Microsleep: How Your Brain Can Fall Asleep for Seconds Without You Noticing

— -- If you're exhausted and in the need to zone out, you might actually be grabbing bits of down time called "microsleep."

The phenomenon can occur most readily when a person is sleep-deprived, when their brain is primed to switch into sleep mode at any second. While a person can appear awake and even have their eyes open, their brain may have slipped into sleep mode, maybe reaching the first or second level of sleep.

Dr. Ilene Rosen, a sleep expert and associate professor of clinical medicine at the University of Pennsylvania, explained if a person appears "zoned out" they might actually be catching some Z's.

"They might have been thinking about nothing," Rosen explained. "Something brings them back to focus attention."

Dan Childs, managing editor of ABC News' Medical Unit, stayed up for 50 hours straight this week -- 40 hours of it on the "Good Morning America" livestream, which was part of a slate of events to commemorate the 40th anniversary of "GMA."

During that period, Dr. Steven Feinsilver at Mount Sinai Hospital in New York, found evidence that Childs' was slipping in an out of consciousness after just a few minutes of sitting quietly. That becomes evident when a person is hooked up to an EEG scan to look at their brainwaves.

Certain brain waves called theta waves showed how quickly a person could be "asleep," Feinsilver said.

Microsleep episodes usually last a few seconds and the person often isn't even aware that the microsleep occurred. It can be dangerous especially if a person is driving, a few seconds of microsleep and a person can miss a red light or not notice a curve in the road, according to the National Institutes of Health.