Last weekend my friend Kevin bagged going to see a late movie with a group of us because he’d promised a woman he’d arrive at her apartment at 10:30 to spend the night. Not for amorous pursuits, he insisted, but because she was terrified of being alone at night. He explained that she’d just moved to the ground floor of a new building after years of living in high-rises, and kept hearing noises in the alley that sent her imagination flying and made it impossible to sleep.

As a feminist, I found her fear irritatingly wimpy. But as a real-life human being, I couldn’t help identifying and believing that her stated reason for wanting him there was probably the truth. Having a fear of the night makes perfect biological sense. Humans are diurnal mammals. We are at our best by day, when our acute vision can detect potential enemies and supply our prodigious brains with the time and the information to plan an escape. But as twilight falls our advantages slowly ebb, and when night takes over we’re left muddled and afraid.

A couple of years ago I was camping alone in the wilds of Theodore Roosevelt National Park in western North Dakota. Bleak and dry, with misshapen rock outcroppings, the landscape’s strangeness was exacerbated by the dark. When I turned off my small flashlight and tried to sleep, the little bluestem grass scratched against the nylon wall of the tent, sounding as though someone or something was trying to get in. I had the clear sense that I absolutely did not belong there. By midnight I was so agitated I packed up my equipment in the dark and ran across the badlands for my car, which I drove to a motel on Interstate 94.

I’m embarrassed by this episode and always will be. At the same time, I think my unplanned evacuation was more natural than my decision to camp in such a place. I was at a distinct disadvantage in the dark. Human beings are inept without light to see by, but many of our natural predators are most comfortable, active, and successful in the dark. Take tigers, lions, and other felines. By day a cat’s vision isn’t as acute as ours. The physics of its lenses have forced a tradeoff, and in the daytime it can only focus on things close to it; resolution is comparatively poor. (If you know anything about photography you can think of feline eyes as being like a camera set at a low f-stop.)

But by night cats rule. Their pupils dilate dramatically, letting any available light flow freely into their eyes. And once the light enters the retina it falls on a great bounty of rods, the photoreceptor cells most efficient at processing images in low light. Cats possess another great trick: our rods also catch light entering our retinas, but in night denizens either the retina or the choroid (the layer right next to the retina) has a mirrorlike adaptation that bounces the light, giving the rods a second chance to absorb it. (It’s this mirror that causes animals’ eyes to illuminate when caught in car headlights.)

So our potential predators see us when we can’t see them. Our other senses are deficient too, intensifying the inequity. Our sense of smell is poor, making it of little use in alerting us to enemies. And our senses of touch and taste are completely useless, as far as I’m concerned–by the time a wild animal or psycho-killer got close enough for me to feel or lick it I’d be busy having a heart attack. The only sense we can rely on for early warning is our hearing. By day noises are put into a context with information gathered from our other senses, particularly vision, but when you hear a terrifying noise at night you have no way of determining its cause unless you can turn on a light. No wonder then that in the absence of any other sensory data every scraping branch or creaking stair conjures up the monster of the black lagoon coming to carry us away.

Human beings pass the terrors of night clustered together, sleeping in dens, huts, villages, and, today, cities. It used to be that no one slept alone in the wilderness unless she was in search of a spiritual vision or wanted to test her bravery. Neanderthals who lacked a healthy respect for nighttime and went out bravely wandering in the dark got killed and failed to reproduce. It’s the timid ones who survived to pass on their genes to us, their panic-stricken, angst-ridden, night-sweating descendants. Given the biological disadvantage we suffer by night, it seems safe to conclude that Kevin’s friend and I felt nervous because we’re evolutionarily justified. It was dark, we were each alone in a strange place with different noises that we couldn’t identify, and every instinct within us wanted to keep us awake and alive to see the morning.

In the modern world the sense of dread is less rational of course, since neither of us was likely to be devoured by a hungry wild beast. But a few generations are insufficient for evolution to catch up with changed circumstance.

Since the ancient reason for our fear has vanished but the fear itself hasn’t, we’ve mostly transferred it to being afraid of criminals. Which is funny in a way, since felons are the same species and just as disabled in low light as their potential victims are. If you were in a city park at night you would probably be most panicked in an unlighted section thick with trees–but then so are muggers. And even if they aren’t afraid, the lack of light makes it difficult for them to assess or even find their potential victims. In 1994 Chicago police officers told a U.S. Forest Service researcher studying crime in Lincoln Park that the vast majority occurred within 100 feet of lighted parking lots.

I keep thinking of the final part of The Silence of the Lambs, where Jodie Foster tries to capture the serial killer in a horrid pitch-black cellar. The detail that made the scene so memorable was that the psychopath puts on infrared night-vision glasses that enable him to watch her when she can’t see him. Ordinarily the two humans would be equally blind in the dark, but in this scene the killer has an unsettling advantage–just as if he were a tiger and Jodie a lost villager.

So when you lie awake afraid at night after hearing a thump, telling yourself that what you fear is crack-addict burglars, you can examine the emotion–and probably decide that the level of dread is crazily out of proportion. Still, you can’t stop the asinine terror that flows in thick, primal channels. It’s there to help keep you alive–and isn’t likely to be diluted or tamed by any amount of midnight reasoning.