Exhausted neurons respond more slowly than usual and take longer to transmit weaker signals, a study found.
Tired brain cells explain why a poor night's sleep is so likely to lead to memory lapses and poor concentration the next day, scientists believe.
Researchers tested 12 tired epileptic patients who had electrodes implanted into their brains to pinpoint the origin of their seizures.
Professor Itzhak Fried, from the University of California at Los Angeles, said: "We discovered that starving the body of sleep also robs neurons of the ability to function properly.
"This paves the way for cognitive lapses in how we perceive and react to the world around us.
"Inadequate sleep exerts a similar influence on our brain as drinking too much. Yet no legal or medical standards exist for identifying over-tired drivers on the road the same way we target drunk drivers."
The study participants were asked to stay awake all night to speed up the onset of an epileptic episode before undergoing surgery. Lack of sleep is known to trigger seizures in vulnerable individuals.
For the test, the patients had to categorise a variety of images as fast as possible while the implants recorded their brain activity.
Lack of sleep caused the neurons to respond to visual stimulus sluggishly, the scientists reported in the journal Nature Medicine. They also fired weakly and their transmissions dragged on longer than normal.
The same effects were likely to occur when a sleepy motorist notices a pedestrian stepping into the road, said the researchers.
Co-author Dr Yuval Nir, from Tel Aviv University in Israel, said: "The very act of seeing the pedestrian slows down in the driver's over-tired brain. It takes longer for his brain to register what he's perceiving."
The team also discovered "slow" brain waves similar to those that occur during sleep in tired regions of the brain. Brain waves are synchronised pulses of electrical activity generated by neurons.
"Slow sleep-like waves disrupted the patients' brain activity and performance of tasks," said Prof Fried. "This phenomenon suggests that select regions of the patients' brains were dozing, causing mental lapses, while the rest of the brain was awake and running as usual."
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