Why Humans Aren't Naturally Nocturnal

Our bodies depend on light and dark for optimal sleep patterns. Here's why sleeping with the lights out is more important that you think.

It's another one of those questions that gets more interesting the more you think about it: Why do we sleep at night instead of during the day? Trace Dominguez, ironically, stays up at night thinking about this stuff -- and he's got your answers in today's DNews dispatch.

The short answer is that we evolved that way. Primates don't have an innate sleep cycle -- we can be either diurnal (day active) or nocturnal (night active), according to a 2008 study in the American Journal of Physical Anthropology. But the way our brains, bodies and eyes are wired suggests homo sapiens almost certainly evolved from a diurnal group of ancient primates.

The human circadian rhythm -- the 24-hour cycle that determines wakefulness and sleep -- is demonstrably geared toward sleeping in the dark. The brain adjusts the circadian rhythm according to how much light enters the eye. When it's dark, the brain floods the body with hormones that lower blood pressure, stress levels and body temperature -- and generally make us sleepy.

RELATED: Do Trees Sleep?

On the flip side, when the brain registers daylight breaking, other chemical switches are toggled to make us more alert and wakeful. In fact, any light coming through the eyelids tends to mess up the brain when it's trying to sleep.

Scientists have generated a ton of interesting research in this area. One intriguing study showed that hamsters developed signs of depression when forced to sleep with the equivalent of a glowing TV in their little hamster bedrooms. Look, these researchers have to spend that grant money on something.

We humans are clearly meant to be active during the day, and sleep during the night -- there's no real argument about that. But it's interesting to think that it didn't necessarily have to be this way, evolutionarily speaking. In fact, move over just a couple clicks on the taxonomy chart -- from primate to mammal -- and we're in a clear minority. Only around 20 percent of our fellow mammals sleep at night. Most are nocturnal, which suggests daytime sleeping held a significant evolutionary advantage.

What if humans had developed as nocturnal creatures? Seems like there's a good science fiction story in that. For one thing, we'd have to rethink the entire notion of the morning person.

-- Glenn McDonald

Learn More:

Sleep Foundation: Lights Out For A Good Night's Sleep

NCBI: Diurnality, Nocturnality, And The Evolution Of Primate Visual Systems

TIME: You Asked: What's The Best Bedtime?