You awake with a pounding heart and clammy hands. Relax, you think to yourself—it was just a bad dream. But are nightmares truly benign? Psychologists aren’t so sure. Although some continue to believe nightmares reduce psychological tensions by letting the brain act out its fears, recent research suggests that nocturnal torments are more likely to increase anxiety in waking life.

In one study Australian researchers asked 624 high school students about their lives and nightmares during the past year and assessed their stress levels. It is well known that stressful experiences cause nightmares, but if night­mares serve to diffuse that tension, troubled sleepers should have an easier time coping with emotional ordeals. The study, published in the journal Dreaming, did not bear out that hypothesis: not only did nightmares not stave off anxiety, but people who reported being distressed about their dreams were even more likely to suffer from general anxiety than those who experienced an upsetting event such as the divorce of their parents.

It is possible, however, that some-thing is going wrong in the brains of individuals who experience a lot of anxiety, so that normal emotional processing during dreaming fails, says Tore Nielsen, director of the Dream and Nightmare Laboratory at Sacred Heart Hospital in Montreal.

But Nielsen’s most recent results, published in the Journal of Sleep Research last June, actually bolster the Australian findings. To tease out how REM sleep—during which most dreaming takes place—affects our emotions, the Canadian researchers showed disturbing images (such as gory scenes or a women being forced into a van at knifepoint) to a group of healthy volunteers just before they went to bed. When the subjects viewed the same pictures in the morning, those who had been deprived of dream-filled REM sleep were less emotionally affected than those deprived of other sleep phases. The same was true for those who experienced fewer negative emotions in their dreams. In other words, having nightmares did not make dreamers more resilient in waking life—just the opposite.

What is not clear from these studies is whether nightmares play a causal role in anxiety or are merely an expression of an underlying problem. Most re­searchers agree that having an occa­sional night­mare is normal and not problematic. But if the dreams give rise to persistent anxiety and concern, something more serious could be going on—and it may be a good idea to talk to a mental health professional about it.