Researchers who study the brain know that it’s far from an immutable object. “It’s much more plastic than most people think,” said Giulio Tononi, a psychiatrist at the University of Wisconsin. “It’s changing all the time.”

One area of change is the synapses, the connections between neurons, which are altered as the brain receives stimuli. “What happens when you’re awake is you produce an overall strengthening of synapses,” Dr. Tononi said. “That’s good, because that’s how you learn.”

Image Credit... Chris Gash

But that is unsustainable in the long run, because stronger synapses require more energy and material, and there’s a limit to how much of both is available. “It’s as if in the morning you start with a V-6 engine,” he said, “and in the evening you find yourself idling, but you’re running a V-8.” Stronger synapses are also bigger, but the brain cannot grow bigger or denser. “If you strengthen synapses because you learn, soon you’d reach a point where you can’t learn further,” he added.