By MARIA KONNIKOVA
SLEEP seems like a perfectly fine waste of time. Why would our bodies evolve to spend close to one-third of our lives completely out of it, when we could instead be doing something useful or exciting? Something that would, as an added bonus, be less likely to get us killed back when we were sleeping on the savanna?
“Sleep is such a dangerous thing to do, when you’re out in the wild,” Maiken Nedergaard, a Danish biologist who has been leading research into sleep function at the University of Rochester’s medical school, told me. “It has to have a basic evolutional function. Otherwise it would have been eliminated.”
We’ve known for some time that sleep is essential for forming and consolidating memories and that it plays a central role in the formation of new neuronal connections and the pruning of old ones. But that hardly seems enough to risk death-by-leopard-in-the-night. “If sleep was just to remember what you did yesterday, that wouldn’t be important enough,” Dr. Nedergaard explains.
In a series of new studies, published this fall in the journal Science, the Nedergaard lab may at last be shedding light on just what it is that would be important enough. Sleep, it turns out, may play a crucial role in our brain’s physiological maintenance. As your body sleeps, your brain is quite actively playing the part of mental janitor: It’s clearing out all of the junk that has accumulated as a result of your daily thinking.
Recall what happens to your body during exercise. You start off full of energy, but soon enough your breathing turns uneven, your muscles tire, and your stamina runs its course. What’s happening internally is that your body isn’t able to deliver oxygen quickly enough to each muscle that needs it and instead creates needed energy anaerobically. And while that process allows you to keep on going, a side effect is the accumulation of toxic byproducts in your muscle cells. Those byproducts are cleared out by the body’s lymphatic system, allowing you to resume normal function without any permanent damage.
The lymphatic system serves as the body’s custodian: Whenever waste is formed, it sweeps it clean. The brain, however, is outside its reach — despite the fact that your brain uses up about 20 percent of your body’s energy. How, then, does its waste — like beta-amyloid, a protein associated with Alzheimer’s disease — get cleared? What happens to all the wrappers and leftovers that litter the room after any mental workout?…