By: David A Fein, MD
Medical Director
Princeton Longevity Center

You live a high-powered lifestyle.  So many things to do and not enough hours in the day for work, your spouse, your kids, getting some exercise and then answering all those work emails in the evenings.  Like many people,  you find the time by getting less sleep.  Maybe you even feel some kind of pride that you can “get by” on just 5 hours of sleep per night.  Or maybe you would really like to sleep more but you spend hours in bed each night tossing and turning agitating about stressors when you would rather be asleep.

But those habits may be raising your risk for dementia and degenerative brain diseases.  Sleep is not just “down time” for the brain – it’s a critical time when our minds strengthen memories and flush out toxins. During deep sleep the brain removes metabolic waste that builds up during the day.

A recently published study published in Nature found that the need to sleep may be the result of the build-up of electrical imbalances in the mitochondria, which are the cell’s energy powerhouse, in brain cells.  Sleep helps to restore the function of the mitochondria.  Without sleep, brain cells may become starved for energy leading to degenerative diseases like Alzheimer’s, Parkinsons and ALS.

Sleep also powers the recently discovered glymphatic system.  The glymphatic system is a waste clearance system in the brain that plays a crucial role in maintaining brain health. Its name comes from a combination of “glial” and “lymphatic”.  Glial cells are non-neuronal cells in the nervous system that provide support and protection for the neurons. Glial cells provide structural support for neurons, maintain the chemical environment necessary for neuronal function, and protect neurons from damage.

The glymphatic system uses cerebrospinal fluid (CSF) to flush out waste products from the brain that have built up while you are awake.   CSF flows into the brain along blood vessels, through channels formed by astrocyte cells (a type of glial cell) and mixes with the fluid that surrounds brain cells, collecting metabolic waste products and toxins along the way. It then drains through channels alongside veins and eventually exits the brain, flowing into the lymphatic vessels in the neck.

The glymphatic system is most active during sleep but it also very dependent on the quality of your sleep.

The time you spend sleeping each night has a complex structure and cycle.  It typically starts with Stage 1 and Stage 2 Light Sleep where you transition from being awake to asleep as muscles relax and brain wave patterns slow.  Stage 3 is Deep Sleep when there are minimal eye movements, muscle activity is at its lowest and brain wave patterns have slowed even more.  Stage 4 is REM sleep characterized by rapid eye movements, increased brain activity, and vivid dreaming.

Typically, a person cycles through these stages of light sleep, deep sleep and REM sleep several times per night, with each cycle lasting around 90 minutes early in the night and getting shorter as the night progresses.

The glymphatic system is most active during Stage 3 Deep Sleep. During that time the space between brain cells expands by up to 60%, allowing for more efficient flow of CSF and removal of toxins.  Sleep deprivation, particularly limited Stage 3 Deep Sleep, has been shown to impair glymphatic clearance, which may contribute to cognitive dysfunction and the accumulation of toxic proteins in the brain.

Amyloid-beta  and tau proteins may be at the heart of Alzheimer’s biology. In healthy brains, these proteins are produced and cleared continuously, but in Alzheimer’s the balance shifts,.  Amyloid-beta forms plaques between neurons, and tau tangles form inside nerve cells. That may lead to loss of brain cells over the years (although some researchers take the position that the build-up of Amyloid may be the result of other processes rather than being the actual cause of Alzheimer’s Disease).  During sleep, the glymphatic “cleansing” system helps remove these proteins. In a landmark study in 2013, Nedergaard and colleagues showed that sleeping mice cleared dramatically more Amyloid-beta from the brain than when awake. In fact, the glymphatic system was shown to be up to 10 times more active during sleep than wakefulness, and sleeping brains cleared far more amyloid-beta than brains kept awake. Likewise, MRIs of people show that deep non-REM sleep is accompanied by large waves of cerebrospinal fluid that appear to flush toxins from the brain.  In another recent study at Washington University just one night of sleep disruption in healthy middle-aged adults led to measurable increases in CSF amyloid-beta, and a week of restricted sleep raised CSF tau levels. As Dr. David Holtzman (senior author of that study) put it, “poor sleep is associated with higher levels of two Alzheimer’s-associated proteins.” These findings help explain a clinical intuition: if you chronically skimp on sleep, the brain’s trash doesn’t get taken out, so amyloid and tau accumulate and over the years can seed Alzheimer’s damage. Researchers therefore speculate that impaired waste clearance from sleep loss may be a key driver of Alzheimer’s disease.

How much sleep is “just right” for brain health? Large studies suggest that both extremes of sleep duration are linked to higher Alzheimer’s risk. In a 2021 analysis of over 4,400 older adults (initially healthy), those who habitually slept 6 hours or less per night had worse memory performance and higher brain amyloid levels than those sleeping 7–8 hours. But people who slept 9 hours or more also performed worse on cognitive tests (especially tasks involving planning and organization), though their amyloid burden was not higher than average. In short, both short sleep and excessive long sleep were associated with worse brain health outcomes, underlining the idea that there is a “sweet spot” (about 7–8 hours) for most adults. Other studies find similar patterns: compared to getting 7–8 hours, regularly getting under 6 hours or over 9 hours is tied to a higher chance of developing Alzheimer’s over time.

Sleep quality matters too. Fragmented sleep, chronic insomnia, and lack of deep sleep have been associated with accelerated memory loss and more Alzheimer’s pathology. Self-reported poor sleep quality in older adults has been linked to higher levels of Alzheimer’s-related tau protein in brain imaging studies. Even in people who do get a typical amount of sleep, frequent awakenings or a night of tossing-and-turning can raise Alzheimer’s protein levels in the short term.

Certain sleep disorders can dramatically raise dementia risk. Obstructive sleep apnea (OSA) – where breathing repeatedly stops during sleep – is a prime example. OSA both fragments sleep and causes low oxygen levels, which may accelerate neurodegeneration. Epidemiological research backs this up.  A 2024 meta-analysis of cohort studies found that people with sleep-disordered breathing had about a 1.5-fold higher risk of later cognitive impairment or dementia compared to those without apnea.  Individuals with untreated apnea often develop mild cognitive impairment roughly a decade earlier than those without breathing problems. Even short of clinical apnea, breathing interruptions (snoring, snooze-heavy breathing) appear to correlate with higher brain amyloid on PET scans in elderly adults. In practice, that means diagnosing and treating sleep apnea is increasingly seen as a possible way to delay cognitive decline.

Other disorders, like chronic insomnia and restless legs, also impair sleep quality and have been tied to memory troubles in smaller studies. For example, people with long-standing insomnia often score worse on memory and attention tests.

Given the evidence, most experts agree that good sleep habits are an easy, beneficial way to support brain health. Aim for a consistent 7–8 hours of sleep per night (short naps are fine, but regular daytime sleep should be no more than 20–30 minutes if night sleep is an issue). Equally important is sleep quality: keep a regular schedule, and create a calm bedtime routine. The NIH’s sleep guidelines recommend steps such as:

• Maintain a consistent schedule. Go to bed and wake up at the same times each day (even on weekends) to keep your body clock steady.
• Wind down before bed. Use the hour before sleep for quiet, relaxing activities (reading, gentle stretching, listening to soft music) and avoid strenuous exercise or bright screens, which can signal your brain to stay alert.
• Optimize your sleep environment. Make your bedroom dark, quiet, and cool. Limit it to sleep activities (avoid working or screen time in bed) so your brain associates the room with rest.
• Watch diet and substances. Avoid large meals, alcohol, and nicotine close to bedtime, and limit caffeine (coffee, tea, soda, chocolate) in the afternoon/evening, as these can disrupt sleep onset and depth.
• Be active by day. Regular physical activity (especially earlier in the day) promotes better sleep at night. Even daily outdoor sunlight exposure helps regulate circadian rhythms.

Protecting the quality and quantity of your sleep, along with maintaining a healthy and active lifestyle, will be an important part of preventing Alzheimer’s in your years ahead.