You know the feeling. You're trying to get through the day, utterly exhausted after a short or fitful night's sleep, and you can barely keep your eyes open. You zone out for a few seconds, losing track of what you're doing until you can rouse yourself into focusing again.

But the truth is you may not have been zoning out. You may have been grabbing a few short seconds of sleep.

Sometimes, when you're totally exhausted, parts of your brain are struggling between consciousness and sleep.

"The physiology of this phenomenon is poorly understood," Medscape says, "but it is hypothesized that the brain takes a 'time out' by refusing further sensory input in order to reorganize excess information in short-term memory."

Microsleeping lasts just a few seconds as your exhaustion wins out and your brain is forced into an incredibly brief nap. Yet you don't recall dozing off, just a little fuzziness.

"Sleep has to last beyond a minute or two for your brain to remember it," Jim Horne, director of Loughborough University's Sleep Research Centres, tells the BBC. "With micro-sleep, you are just left with a feeling of not knowing if you are coming or going."

Microsleeping in the lab

To see what happens when a sleep-deprived brain battles to stay alert, researchers in Singapore kept volunteers awake in a lab for 22 hours. Then they were placed in a dark fMRI machine and told to keep their eyes open. When they closed their eyes (which they couldn't help doing) a recorded voice told them to "Please keep your eyes open."

Each time the volunteers shut their eyes, their brains were scanned. The researchers compared those scans to scans of normal, well-rested people who just closed their eyes. The results were published in the journal NeuroImage.

The study found that during those tiny bits of microsleep, there was reduced activity in the thalamus, which is the part of the brain that regulates sleep. More surprising, there was increased activity in the parts of the brain associated with paying attention and sensory processing. This suggests a battle between the awake brain and the sleeping brain.

Microsleep behind the wheel

man yawning while driving In a recent study, nearly half of men in the U.K. admitted to driving while drowsy. (Photo: pathdoc/Shutterstock)

Taking microscopic naps may not be a big deal at your desk or while watching TV, but they can be horrific when you're driving. According to Medscape, microsleep is implicated in more than 1,500 road deaths in the U.S. each year.

According to the U.K. road safety charity, Brake, 45 percent of men and 22 percent of women admitted to microsleeping while driving, reports the BBC.

A small study in Germany found that driving performance and accidents increase as microsleeping episodes increase. Volunteers drove a simulator for 40-minute intervals between the hours of 1 a.m. and 7 a.m. without being allowed to take naps. As the night wore on, the drivers had more episodes of microsleep and were more likely to swerve out of their lane and had more "crashes."

Stopping microsleep

The best way to prevent microsleep is to make sure you get enough real sleep. The National Sleep Foundation says adults need 7 to 9 hours of sleep each night for overall health and well-being.

If you get a good night's sleep and still find yourself microsleeping, talk to your doctor. It may be a sign of an underlying medical condition.

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.