The Self-Cleaning Brain

How sleep clears toxic metabolic byproducts from the brain.

Posted Oct 27, 2013

Sleep deprivation is well-known to lead to poor cognition, decreased attention, and increased irritability. On the more serious side, it can spur manic episodes in bipolar disorder, increase the risk of migraines and seizures, and with enough sleep-deprivation we will die (1). Many factors in our modern world decrease and disrupt our sleep, including late night reading from blue-lit electronic screens, television, and simply getting too little sleep as we try to squeeze too much into each day. But why is sleep so important? As much as we know about the brain and the importance of sleep, much of what goes on to make sleep replenishing and vital remains a mystery.

Last week, a new paper was published in Science, “Sleep Drives Metabolite Clearance from the Adult Brain” that may give us enormous insight into some of the basic physiology of sleep. A major caveat is that the study was done in mice, but it may be vital to understanding human brains as well.

Body interiors are self-cleaning via our circulatory and lymphatic systems. Nutrients are carried to the cells, waste products are carried away and disposed of in this manner. Because some waste products are toxic, leading to cell death, poor circulation and poor clearance will cause health problems. In the brain, there is no “lymphatic system” per se, despite the fact that the brain is one of our most energetic organs and makes a lot of toxic waste products. Instead, the cerebral spinal fluid filters and exchanges with interstitial fluid that bathes our brain cells, clearing these metabolic products, including the amyloid proteins that build up in the case of dementia. This system is called the “glymphatic system” due to the dependence of clearance on the rate of convection and flow of the fluid and the important role of nerve cells called glial cells.

Most of our living brain studies in animals and in young children are done while the subjects are anesthetized or sleeping, often with little consideration for the fact that there could be major differences in metabolism and function during sleep vs wakefulness. In mice, researchers used fluorescent tracers to track the flow of fluid between brain cells and through the central nervous system during waking and sleep. They found that the space between cells increased from 14% of brain volume to 23% of brain volume during sleep and anesthesia. That is a 60% increase in space with huge ramifications on the rate of flow during this time. They also injected mice with labeled amyloid protein and found that amyloid was cleared much more efficiently in sleeping brains than wakeful brains.

Leftover neurotoxic products of cellular activity is responsible for inflammation, cell death, and causes problems with synaptic calcium transmission. Excitotoxic calcium flow is implicated acutely in migraines, seizures (2), manic episodes (3), and chronically in various conditions such as major depressive disorders, dementia and Huntington’s Disease (4), and even autism. If these findings hold for humans, we now have a lot of insight as to why sleep is so very important, and it might enable us to find different tools to help people with all sorts of neurodegenerative conditions.

In the mean time, it certainly doesn’t hurt to use various methods to increase sleep quality now. I recommend wearing blue-blocking glasses or using free blue-blocking software while using computers or tablets after sunset. If you are a late shopper or go to the gym after work, those bright overhead fluorescent lights might interfere as well, so blue-blockers can be useful in these situations. This simple intervention can decrease wakeful messages to the brain when we are gearing up for sleep. 

Bright light in the morning can strengthen circadian rhythm patterns, improve sleep, and reduce seasonal depression. Those who live fairly far north (or south, in the Southern Hemisphere) can use 10,000 lux lamps from September to March as a substitute for the low winter sun. Anyone with insomnia should avoid caffeine and other stimulants after noon, and some people are slow metabolizers and will do best going caffeine-free altogether. Certain natural products, such as magnesium and melatonin (best used for short periods) can help bring on a restful sleep, as can certain breathing exercises, mindfulness practices, and yoga can quiet the mind.

Stronger sedatives and alcohol will induce a deep sleep at the beginning of the night, but tend to cause a lighter, broken sleep in the second half of the night, decreasing overall sleep quality. However, sometimes the acute insomnia brought on by a particular stress or a death in the family can become a difficult, chronic insomnia. Judicious use of stronger sedatives for very short stressful periods can be useful to prevent chronic insomnia, but one should take care due to side effects and dependence on these medicines.

Finally, make sure the bedroom is dark and cool, but not cold, and wear loose-fitting clothing.

If human-brains are self-cleaning during sleep, you will be doing a lot to preserve cognition and brain health just by getting your sleep. Though there is not enough time during the day, a well-rested brain can be productive much more efficiently, so you can feel good and get a lot done at the same time. Sounds like a win win to me.

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Copyright Emily Deans, MD