[MUSIC]
For many of us, the worst moment of every day goes something like this…
[ALARM CLOCK NOISE]
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 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, 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 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.
I still love you though, coffee.
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.
We’re definitely less active at night, but considering we only burn about 100 fewer calories
while sleeping, it’s not a very good energy-saving strategy overall. We definitely do a lot of
cellular repair, protein synthesis, and general biological upkeep while we’re in dreamland,
but it’s not like we don’t do that stuff when we’re awake too.
Another theory says that our bodies use time asleep to flush out all the neurogarbage,
removing waste products that build up in our neurons and brain cells. And, decision-making
regions of our brain like the prefrontal cortex, well they don’t get any downtime while we’re
awake, like even if you’re totally relaxed and you think your mind is clear, your prefrontal
cortex is still prefrontal cortexing. Just try and think about nothing. Go ahead. See?
You’re thinking about not thinking. Sleep seems to be the only time for this region
to power down and get a break.
The greatest benefit of sleep may lie in processing information and consolidating memories from
throughout the day, letting the brain do all the rewiring that is necessary for thinkin’
better. Sleep deprived people do worse when learning new tasks and they're less able to
process new information, whereas a good night’s rest appears to make us more creative so we
can come up with solutions to new problems that we haven’t seen before.
Perhaps the biggest mystery is how sleep evolved in the first place. Snoozin’ animals are
easy targets for predators, so you’d think evolution would have come up with something
better. But it hasn’t. There’s no way to get around the need for sleep.
Some animals have come up with interesting ways to deal with the inconvenience of sleeping,
though. Dolphins obviously can’t nod off without drowning, so they only sleep with
one half of their brain at a time, swimming along using the half of their body that’s
still awake. Before baby dolphins learn that trick, they take adorable little dolphin naps
while their parents keep them afloat.
Sleep or similar patterns of rest are seen so universally throughout the animal kingdom
that they must have an ancient origin, and one clue comes from a tiny, ocean-dwelling
worm.
Every night, these worms swarm near the surface of the ocean to feed, and every day they sink
down deep to avoid light and predators. The worms have special daylight-sensing cells
on their back, just like the ones in our eyes. When it’s dark, those cells trigger the
production of melatonin, just like in our brains. As the melatonin builds up, tiny hairs
on their bodies stop beating and the worms begin to sink, just in time for the sun to
come up. As the melatonin disappears throughout the day, the hairs begin beating again and
they swim back up to the surface to do it all over again.
Sleep might have evolved 700 million years ago, the last time we shared a common ancestor
with that tiny worm. It's pretty important, so maybe we should all make a little more
time for it.
If you want to learn more about the science of sleep, one book that really helped me is
“Dreamland: Adventures in the Strange Science of Sleep” by David Randall. And also, be
sure to check out this half-hour playlist from our friends at The Good Stuff that digs
even deeper into what are brains are doing while we’re asleep, plus Craig goes to a
sleep lab to find out how to get a better night’s rest. Sleeping on the job, Craig,
real professional. Oh and over at BrainCraft, Vanessa has a video with some scientific tips
on how to beat jet lag. In fact, just make sure you’re subscribed
to The Good Stuff and BrainCraft, they are awesome. Links to all that down in the description.
Stay curious.