Darkness: Not Our Old Friend

Want your Halloween party to be really scary? Turn out the lights.

Researchers at the National Institute of Mental Health have found a powerful link between stress and anxiety, indicating that even moderate stress might make you freak out when something happens that's just a little spooky. Especially if it happens when it's dark.

Of course it's no secret that humans have an inherent fear of the dark. Scientists, and even us common folks, have known that for a long time. With apologies to Simon and Garfunkel, we know that darkness is really not our old friend.

Many years ago, my grandmother probably came closer to the truth when she jokingly warned me that men crave the darkness because their deeds are evil. So when Christian Grillon and colleagues at the National Institute of Mental Health decided to look into how stress affects anxiety, or fear, they added an interesting element to their project.

Twenty volunteers of sound body and mind were tested to see whether a little stress made them more likely to flinch at the sound of a loud noise when the lights were on or when the lights were off. As expected, stress increased their level of anxiety in both cases, but especially when they were tested in the dark.

"Our main objective was to examine whether stress increases unconditioned fear in humans," the researchers said in their study, to be published Nov. 15 in the journal Biological Psychiatry. They were especially interested in how something called the "startle response" was influenced by stress, particularly in the dark.

Interestingly, although rats are often used as a tool to study humans because they are similar in so many ways, rats and humans are opposites in how we react to daylight and darkness. Rats are more likely to have a startle response in daylight than at night, because rats are "a nocturnal species naturally afraid of brightly illuminated environments," the researchers say. If you want to scare a rat, shine a light on it.

Humans, on the other hand, are diurnal, meaning we spend most of our working time in the daylight, and thus are more averse to darkness. So if you want to scare a human, turn out the lights.

To test their assumptions, the researchers instructed their 20 volunteers to relax for a while, then prepare and deliver an eight-minute speech on abortion to two judges, one male and one female, in white lab coats, looking stern and intimidating, no doubt. The level of stress was measured in several ways, including blood pressure, the level of cortisol (a stress indicator) in saliva and perspiration. Those measurements were taken several times before and after the stress test.

Twenty-five minutes after the stress test, the participants were blasted with a loud noise, delivered through headphones, to measure their level of anxiety, or fear. As a control, however, sometimes the participants were tested for anxiety without the stress test. They were measured again for physical symptoms, as well as for how quickly and for how long they blinked. As expected, the reaction was more pronounced after stress, and it was greatest in the dark.

"To our knowledge, this is the first report showing that unconditioned anxiety is enhanced by prior stress," the researchers said.

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