For many of us, the worst moment of every day goes something like this…

That noise marks our daily return from the mysterious world that we call sleep. We spend a third of our lives asleep, yet, other than the odd snapshot of a dream here and there, most of us have no idea what happens after we close our eyes.

Luckily we're in good company, because there's also a lot scientists don't know about sleep too. For a long time, it was just something that happened, everyone assumed that our brains were hitting the reset button and just turning off for a while. But in the past few decades, it's become clear that sleep might be the single most important behavior that humans and other animals experience.

It might seem like we don't do much while we're asleep, but neuroscience tells a different story. Human sleep patterns are controlled by two competing networks of chemical and electrical signals in the brain.

During our waking hours, neurotransmitters released deep within our brain keep our cerebral cortex alert and primed for consciousness. But throughout the day, as our neurons break down ATP for energy, the byproduct adenosine builds up and activates sleep control neurons near the hypothalamus.

A special region in the center of our brain acts as our master biological clock. Light-sensitive cells in our retinas feed signals deep into that brain region, training neurons to sync up with Earth's 24-hour cycle of day and night. These circadian rhythms are the control switch that tells us when to feel sleepy or awake.

As the world goes dark, this master switch tells our pineal gland to increase levels of the hormone melatonin in the bloodstream, sort of like a chemical lullaby. Feelings of fatigue set in, body temperature lowers slightly, that heat loss is actually why many of us like to fall asleep with our feet sticking out of the covers, true story! Together all this neurochemistry sends one clear message to our bodies: when it's dark, it's time to go to bed.

Unfortunately, in modern times, darkness is increasingly rare. In the United States, 99 percent of people live in areas that meet standards for light pollution, and we've got one person to thank for that: Thomas Alva Edison.

Edison thought sleep was lazy, unhealthy, or inefficient, even though he took several naps a day. But despite that hypocrisy, his work more/sleep less view changed our world forever. Illuminating the night became a sign of economic progress, and humankind was no longer at the mercy of nature's clock. Or so we thought.

Artificial light can have serious effects on our sleep cycle. When we're exposed to bright light at night, our brain doesn't know better than to think the sun is shining. This can be very confusing, preventing the release of melatonin and the onset of sleep. Depression, heart disease, diabetes, and cancer have all been linked to chronic overexposure to artificial light.

Until just a couple of hundred years ago, it was common for people to fall asleep right after the sun went down, snooze for a while, wake up around midnight, where they would read or study or do other stuff, then go back to sleep until morning.

Modern experiments have suggested that if people are kept away from artificial light, their bodies will return to this pattern of first and second sleep, yet most of us insist on sleeping the whole night through. What's worse, our circadian rhythms are so tuned to day and night that if we stay up past our usual bedtime, we don't wake up later, we just tend to sleep less.

As a result, we're massively sleep-deprived. Most adults average just six and a half hours a night. Teenagers average just five hours on school nights, which is half of what they need. To fight this chronic exhaustion, we turn to stimulants like caffeine to help our brains ignore that buildup of adenosine, and then to fight the stimulants, many people rely on alcohol, which just sedates us, it doesn't even help with real, restful sleep.

This vicious cycle is worth literally billions of dollars a year. It's kinda messed up.

So what is sleep for? In short, we're not really sure, but we know it's essential to life. Animals deprived of sleep for a long enough time will have seizures and can literally die from exhaustion, plus a whopping 15% of our genes are linked to circadian rhythms. Still, there's no consensus on exactly why our bodies need sleep.