According to a study made by the researchers at the University of California-Berkeley and Harvard Medical School, the lack of sleep causes the activity of the brain’s emotional centers become more reactive. The study was the first one to use MRI technology and was published in the journal Current Biology. It was made to show what parts of the brain are affected by the lack of sleep.

The research was made on a group of 26 people ages 18 to 30 who were divided into two groups, one that slept normally and the other one that was deprived of sleep for 35 hours.

The researchers scanned the activity of the brain using functional magnetic resonance imaging. At first they were neutral and gradually became negative and disturbing.

The group that slept responded normally, while the group that was deprived of sleep overreacted. The emotional centers of the brain became more than 60 percent more reactive to the negative stimuli. The activity in the part of the brain that puts the body in alert registered a high activity, and the activity of the prefrontal cortex, which controls logical reasoning, registered a low activity.

The result means that the people deprived of sleep overreact at emotional challenges, challenges that they would normally be able to cope with.

In the future the research can concentrate on the components of the sleep that restore emotional stability.

The fact is that the lack of sleep can trigger psychiatric disorders such as depression.

According to a 2005 poll conducted by the National Sleep foundation the Americans are the most sleep deprived people in the world, with about 40 percent of them getting less than seven hours of sleep a night, and 75 percent having some sleep disorder.

"It's almost as though, without sleep, the brain had reverted back to more primitive patterns of activity, in that it was unable to put emotional experiences into context and produce controlled, appropriate responses," said Matthew Walker, director of UC Berkeley's Sleep and Neuroimaging Laboratory and senior author of the study.

"Emotionally, you're not on a level playing field," Walker added.