dk – Danish public health portal

Do not chase a set number of minutes of deep sleep on your watch. Consumer devices estimate sleep stages imprecisely, and worrying about sleep can itself keep you awake. Check in with yourself instead: do you wake up reasonably rested, and can you keep your focus in the afternoon without nodding off? Then you are most of the way there. Danish Health Authority (Sundhedsstyrelsen) sundhed.dk – Danish public health portal

• Weeks 1–2: Choose a fixed wake-up time – weekends too. Note bedtime, awakenings and energy each day.
• Weeks 3–4: Only go to bed when sleepy. If you lie awake more than about 20 minutes, get up and do something calm.
• Weeks 5–6: Switch off screens and skip alcohol in the last 1–2 hours before bed. Get daylight in the morning.
• Weeks 7–8: Assess: are you more rested? If not, or if your partner has noticed breathing pauses, book an appointment with your doctor.

When should you see a doctor?

See your GP if you have slept badly for more than three months, if you fall asleep during the day – for example at the wheel – or if you snore loudly with pauses in breathing. Sleep apnoea is common after 50 and can be treated. PMID 33879784 sundhed.dk – Danish public health portal

Also seek help if you or those close to you notice increasing forgetfulness, getting lost in familiar places or difficulty managing daily life. This is rarely due to sleep alone. For sudden confusion, paralysis or speech problems, call 112 (the European emergency number). PMID 33879784 sundhed.dk – Danish public health portal