The result is a compromised ability to respond to things, along with a faulty tendency to do things you shouldn't have done. When the frontal lobes aren't working efficiently, people also have more difficulty paying attention during boring tasks, such as driving a car on a highway or operating a morning commuter train.
Early morning hours, like when the Metro-North train crashed, are some of the most vulnerable times for sleepy accidents, Howell said, especially for people whose circadian rhythms favor a later sleeping schedule and make it biologically difficult to function well after waking up with an alarm clock at 5 a.m.
Reports that Rockefeller had been driving for 20 minutes since his last stop and felt zoned out before the accident suggest that he probably fell asleep before the crash, Howell added.
Recently, scientists have begun to piece together an even more nuanced understanding of why sleep is so restorative. In a study published in Science this fall, Nedergaard and colleagues injected mice with a green dye that allowed them to track the movement of cerebrospinal fluid, the liquid that surrounds the brain.
As our brains do their work throughout the day, previous work had shown that cerebrospinal fluid collects the waste products of normal metabolism and functioning. Then, a network of tiny channels works like a dishwasher to regularly flush out the dirty fluid and send it to the liver for detoxification.
The new study found that sleeping facilitated the flushing of this toxic fluid, which was much slower to drain in sleep-deprived rodent brains. Nerve cells are very sensitive to the presence of waste, Nedergaard said. When surrounded by contaminated fluid, communication at the cellular level likely slows down.
"What we described is that this microscopic cleansing system turns on as soon as we fall asleep and washes the brain clean," Nedergaard said. "From our standpoint, when you're sleep deprived, you get a dirty brain."