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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.
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 sleep, 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.Together all of 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.Artificial light can have serious effects on our sleep cycle. When we’re exposed tobright 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 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. 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.