Tonight, and almost every night, something amazing will happen inside your brain. As you turn off the light switch and fall asleep, you will be switching on the neurological equivalent of a dishwasher deep-clean cycle. First, the activity of billions of brain cells will begin to synchronise, and oscillate between bursts of excitation and rest. Coupled with these “slow waves”, blood will begin to flow in and out of your brain, allowing pulses of the straw-coloured cerebrospinal fluid (CSF) that usually surrounds your brain to wash in and be pushed through the brain tissue, carrying the day’s molecular detritus away as it leaves.
Most people recognise that if they don’t get enough sleep, their mood and memory will suffer the next day. But mounting evidence is implicating this “brainwashing” function of sleep in longer-term brain health.
“Sleep is not just a state where things turn off. Sleep is a very active state for the brain – and it seems to be a special state for fluid flow within the brain,” says Laura Lewis, assistant professor of biomedical engineering at Boston University, in the US, who has imaged this pumping process in sleeping humans.
If we don’t get enough regular sleep, these toxic byproducts can accumulate, gradually increasing our risk of dementia and brain diseases. We tend to get less deep sleep as we get older, making it harder to clear out the debris. Fortunately, scientists are homing in on ways to boost this kind of sleep, which could ultimately help to keep our brains healthier for longer.
Doctors have long recognised the restorative properties of sleep, but it wasn’t until 2012 that Prof Maiken Nedergaard at the University of Rochester Medical Centre, in the US, and her colleagues identified a previously unknown plumbing system in the brain that springs to life during sleep, and enables the organ to clean itself.
They found a series of tiny channels surrounding the brain’s blood vessels that allow CSF to filter in, and get pushed through the brain tissue by the pulse of blood alongside – and dubbed it “the glymphatic system”, because it is similar to the body’s lymphatic network except managed by the brain’s glial (support) cells. Having such a system is important because your neurons are extremely active during the day, and produce waste that needs to go somewhere.
“Just as if you don’t have a filter in an aquarium, the fish will die in their own dirt, all this stuff accumulates in the brain that needs to be removed,” Nedergaard says.
One such molecule is beta-amyloid, a toxic protein that accumulates inside the brains of Alzheimer’s patients, and disrupts brain cell function. Nedergaard showed that significantly more beta-amyloid was removed from the brain during sleep. Other studies have found an association between lifelong sleep disruption, elevated levels of amyloid, and Alzheimer’s risk.
However, Nedergaard believes the system could be important for the clearance of many other molecules; from the tau protein that accumulates in Parkinson’s disease to lactic acid, which builds up in the brain when we are awake and has been linked to seizures, to inflammatory molecules produced by immune cells resident in the brain.
Other researchers have suggested that the glymphatic system could provide a missing link between disrupted sleep and mood disorders such as bipolar, or psychiatric diseases including schizophrenia.
Lewis has expanded on Nedergaard’s studies by persuading human volunteers to have their brains imaged while they sleep. “We saw these large waves of fluid flow that started to wash over the brain about every 20 seconds or so, and could travel quite long distances inside the brain,” she says. “As soon as people woke up, this flow pattern would disappear.”
This system seems to be most active during slow-wave sleep – the deepest phase of non-rapid eye movement sleep, predominating during the early hours of the morning.
For reasons that aren’t yet fully understood, people experience less of this kind of sleep as they get older. The glymphatic system also shows a dramatic decrease in efficacy as we enter our later years. “Your dishwasher only works at 20% capacity,” Nedergaard says.
Deep sleep isn’t only important for keeping the brain clean. We release growth hormone during it, helping to repair muscles, bones and immune cells. Deep sleep is also considered important for memory consolidation and the regulation of blood glucose.
So, what if scientists could find a way to reboot deep sleep as we get older? Prof Penelope Lewis, a sleep researcher at the University of Cardiff, believes this could be possible. Deep sleep is characterised by the brain’s neurons firing together in bursts of electrical activity, followed by periods of relative inactivity – which can be visualised as “waves” on a recording of brain activity called an electroencephalogram (EEG). The Cardiff team has demonstrated that playing a “click” sound to sleeping volunteers as they approach the peak of each oscillation can enhance this neural synchrony, resulting in higher peaks and deeper troughs.
“If you keep doing that again and again, it can boost the amount of slow-wave sleep that you get, and it may boost the extent to which memories are consolidated across that sleep as well,” Prof Lewis says.
Extending this to older adults appears to be more challenging. Although the Cardiff team has managed to boost their slow waves, “when we compare it to what happens in a younger group, the effect is puny,” she says. Researchers are now investigating whether targeting the sound to a particular time point during the oscillation could have a more powerful effect.
Whether such approaches will be fruitful remains to be seen. In the meantime, there is plenty we can do to optimise the amount of deep sleep we get, regardless of our age.
The key thing to focus on is sleep quality, which means avoiding coffee, alcohol, exercise and electronic devices in the run-up to bed, and maintaining a dark bedroom overnight. “If light is coming in through the window, or from pilot lights on electronic devices, even if it doesn’t wake you up, it may kick you into a lighter sleep stage and you won’t feel as well rested,” say Prof Lewis.
Sleep may look like a passive process, but as your consciousness checks out, the glymphatic system kicks in, helping to keep your brain fresh and clean. Just as with housework, if you miss the occasional session, no one may notice, but if you scrimp too much, the clutter will gradually accumulate and eventually come crashing down.
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