DON’T LET ME KEEP YOU AWAKE…
Practically everything that moves and breathes needs to sleep. But in a world in which it’s ‘survival of the fittest’, sleeping doesn’t make much sense – if you don’t want to get eaten by a predator, that is. Therefore, there must be a good reason why so many living things spend a good chunk of their lives snoozing. But the exact purpose behind what Edgar Allan Poe called “those little slices of death” has eluded scientists for decades. Many sleep experts think that sleeping
strengthens the memory and slumber is a time when the day’s events are moved into ‘long term’ storage. That may be true, but judging by the devastating effects sleep deprivation can have on our mental abilities (see ‘Dying to Sleep’ on page
32), there’s little doubt that we need a regular dose of Zs to keep our emotions and our minds balanced. Exactly how sleep weaves its restorative magic has been something of a mystery – but now
new research says that one of the main reasons we sleep is to physically ‘clean’ the brain. Every biological process – and every chemical reaction – in the body produces by-products. The brain is no exception: it churns out waste, such as damaged proteins. This waste needs to be cleared away or else it will cause damage. Until now, the only way we thought the brain did this was by breaking down and recycling the waste within brain cells. Last year, however, researchers led by Maiken Nedergaard at the University of Rochester Medical Centre, New York, discovered a completely new
brain waste disposal system. They found it using a cutting-edge imaging technology called two-photon microscopy, which offers a new way to look inside a living brain. Nedergaard and her colleagues injected a special dye into the brains of mice and, using the microscope technique,
spotted a drainage system that pumps liquid (cerebrospinal fluid) through the brain. This flow of fluid seems to ‘wash away’ accumulated waste before emptying the ‘dirty’ liquid out of the brain and into the blood, where the waste can ultimately be destroyed in the liver. Nedergaard and colleagues dubbed this the ‘glymphatic system’ (due to its similarity with another of the body’s waste removal systems, the
lymphatic system). The researchers also thought that because this newly-discovered glymphatic system uses a lot of energy, it should be less active while the brain is awake and busy. To test their idea, they compared awake and sleeping mice, and found that the glymphatic system was actually ten times more active during sleep. They strongly suspect that this happens because the tiny channels between blood vessels and cells that the fluid flows through get wider during sleep, allowing fluids to pass more freely and “take out the trash”.
A dream answer for Alzheimer’s?
Finally, the researchers went a step further by injecting a toxic protein that has been strongly linked with Alzheimer’s disease, called betaamyloid, into mice’s brains. They saw that this waste protein was flushed from the mice’s brains twice as
quickly when the mice were asleep. This could be an extremely important discovery because almost all neurodegenerative diseases (such as Alzheimer’s) are associated with the build-up of waste products. This research may therefore help us understand how sleep aids the brain, and ultimately how we might treat these conditions. We can’t be certain whether these mouse results will apply for us, so the next step is to check whether the same things happen in human brains. By pure coincidence, just days after these results came to light, research was published which shows that older people who sleep poorly have higher levels of the harmful beta-amyloid waste protein in their brains, as measured by PET scans. The researchers behind this work urge caution, insisting that their
results don’t prove that poor sleep causes Alzheimer’s (it could be the other way round, for example). Sleep is, however, starting to look like an extremely plausible factor in the development of Alzheimer’s. So not only will plenty of sleep make you a happier, more alert person this Christmas, it may also improve your long-term mental health. How many more reasons do you need to catch up on all those Zs these holidays?