The researchers from Harvard Medical School and the University of California at Berkeley, while keeping volunteers awake for 35 hours, found huge increases in brain activity when these study subjects were shown images created to provoke emotional responses.
Using functional magnetic resonance imaging (fMRI) on the brains of the sleep-deprived volunteers, the scientists were able to look at blood flow in the brain and in real-time, which shows where the brain is most active.
As part of the study, the volunteers stayed awake for an extended period. They were then scanned while being shown anger- or fear-stimulating picture cards.
The researchers led by Matthew Walker found that the parts of the brain linked to emotional reactions (the amygdala) showed more reactions (over 60% more) to the cards, compared with a normally-rested volunteer. The brain images depicted a sort of re-wiring of brain activity, from the calming and rational prefrontal cortex to the "fear center" ; the amygdala. The subjects were noticed to react more emotionally to the pictures than the well-rested control group.
Says Walker:" The size of the increase really surprised us.
"It is almost as though, without sleep, the brain reverts back to a more primitive pattern of activity, becoming unable to put emotional experiences into context and produce controlled, appropriate responses."
Walker added: "We found a strong overreaction from the emotional centers of the brain. It was almost as if the brain had been rewired, and connected to the fright, flight or fight area in the brain stem."
Walker suggested that the findings might provide clues to the connection between sleep and psychiatric disorders.
Meanwhile in UK, Professor Jim Horne (Sleep Research Center, Loughborough University) welcomed the research, while commenting it would be difficult to use it to unravel the relationship between mental health and sleep.
"This is a complex area - the big difference is that people with mental illness might not be aware that they are over-reacting or behaving irrationally, whereas someone with sleep deprivation would be more aware of this overreaction", he said.
Professor Derk-Jan Dijk, a sleep researcher from the University of Surrey, describes the findings as "interesting".
"While there have been a lot of studies into the effects of sleep deprivation, this is the first to show what is happening in the brain in response to these emotional stimuli", he says.