• Glenn S. Rothfeld, MD

The Dementia-Fighting Powers of Sleep

What if I told you that there was something safe and effective that you could do while you sleep, that wards off Alzheimer’s and Parkinson’s Diseases, improves memory and boosts your immune system? Hint: it’s a trick question, the answer is … sleep!

Image created by Katherine Streeter of NPR

Every living thing requires sleep, from the mighty elephant to the tiny fruit fly. Even sharks, who have to stay moving in order to breathe, shut off part of their brains for “rest” periods.

So what’s so important about sleep? For one thing, researchers have long known that sleep is necessary for proper memory function. In a 2013 paper published in Nature Neuroscience, researchers at Harvard Medical School and University of California Berkeley described the complex process that our brain engages in to improve memory while we sleep. The researchers call it a triage process. All the day’s memories are reviewed, and the ones that are most impactful and necessary are reinforced, linked to existing memories, and even “tagged” for later recall. The memories that are not deemed important are not reinforced, and thus forgotten. This type of study was repeated in many forms, all with the same results.

As we age, the ability to remember becomes more challenging. Diseases such as Alzheimer’s frequently start with forgetting simple names, facts and basic orientation. Much of the ability to remember things depends on good quality sleep. As one sleep researcher said, perhaps lack of sleep does not cause Alzheimer’s, but if there were already a tendency toward it, lack of sleep could mean the difference of decades in its appearance.

We also know from current research that good sleep is critical for proper immune system function. T cells (the large family of lymphocytes that form the cornerstone of our immune response) are significantly decreased if we are sleep deprived. Even healthy people with robust immune systems demonstrate a loss of immune system function when they are sleep deprived. This can lead to colds and flu, to an inability to ward off chronic conditions such as shingles, and even to a lack of proper immune vigilance against cancer cells.

In addition to its effects on lymphocytes and other white blood cells, sleep deprivation raises the level of inflammatory messengers in the body. Several studies have demonstrated that sleep deprived people have higher levels of inflammatory cytokines such as IL-6 (interleukin-6) and inflammatory markers such as CRP (C-reactive protein). The presence of inflammation is implicated in heart disease, brain diseases such as Alzheimer’s and Parkinson’s, and even cancer.

But, as important as memories and immune function are, we are just beginning to understand the critical function that sleep plays in maintaining our health.

Dr. Maiken Nedergaard, a sleep biologist who runs the sleep laboratory at University of Rochester, has been doing research into an exciting new role for sleep in proper brain function. To Dr. Nedergaard, it did not seem plausible that sleep, which made us so vulnerable to external forces, existed just so that we would remember things and not catch colds as easily. So her laboratory has been digging for a more fundamentally important function for this universal activity. And in time, they began performing experiments outlining a remarkable function of sleep: Brain Cleansing.

Our bodies have an intricate system of cleaning out toxins, known as the lymphatic system. Lymph channels course throughout the body, weaving between cells. They carry macrophages and other immune cells, which can move into the body tissues and engulf debris and the toxic byproducts of metabolism.

But the brain, with the blood-brain barrier making it impenetrable to lymph cells, is different. It uses around 20% of our total energy to perform its function of thinking, and all of that energy expenditure creates waste products, just like energy production elsewhere in the body. Some of those waste products, like tau protein and beta-amyloid, can be toxic to brains, leading to Alzheimer’s and similar dementias if not removed.

When we’re awake, our brains are too busy and active to be heavily involved in cleansing. We have “clean-up” cells, called glial cells, whose job it is to move through the fluid spaces of our brains (about 20% of the volume of our brains during the day). But by day, those cells only clean up the surface of the brain. That’s where Dr. Nedergaard’s discovery, derived from mouse studies, comes in.

It seems that, when we sleep, our brain cells actually shrink. This allows the channels in between those cells to swell and fill up with cerebrospinal fluid, providing a medium for the brain’s glial cells to move in and around each cell, removing any debris they discover. In essence, our sleeping brains are like those self-cleaning toilets in Europe, flooding and washing away the garbage from the day’s metabolism!

The study was done by tagging markers that were injected into the mouse’s cerebrospinal fluid. They were found to follow specific pathways through the brain and out again. When the mice were asleep, the fluid exchange actually increased by 20 times! To put it another way, the researchers measured a 60% increase in the flow through the interstitial fluid, the channels around and between the brain cells, channels that were inactive when the brain is busy in activities of wakefulness.

This was so shocking that the researchers thought at first that they had calculated wrong. But on repetition it became obvious: the sleeping brains cleared twice as much waste as the waking brains! And much of that waste was identified by the researchers as beta-amyloid, the toxic substance implicated in the genesis of Alzheimer’s Disease.

The glial cells actually gobble up any of the waste products, washing them through the channels of the cerebrospinal fluid. So far, similar results have been studied in dogs, goats and baboons. And human studies are being planned as well.

Stay tuned for next week, when I discuss barriers to a good night's sleep, as well as tips for catching the elusive Zzz's!