Why Do We Have To Sleep?


[MUSIC] [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.