Can You Be Awake And Asleep At The Same Time?

"I was in a daze . I don't know what I was thinking about and the next thing I know I was hitting the brakes."

Those were William Rockefeller's words to a law enforcement source, the New York Daily News reports, as investigators examine the moments leading up to the Metro North train derailment in the Bronx on Sunday that killed four and injured 75.

Unlike previously reported, Rockefeller seems to have been well-rested, officials say. "There's every indication that he would have had time to get full restorative sleep ," Earl Weener of the National Transportation Safety Board told the Daily News.

While zoned out, it's been suggested that Rockefeller may have slipped into what's known as microsleep , when you nod off for just a few seconds, often without even knowing you're doing so. People in microsleep might even still have their eyes open, or still carry out tasks "as if on a kind of auto-pilot," ABC News reported.

That's because during a microsleep, parts of the brain remain alert and awake while others doze off, according to a 2011 study in rats. Specific nerve cells in the brain entered a sleep-like state , according to the study, "with negative consequences on performance."

When sleep doctors look at a brain at rest during a sleep study, they look at the activity in 30-second chunks, Dr. W. Christopher Winter, M.D., medical director at the Martha Jefferson Sleep Center in Charlottesville, Va., tells HuffPost Healthy Living. "Sleep scientists define sleep onset as the first time you have 15 seconds of sleep within a 30-second period," he says. "It's not that the lights just cut off, the light flickers and flickers and then it's off." During that flickering, we might have microsleeps of a few seconds, while during the larger 30-second period we are technically awake.

But we're generally not aware it's happening, he explains. Someone driving or doing another monotonous task may suddenly notice they weren't all there for a brief moment, a realization that's often startling, says Winter, who has not evaluated Rockefeller.

Microsleep is evidence of how powerful our urge to sleep is, Dr. Charles Czeisler, M.D., Ph.D. , director of the Division of Sleep Medicine at Harvard Medical School, told ABC News. "We often delude ourselves into thinking that we decide whether or not we're going to go to sleep," he said. "'I'm just going to go another 10 miles. It's only half an hour to my house.' When you build up enough sleep pressure, you automatically make that transition to go to sleep. It can happen in the blink of an eye."

Unfortunately, those few seconds of unconsciousness can be long enough to wreak havoc. A car traveling 55 miles per hour will cover the length of an entire football field in just 4.6 seconds , the average amount of time distracted drivers spend reading or sending texts, according to the National Highway Traffic Safety Administration. To be asleep for the same period of time -- or even less -- could spell disaster.

While it can happen to anyone, says Winter, it would be unusual for a well-rested person who just got behind the wheel to experience a microsleep. "The people who are most susceptible are people who are sleep deprived and people who are in a monotonous situation for longer periods of time," he says.