Past research by Walker and colleagues at Harvard Medical School, which was published in the journal Current Biology, found that in people who were sleep deprived, activity in the prefrontal lobe - a region of the brain involved in controlling emotion - was significantly diminished. He suggests that a similar response may be occurring in the nap-deprived volunteers, albeit to a lesser extent, and that it may have its roots in evolution.

REM sleep appears to not only improve our ability to identify positive emotions in others; it may also round out the sharp angles of our own emotional experiences. Walker suggests that one function of REM sleep - dreaming, in particular - is to allow the brain to sift through that day's events, process any negative emotion attached to them, then strip it away from the memories. He likens the process to applying a "nocturnal soothing balm." REM sleep, he says, "tries to ameliorate the sharp emotional chips and dents that life gives you along the way." '

"It's not that you've forgotten. You haven't," he says. "It's a memory of an emotional episode, but it's no longer emotional itself."

That palliative safety-valve quality of sleep may be hampered when we fail to reach REM sleep or when REM sleep is disrupted, Walker says. "If you don't let go of the emotion, what results is a constant state of anxiety," he says.