We've long known that sleep is as important to our health as good nutrition and regular exercise. Not getting enough sleep is detrimental to daytime functioning – to our mood, energy, concentration and reaction time. Over the long term, it contributes to obesity and the risk of serious illness.
But research has found that sleepless nights have implications well beyond making us sleepy the next day. When we go to sleep, our brains go to work, performing critical functions that affect cognition and memory.
During waking hours, the brain is bombarded with more stimuli than it can process. When we sleep, the brain makes order out of chaos and archives memories for later retrieval. It does this by strengthening critical neural connections, discarding unimportant ones, and solidifying new memories.
This explains why “sleeping on it” helps us recall a newly learned task. It also helps explain why people suffering from memory deficits can recall a name from 40 years ago but not what they had for lunch yesterday. Their brains have become less efficient at making new connections and storing new memories.
The other critical function that occurs during sleep can be thought of as clearing out the brain – actually washing away the waste products that build up all day, including toxins like beta amyloid, which is implicated in Alzheimer's disease. All those stimuli coming in during the day keep the neurons in the brain firing on all cylinders and as with every organ in the body that converts fuel into energy, that energy produces waste proteins that accumulate during waking hours and have to be cleared out when we sleep.
New research has provided insight into how this works and may lead to a road map for preventing cognitive decline and degenerative diseases such as Alzheimer's. Over the course of the night, our brains cycle through several phases from lighter sleep, to the deep, almost unconscious restorative sleep characterized by slower brain waves, to rapid-eve-movement (REM) sleep when the brain becomes more active and we are most likely to dream. Researchers have focused on the deep, non-REM phase of sleep during which brain activity slows and memory retention takes place. A 2013 study on mice showed that during this phase, cerebrospinal fluid, a watery liquid that surrounds the brain and spinal cord, washes away accumulated toxins but it wasn't clear how or why this occurred and a similar effect hadn't been shown in humans.
Recently, in a study published in the journal Science, a team of Boston-based researchers, using non-invasive techniques on human subjects, demonstrated that the electrical activity of brain waves plays a role in this process.
Brainwaves are produced by electrical pulses from masses of neurons firing together in rhythm. Their activity is faster when we are awake and slows when we sleep. Subjects in the study wore EEG (electroencephalogram) caps that tracked electrical activity in their brains, showing which phase of sleep they were in while they slept inside an MRI machine that measured blood oxygen levels and the flow of cerebrospinal fluid in their brains. What the researchers found is that during non-REM sleep, neurons would synchronize and all stop firing at the same time. During this “quiet time,” blood would flow out of the brain since the lack of activity means the brain needs less oxygen. As blood flowed out, cerebrospinal fluid would rush in, occupying the empty spaces and washing toxins out of the brain.
This finding illustrates once again the importance of sleep. Neurons are busy when we're awake; they don't stop firing in unison and they need oxygen to do their work. So blood isn't vacating the brain and there's no room for the cerebrospinal fluid to flow in and wash away the waste products of all that activity. We know that people suffering from neurodegenerative diseases like Alzheimer's have weaker and slower brain waves, so there might be fewer and slower waves of cerebrospinal fluid clearing toxins from their brains. The implication is that focusing on a way to increase the flow of cerebrospinal fluid might be an effective way to prevent or treat Alzheimer's.
Many attempts to develop drugs to treat Alzheimer's have focused on beta amyloid itself without notable success. Improving the “wash cycle” in the brain might clear away not just beta amyloid but other toxins as well, such as tau, a protein that has been found tangled in the brains of Alzheimer's sufferers. Research will continue while we all recognize once again how important it is to catch our zzz's.