The findings offer a compelling explanation for why people with post-traumatic stress disorder (PTSD), such as war veterans, have a hard time recovering from painful experiences and suffer recurring nightmares.

"The dream stage of sleep, based on its unique neurochemical composition, provides us with a form of overnight therapy, a soothing balm that removes the sharp edges from the prior day's emotional experiences," said Matthew Walker.

Walker is an associate professor of psychology and neuroscience at University of California, Berkeley, and study co-author, the journal Current Biology reports.

For people with PTSD, Walker said, this overnight therapy may not be working effectively, so when a "flashback is triggered by, say, a car backfiring, they relive the whole visceral experience once again because the emotion has not been properly stripped away from the memory during sleep".

The results offer some of the first insights into the emotional REM sleep, which typically takes up 20 percent of a healthy human's sleeping hours, according to a Berkeley statement.

"During REM sleep, memories are being reactivated, put in perspective and connected and integrated, but in a state where stress neurochemicals are beneficially suppressed," said Els van der Helm, doctoral student in psychology at Berkeley. He led the study.