(Intro)

H: Sleep.  We spend a third of our lives doing it, and we literally can't live without it.  It's a complex phenomenon and it still isn't well understood.  After more than 100 years of research, we still don't really know why we sleep, but over the years, scientists have made plenty of fascinating discoveries, like that sleep is important for memory and that not sleeping can make pain worse and make it harder to heal.  So here are nine discoveries that changed the way biologists think about sleep.

1) Humans willingly delay sleep all the time.  We stay up late for parties and we pull all-nighters to study for tests, but going without sleep for too long will actually kill you, and we know this partially because of one particular Italian family.  In the 1800s, a Venetian man named Giacomo fell mysteriously ill.  He suffered from dementia and an inability to sleep, and then he died.  He passed on his condition to his descendants, and the disease still runs in the family today.  

When scientists were finally able to identify the cause in the 1970s, they uncovered an extremely rare, incurable genetic disorder that they called fatal familial insomnia.  Its main symptom is a progressive insomnia which causes hallucinations, delirium, and is ultimately fatal.  Even though it is super rare, we know which genetic mutation causes the disease.  Researchers think this mutation causes damage to a brain region called the thalamus, which is known to regulate sleep and consciousness.  The progressive inability to sleep then leads to death, because the body is never able to rest.  With metabolism in high gear all the time, organs start to fail.  It's possible that patients with FFI die from more than just sleep deprivation, but studies done on rats in the 1980s confirmed that sleep deprivation on its own is fatal.  Rats who were kept from sleeping developed all kinds of metabolic problems, like weight loss, increase metabolism, and fluctuating body temperatures, and within 2-3 weeks, all of the sleep deprived rats were dead.  

Researchers still aren't sure quite why we sleep, but we do know that not sleeping is deadly.

2) We also know that it helps you consolidate memories.  That might seem obvious.  Who hasn't had a teacher remind them to get a good night's sleep before the big test, but biologists didn't realize that sleep was critical for helping store memories until about 100 years ago.  In the 1920s, a group of researchers taught subjects a list of nonsense words.  People who got a good night's rest were better at remembering the list than people who didn't get to sleep between learning the list and taking the test.  This research sparked a wave of studies that found strong connection between certain kinds of memory and sleeping.

See, there are different kinds of memory which rely on different brain circuits.  Declarative memories are memories of events and facts and they rely on an area of the brain known as the hippocampus.  They're separate from non-declarative memory, the kind of memory that builds habits and motor skills like playing the piano, and it turns out that sleep is especially important for helping the brain retain declarative memories.  So the teachers who tell you to sleep the night before the test are right.  It's really important for remembering facts and figures.  Other studies have discovered that the type of sleep that's important for this process is what's called non-REM sleep.  It's the non-Rapid Eye Movement sleep.  Even though you might relive your classes in your dreams, it's actually when you're not dreaming that your brain is hard at work solidifying those memories.

3) Your brain is super active when you're sleeping, which also might seem like old news, but biologists didn't know that until the 1920s either when they stuck electrodes on peoples' scalps to noninvasively record brain activity.  Since it feels like your mind is shutting down when you're sleeping, scientists had figured that the brain basically goes silent, but once they started actually monitoring brain activity, they learned that that is not what's happening at all.  They used a new technique called the EEG, which records the electrical signals that come from neuron activity in the brain.  When they recorded peoples' brains while they slept, they discovered that instead of silence, the sleeping brain is dominated by big slow waves of activity.  More research has picked apart how different stages of sleep are associated with different wave patterns and helped clarify the difference between REM sleep and non-REM sleep.

The idea that the brain is active during sleep and that different sleep states correspond to different kinds of wave patterns changed the way biologists thought about sleep and helped drive lots of future sleep research.

4) Around the same time, scientists were discovering sleep waves.  A doctor named Constantin von Economo was treating patients stricken with encephalitis lethargica.  Symptoms of the disease include brain inflammation, lethargy, and in extreme cases, an irreversible coma-like state.  von Economo connected the dots between the location of the brain inflammation and the sleep problems seen with the disease and realized that particular brain regions might be involved in sleeping and waking.  He correctly proposed that the brain stem is the center of the arousal system and sends the waking signal to the forebrain.  He also guessed that the hypothalamus is an important sleep center that helps regulate the transition between being awake and asleep.  Knowing how the brain switches between sleeping and waking is important for lots of areas of research like treating narcolepsy and other sleep disorders and von Economos' careful study of this disorder and his hypothesis about how the brain might control sleep laid the groundwork for future generations of scientists to better examine the circuits and signals that control sleep.  

5) Doctors have known for a while that insomnia and chronic pain go hand in hand.  Obviously, being in pain all the time would make it pretty hard to sleep, right?  But it turns out that sleep deprivation actually increases pain sensitivity, so if you stub your toe while sleeplessly pacing the room in the middle of the night, it's gonna hurt more than normal.  In the 1970s, a group of researchers discovered that if you kept rats from getting any REM sleep, they were more sensitive to painful shocks than rats who were allowed to sleep normally.  Aroun the same time, another gorup of scientists found that patients who were sleep deprived developed muscle soreness and a study in the mid-2000s found that patients who self-reported symptoms of insomnia were more sensitive to pain, too.  So patients dealing with chronic pain might have it even worse if they're struggling with insomnia.

There's evidence that sleep and pain systems use some of the same brain circuitry, so if something goes wrong with one system, it could be going wrong with the other.  The discovery of this connection was really important for understanding and treating chronic pain conditions like fibromyalgia, which can be debilitating and have big impacts on quality of life.

6) Being sick is exhausting, but why is that?  Well, it turns out that sleep isn't just necessary for our survival, it's important for proper healing too.  In the late 1990s, researchers found that when rats were sleep deprived, their immune systems were hyperactivated, meaning that they produced lots of inflammatory molecules in their blood.  An overactive immune system can lead to all kinds of problems like chronic inflammation and autoimmune disorders where a person's immune system begins to attack their own body.  More recently, in the early 2000s, scientists found that in rats, sleep deprivation makes it harder for burn wounds to heal.  All of this evidence has led scientists to believe that our immune systems need sleep to function properly and fight off illness.  So you should be getting plenty of rest if you're trying to kick that cold.

7) Technology has really exploded in the last couple of decades and it's happened so fast that we still really haven't figured out how our devices are affecting our health, but because of research that started in the 90s, scientists are starting to suspect that technology could be interfering with our sleep.  Blue light, which is the main wavelength of light emitted by devices like smartphones and laptops, has a really strong effect on the production of melatonin, a sleep hormone.  The effect is even stronger than natural light.  Messing with melatonin production can also mess with your sleeping patterns and ciradian rhythms, which we know can have all kinds of health effects.  Studies have found that the effect is measureable.  When people are exposed to blue light for a couple of hours before bedtime, it disrupts their sleep cycles and in 2014, a group of researchers found that when people reading before bed used ereader devices that emitted light instead of a paper book, they had a harder time sleeping and were less alert the next morning.  We still don't know what that kind of disruption could mean for your long-term health, but since not getting enough sleep seems to be a common theme here, we can probably guess that it's not good.  There are lots of apps out there which will shift the light in your laptop or phone to warmer tones which could help reduce some of the effects of watching all those cat videos in the middle of the night. 

8) One of the most exciting sleep discoveries in the last five years came from a study in 2013.  Researchers demonstrated in mice that during sleep, brain cells called astrocytes contract, expanding the space between all of the brain cells by almost 60%.  This allows more cerebrospinal fluid, or CSF, to flow through the cracks in the brain.  CSF is a clear fluid produced by the brain that fills and cushions the brain and spinal cord.  It's pretty similar to blood plasma and it plays important roles in maintaining the right chemical balance in the brain.  When CSF flows between brain cells during sleep, it also acts like a cleaning fluid, pushing out all of the debris that collects during the day.  Clearing out all these toxic waste products, like a type of protein called amyloid beta, may be really important to brain health, since too much debris is thought to cause diseases like Alzheimer's.  So a better understanding of what's basically your brain's dishwasher mode might help scientists come up with better treatments for neurodegenerative disorders.

9) If you had to guess how much our ancestors slept, you'd probably figure that they slept more than we do now.  After all, they had no electric lights to keep them active after dark.  It turns out that might not actually be true.  In 2015, researchers working with hunter-gatherer tribes published a study showing that they sleep less than their Westernized counterparts.  The study involved giving tribe members activity trackers to monitor their movement and sleep.  They found that the subjects slept for around 6 to 7 hours per night and that they moved around and woke up more than people in industrial societies do.  Our ancestors' lifestyles were similar to the hunter-gatherer lifestyles in a lot of ways, which has led scientists to believe that these results mean our ancestors probably slept about the same amount of time that we do now, and that their sleep wasn't always as restful.  So paleo sleeping probably would not be the healthiest trend.

All these discoveries have had big effects on how scientists understand and study sleep.  We know that not sleeping at all can be deadly and that sleep deprivation can lead to all kinds of health problems, but there's still a lot left to learn and maybe with future studies about sleep, scientists will finally put together all of the pieces of the puzzle and understand completely why we do it.

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