As the patients slowed down, their brain cells did, too.
"The acute effects of a lack of sleep can also cause cognitive and behavioral lapses that contribute to accident-induced injury or death", the researchers write.
The UCLA team then asked the patients to categorise a variety of images as fast as possible while their brain activity was measured in real time.
"But it has been hard to determine precisely how sleep deprivation influences neural activity within the human brain owing to the invasive techniques required to record neural activity".
"We were fascinated to observe how sleep deprivation dampened brain cell activity", said Yuval Nir from Tel-Aviv University.
A study has found a lack of sleep makes us feel foggy for a reason: brain cells lose their ability to communicate with each other when we're exhausted. In total, the activity of nearly 1,500 brain cells was recorded across the 12 participants.
Most alarming about the research, however, is that brainwaves seemed to slow down, meaning certain parts of the brain were attempting to sleep - not something you want to happen while driving. Neurons fired more weakly, Fried said, and communications lagged.
The scans suggested a lack of sleep was interfering with the neurons' ability to translate what was being seen into coherent thoughts, in the same way that a exhausted driver takes a moment to react to a pedestrian stepping out into the road. Fried explained that for example if a person is driving a auto and another person jumps in front of it, a sleep deprived person would have a different response than one who has had adequate sleep. "It takes longer for his brain to register what he's perceiving", says Dr. Nir.
"These are the very neurons [that] are responsible for the way you process the world in front of you", Fried says.
This examination will also help the researchers to understand why seizures develop on sleep deprivation. Thus a drowsy driver could be slower to respond and hit the brakes. This could raise concerns over exhausted commuters driving into work after failing to get enough sleep, should someone cross the road in front of them.
The researchers also discovered that slower brain waves accompanied sluggish cellular activity in the temporal lobe and other parts of the brain. But, as Fried said in a university release, "no legal or medical standards exist for identifying overtired drivers on the road the same way we target drunk drivers". Fried says this suggests that certain regions of the brain were "dozing, causing mental lapses" while the rest was trying to stay awake and run as usual.
The research received support from the National Institute of Neurological Disorders and Stroke, the National Institute of Mental Health and other organizations.