If you're anything like me, you might still be cursing daylight savings time from a few weeks ago. Why is it so hard to make your body wake up an hour earlier, or adjust to an entirely different time zone? It turns out that you're not just lazy, your body has an internal clock that has its own plan in mind.

Maybe you're a world traveler and you think nothing of crossing half a dozen time zones in a couple hours, or maybe you work at night and it's finals week and sleep is less important than passing calculus, or maybe you just hosted a 40-hour live stream with your brother. Hey, it's your choice, but if taken to the extreme, living the night life can mess you up, leaving you exhausted and confused, and probably sick. That's because you, like most other inhabitants of the earth, have a special biological system that keeps your body in sync with the cycles of day and night.

It's your circadian rhythm, and unlike your alarm clock, it doesn't have a snooze button. Our internal timekeeping device is, logically enough, synchronized to the rising and setting of the sun, many of your body's systems are calibrated to the appearance and disappearance of natural light, and when we mess with that, things can get out of whack in a hurry. Circadian comes from the Latin "Circa Diem", or "approximately a day", and pretty much all living organisms down to algae and bacteria have their own circadian rhythms.

Whether you're an insect, bird, or a mammal, those rhythms affect the three big necessities: eating, sleeping, and mating. Everything from testosterone secretion to bowel movement suppression is controlled by this daily cycle. In humans, scientists are slowly beginning to understand how these natural oscillations take place every day.

What we're pretty sure about is that the main regulator of circadian rhythm can be found in the hypothalamus, a small area at the base of your brain that's responsible for connecting the nervous system to your endocrine system. Our biological clock is dictated by a group of nerve cells within the hypothalamus called the Suprachiasmatic Nucleus, or SCN. This is connected to our optic nerves, allowing the SCN cells to respond to light and dark.

So in the morning, when our optic nerves sense light, the SCN sends signals to raise our temperature, heart rate, blood pressure, and delay the release of hormones like melatonin. That helps us with the sleeping.

Researchers have found that as our body temperatures rise throughout the morning, our memory, alertness, and concentration also sharpen. So we tend to be at our cognitive best in the late morning. And that is generally followed by an afternoon lull.

In fact, while our desire to sleep is strongest from 2 a.m. to 4 a.m., a close second is between 2 p.m. and 3 p.m. That may sound counterintuitive, unless you have ever had a class at 2 p.m. in a nice, warm room and suddenly, that wooden desk starts to feel a lot like a featherbed. This suggests that napping is an important and natural part of our daily rhythms. So don't feel bad about it. 

No other species exhibits the same, once-a-day sleep pattern that most humans have become accustomed to. And there's growing evidence that mid-afternoon napping might be in all of our best interests. That afternoon lull is followed by another period of alertness, but in the evening, as the sun disappears, the SCN again picks up those signals of changing light from our eyes, organs shift into low gear, our body's temperature cools, and sleep-induced hormones are activated. 

Problem is that our daily schedules do not correlate with sunrise and sunset anymore. What with working and studying and late night redditing, we're constantly fighting that master clock that's been trying to keep us in sync since we were babies. 

Scientists have linked disruptions to our natural rhythms to health issues ranging from diabetes and obesity to depression and dementia. It's believed that up to fifteen percent of our genes may be regulated by circadian rhythms. 

So when you get off that next transatlantic flight or finish pulling that all-nighter, do yourself a favor: Listen to your body and take some more naps! I'm gonna go down for one right now, cuz we're approaching the 2 p.m. thing, so...

I'm not sure how this is gonna look with the green screen, you  guys, but I don't care. I'm sleepy!

Michael:
While Hank catches some quick Z's on the floor there, I'd like to bring up a question that's been bothering me. 

Why do we even need sleep? Some animals like dolphins can just turn off half their brain at a time, and people are known to pull all-nighters to squeeze in extra work or party time. So how important is sleep to humans, really?

Hank:
Even though the average person will spend 25 years of their life asleep, there's no scientific consensus as to why exactly we do it. Only thing we know for sure: our brains definitely think that sleep is important.

Deep in your hypothalamus, the tiny nut-sized region at the base of your brain, you have a little cluster of cells that acts like a timer, called the supra-chiasmatic nucleus. When you're exposed to light, this little cluster busily releases awake hormones, like cortisol, and suppresses the release of sleepy hormones, like melatonin. When it's dark, it does the opposite. 

A second trigger for sleep is believed to be the buildup of the compound adenosine in the brain. Adenosine is a byproduct of your neurons and other cells when they burn up adenosine triphosphate, the main molecule that our bodies use to store energy. Research suggests that when a bunch of leftover adenosine accumulates in your brain, you get sleepy. 

We talked about adenosine before, when we went into the science of caffeine, because caffeine works by bonding to the same receptors as adenosine, tricking the body into thinking it's not tired. 

But when you do sleep, those adenosine levels drop, as it's gradually reabsorbed by your neurons. This is partly what makes you feel rested when you wake up.

So, we sleep when our brains tell us to sleep, but that doesn't answer the larger question: why are we wired to sleep? It seems like a kind of terribly inconvenient thing to have to do. Also, super dangerous, if you're surrounded by jaguars or something. 

There are lots of theories out there, and it's unlikely that any of them alone is the single answer. Instead, they may all contribute to this weird urge that we have to lapse out of consciousness. 

For starters, all mammals and birds sleep, and other critters like reptiles, insects, and fish exhibit some kind of sleep-like behavior. That even includes the millimeter-long nematode worm, which experiences stress when denied rest. 

Some scientists suggest that inactivity at night is an evolutionary adaptation that boosts...

...an animal's survival rate by keeping it out of danger when it would be most vulnerable. Basically, sleep could be a way to keep still so you attract less attention. And yet, lions sleep a wopping fifteen hours a day, while Mr. Giraffe, arguably a tasty meal for said lion, gets less than two hours a day. 

So another theory is that sleeping might be a way to conserve energy. Much of life, at least in the wild, is about procuring calories to keep going. So going dormant for about a third of your day could be a smart move. Humans use about ten percent less energy when they're sleeping, as our breathing and heart rate and body temperature all take a dip.

But the broadest support out there for sleep theories is that it provides restoration. Sleep, after all, is when you grow muscle tissue. Your cells synthesize proteins, your tissues repair themselves, and growth hormones are released.

But surely we could take care of all of that without having to be unconscious, right? Like why can't our cells fix themselves while we're sitting on the couch watching Real Housewives of Milwaukee? 

Because our brains need sleep as much as our bodies do. Emerging research suggests that sleep allows the brain to rejuvenate, and maybe more importantly, reorganize. This theory is known as brain plasticity. 

We all do and see a lot of different stuff every day, and we'd probably like to remember most, but not necessarily all of it, when we wake up. Brain plasticity theorizes that sleep is when our brains replay and store the events of the day, providing eight hours or so for the processing and consolidating of new memories.

This theory has been supported by tests on human subjects. In one experiment, a set of volunteers memorized sequences of patterns in the morning, while a second set memorized them in the evening. The morning group had their memories tested twelve hours later without sleeping, and the evening group was tested twelve hours later, too, but after they slept. And the evening group proved better at recalling the patterns. 

It may actually be good advice to take a nap while you're stuck on your problems or sleep on a big decision that you have to make. Your brain may need that time to process everything you've observed. 

But if sleep helps reinforce memories, what about the stuff we'd like to forget. Like, I don't have any reason to remember the color of the car that cut me off this morning or the words to the radio commercial where the guy's singing about furniture. Luckily, sleep can help clear out all that excess junk from our brains. 

When you form memories during the day, your brain strengthens the synapses, or junctions, between neurons. Learning new things often causes neurons to create entirely new synapses.

By tracking the bursts of electrical activity that happen a thousand times each night among your billions of neurons, scientists have discovered that, during sleep, both high frequency and low frequency bursts increase, but moderate frequency bursts decrease. In other words, your brain is choosing to either rev up or calm down the firing between each of those synapses you made while you were awake, ultimately strengthening or weakening each connection. 

So, though it's a little sad when you think about it, when you wake up, the insignificant details about the previous day are probably lost forever. But keep in mind that, without this daily cleaning, your brain would face a major energy shortage and space crunch. In a way, this function of sleep is kind of like defragging a harddrive. 

Problem is: People, Americans in particular, don't get enough sleep. In a survey conducted by the National Sleep Foundation, Americans on average get over an hour less than the recommended eight hours a night. And teenagers need even more. 

In addition, nearly twenty percent of Americans report problems sleeping. More than 200,000 car accidents each year are caused by sleepy drivers, killing more people than drunk driving.

It doesn't take long for the brain and body to feel the effect of sleep-deprivation, and the problems compound with time. Just go one night without sleep, and your brain quickly starts trying to scramble, beginning with the amygdala, the part of the brain that tells the body to be prepared for danger. Short-term sleep-deprivation throws the amygala into overdrive, which in turn shuts down the prefrontal cortex. That's the part of the brain that controls logical reasoning, among other things. 

So a single all-nighter can put you in a state that one researcher has called "emotional Jell-o." Because when it bypasses the prefrontal cortex, the sleep-deprived brain connects instead to another part, that evolutionarily speaking, is one of the oldest and most primitive regions. It's called the locus coeruleus, or the blue spot of your brain, because for some reason, the tissue is actually blue. Its job is to make you respond instinctively to stress and panic. Only it can interpret pretty much anything as a threat:

 

a swerving car, a terse e-mail, an offhand remark by a boyfriend, leaving you anxious and suspicious of everyone and everything. 

And the longer you go without sleep, the worse things get. Memory and speech control are the next to suffer, but after several days, things really start to get weird. General paranoia can give way to increasingly vivid hallucinations. Some theorize that this is your brain actually forcing you into a waking sleep.

But the question is: can a lack of sleep actually kill you? And the answer is almost definitely yes. 

Sleep is closely tied to immune health. Studies have shown a fifty percent decrease in antibodies in test subjects who are only moderately sleep-deprived for one week, exposing them to a host of illnesses. And in a famous sleep study from the 1980s conducted by sleep researcher Allan Rechtschaffen, a group of rats deprived of sleep all died within two weeks. The cause, as far as Rechtschaffen could tell, was simply exhaustion. Nothing physically was wrong with them. A follow-up experiment conducted in 2002 similarly failed to find an unambiguous cause of death. 

So probably you wanna know how long can you possibly go without sleep. Well, the longest documented case of a person voluntarily staying awake is 264 hours, or roughly eleven days.

It happened in 1965 when 17-year-old Randy Gardner set the record as part of his science fair project. Gardner emerged relatively unscathed physically, most likely a result of his age, but he was described as being "cognitively dysfunctional" at the end. While awake, he experienced blurred vision and involuntary eye movements, and his hallucinations included seeing fog around street lights, feeling a band of an imaginary hat, and believing he was a running back for the San Diego Chargers. 

The lesson here, I think, is don't try to beat Randy Gardner's record at your next school science fair. There's really nothing good that can come of it. 

Michael:
Okay, so sleep isn't going away any time soon. Feel free to take that post-feast nap now. Or after this episode. 

But sometimes, falling asleep is easier than it sounds. If it's so important, why can it feel almost impossible?

Michael from the past:

As many as 60 million Americans suffer from insomnia, so it's not much of a surprise that when Google sent us their list of most-asked questions in the English-speaking world, "how can I fall asleep?" was pretty close to the top. 

Sixty million people. That's weird, since, you know, it's literally impossible not to sleep. And if you're one of those people who's up in the middle of the night googling sleep solutions, it might help you to learn a little about the science of sleep. 

The first thing you should know if you're having a hard time getting some shut-eye is that you're wired to sleep regular hours, going to bed the same time each night and walking up the same time each morning. Having a regular wake-up time seems to correlate pretty highly with the ability to fall asleep consistently.

This is because it keeps you aligned with what's called your circadian rhythm, your body's natural tendency to stay in sync with the cycles of day and night. And you know what controls your body's circadian rhythm more than anything else? Light. 

A lot of the help you get falling asleep comes from hormones. They lower your heart rate and reduce your blood pressure and basically let you relax. The key player here is the hormone melatonin, and it's regulated by your exposure to light. In darkness, it flows freely. But when you're exposed to light, whether natural or artificial, the release of melatonin stops. 

So you know what that means? No phones or laptops in bed! The light emitted by electronics confuses your body into not knowing that it's time to sleep. So scientists suggest at least an hour of screen-free time before bed, though I am completely incapable of that myself. 

Another obvious enemy of sleep: caffeine! Even though you think that cup of coffee after dinner might only affect you for an hour or so, studies have shown that caffeine consumption as much as twelve hours before bedtime is linked with insomnia. 

And even the way you think about sleep can affect your sleep patterns. Worrying about not getting enough sleep is a common enough cause of insomnia that it has its own name: sleep onset insomnia. 

But you know what's really weird? A lot of the time when we feel like we can't sleep, we actually are sleeping. When scientists rouse patients in the first or second stages of sleep, more than sixty percent of them say that they weren't sleeping. Even though they were. 

Now, of course, there's a whole class of medications that will help you sleep, from antihistamines to pharmaceuticals known as hypnotics, which include Ambien and Lunesta. 

But research has shown that, while patients can fall asleep fast on hypnotics, the effect is small, only adding about fifteen minutes to their sleep times.

Other studies indicate that our minds are significantly more powerful than any medications. In double-blind studies, patients who were simply told that they were taking a sleep drug ended up sleeping far better than patients who were told they weren't. So if you want to know how to sleep, the answer is right there in your head. 

Now, as part of our work answering the world's most-asked questions, we asked you, our scishow viewers, a few questions, and one was how many hours per night you sleep. And it's bad news. Only ten percent of you are sleeping more than eight hours per night. And eight and a half is the doctor recommended amount. And over half of you report having trouble getting to sleep at least once per week. 

And now it's time for Meaningless Correlations!

The best sleepers, for countries where we had enough data to make any sort of judgment, were Saudi Arabians, with 76 percent reporting that they experience insomnia infrequently or never. Most of Europe scored better than average, with the Netherlands, Russia, and Spain all sleeping relatively soundly. The English speakers in the U.S., UK, and Australia all had some of the worst scores.

And finally, unsurprisingly, our staggeringly unscientific survey reports that people who commonly drink coffee, soda, energy drinks, or tea are all more likely to suffer from insomnia. 

Michael:
Oh, right, don't look at your screens before bed. That's doable. 

Sometimes, though, sleep problems aren't all in our heads, or entirely in our hands. Insomnia is complicated, and it's not the only thing that can go wrong while you slumber. 

Hank:
Say you've been nappin', like between classes or after a long night out or, I don't know, after you're broadcasting on YouTube for 48 straight hours to raise money for charity. Now imagine you're waking up, and suddenly, you discover that you can't move. 

You want to speak, but you can't. Your mind is acutely aware of what's happening but you are powerless to get your body to do anything. It may last a few seconds, it may last a few minutes. In rare cases, it can last more than an hour. 

It's called sleep paralysis. And you might not have to imagine it, because up to 40 percent of us have experienced this sleep disorder at some point in our lives. 

I am one of them.  

We don't like to think about the bad things that can go on while we're in dreamland, just as we hate the disorders that keep us from even falling asleep. Hello, insomnia! 

But even though we've talked a lot on this show about the science of sleep--why we need it, why we dream, and where dreams come from--there is a whole other polymorphously messed up realm of human biology that explains what happens when sleep turns against us. 

We can't turn our brains off. We forget to breathe. We have waking hallucinations. Some of us even walk, eat, run, and have entire conversations when we're asleep. 

The half-asleep brain is a crazy place, and once you understand it, you may never see the back of your eyelids the same way again.

When most people think of the things that cramp our sleep style, they think "insomnia." But defining, diagnosing, and treating this most common sleep disorder can be tricky. In fact, for a long time, most scientists considered insomnia to be a symptom of another problem, like depression, anxiety, asthma, stress, substance abuse, a traumatic injury, even jet lag. Though today, insomnia is considered by many to be a chronic disease of its own that interacts with other medical conditions. 

So if you've ever had prolonged trouble falling asleep or staying asleep but you don't have any other health issues, then doctors would probably say that you have primary insomnia. If you do have something else going on, like a physical or psychological condition, then you've got secondary insomnia. And most cases of secondary insomnia are chronic, meaning it lasts for more than a month. 

There are also cases of acute, or short-term, insomnia, which is usually triggered by stress or some specific life event. Whatever the cause, scientists believe these insomnias are the result of the simple but eternal struggle between arousal and sleepiness. 

More and more research is suggesting that a condition called hyper-arousal, where the nervous system remains in a constant state of alert, may be the main reason for chronic insomnia. Hyper-arousal is basically a prolonged version of your body's fight-or-flight response.

You know that feeling. Your stress hormones are amping up. Your heart and respiration rates increase. 

For most people, this response disappears pretty quickly. But for many insomniacs, it can last all day into the night, and that makes it difficult for the body to relax, obviously. 

But what's there to be afraid of once you have dozed off?

Well, sleep apnea is one of the more serious disorders. 

Simply put, it's when your breathing is interrupted while you're sleeping. Apnea comes from the Greek word for "want of breath." For some people, it happens once or twice per night. In serious cases, it can happen more than a hundred times per hour. 

Though relatively common--more than 12 million Americans have it--the most severe forms of sleep apnea can lead to heart failure and stroke. It's also been linked to diabetes. 

One of the more troubling aspects of sleep apnea is that the victims don't always realize they have it. The most common form, obstructive sleep apnea, happens when the muscles at the base of the tongue and the uvula, that fleshy piece of tissue at the back of the throat, collapse. If this collapse blocks the airway only partially, then you get snoring as air tries to squeeze through. But apnea occurs when the airway is completely blocked. When this happens, blood oxygen levels drop fast, which in turn tells the brain that breathing has stopped and that those muscles have to reopen the airway posthaste!

But this whole conversation between your body parts can last from a few seconds to a few minutes, usually without the person ever waking up. Needless to say, though, this does not make for very restful sleep.

And there are all kinds of treatments for this kind of apnea, but most people end up using a continuous positive airway pressure, or CPAP, machine. It has a mask that fits over the mouth and nose and gently blows air into the throat, helping to keep the airway open, which doesn't sound very restful, either. 

Another type of disorder, central sleep apnea, is much more rare. Instead of a physical blockage in your throat that prevents breathing, this is an interruption of the brain signals that tell the body to breathe. 

Until recently, there were few effective treatments, but scientists are experimenting with a new pacemaker-like device that sends an electrical impulse to stimulate the diaphragm to breathe during sleep. 

Okay, so forgetting to breathe is pretty bad, but sleep disorders get downright dangerous when it comes to sleepwalking. Partly because, though it affects about fifteen percent of people, it is more common in chidlren between the ages of 3 and 7. 

Sleepwalking, also known as somnambulism, almost always occurs during the first third of the night. That's the part of the sleep cycle we call non-rapid-eye-movement sleep, and it's generally a period of deep sleep characterized by slow brainwaves. While there's no consensus as to what causes this condition...

...there are a few theories as to why people begin walking, eating, running, even having conversations while the brain thinks it's asleep. 

Some scientists think it's caused by the brain trying to circumvent other stages of sleep and go directly from deep non-REM sleep to full-on wakefulness. Others point to a chemical messenger in our bodies known as GABA, which normally serves to slow down activity in the brain's motor system. It turns out that children lack the fully developed neurons that release GABA, so without a full network of connections in the brain to keep motor activity under control, it may be easier for young brains to instruct the body to move around while it's asleep.

Which is crazy, because have you seen how much children move around when they're awake?

Sometimes the causes of sleepwalking are external and pretty controllable, like sleep deprivation, stress, and drug use. 

But scientists have also discovered that some people are genetically predisposed to it. A study conducted on families with at least four generations of sleepwalkers have traced the condition to a defective section on chromosome 20. And a mother or father who carries this defect has a fifty percent chance of passing it on to their children. 

But this is not the case with sleep paralysis, which I experience sometimes. It's a little weird, sometimes even a little scary, though I know it's temporary. But why does this happen? Why does my brain and my body decide to freak me out first thing in the morning?

In some cases, sleep paralysis is a symptom of narcolepsy, a disorder that causes people to have an overwhelming need to sleep. But it also regularly happens to non-narcoleptics, like me. The weird feeling of being conscious but unable to move generally occurs as you're falling asleep, but it can also strike you as you're waking up. 

Scientists believe that a main cause is entering REM sleep soon after lying down, bypassing the stages of non-REM sleep that normally happens first. Typically, REM dominates the later stages of sleep, which are also when the most vivid dreams occur. During this deep sleep, your muscles are essentially paralyzed, most likely, to prevent you from acting out whatever crazy, space-unicorn-riding dream you're experiencing. 

Sleep paralysis occurs when you wake up in the middle of all of this, and suddenly you're aware of the fact that moving is impossible. 

Recent studies have also found that the same neurotransmitters that affect sleepwalking may also cause sleep paralysis. 

University of Toronto scientists discovered that both GABA and glycine switched off the brain cells that allow muscles to be active during REM sleep, at least in lab rats. 

Now, in the most severe cases of sleep paralysis, victims also experience frightening and bizarre hallucinations. They can be visual or auditory and often involve shadows or hearing voices or footsteps, or seeing images of humans and animal figures. Occasionally, victims will also feel pressure on their chest and have trouble breathing. Scientists have described this phenomenon as "dreaming while awake." 

Me? I prefer to do my dreaming while asleep, thank you very much. 

And hopefully, I didn't make you afraid to go to bed tonight. 

Michael:
Okay, Hank keeps talking about those 48-hour livestreams, which seems like a tease. If you want to join him this year, Project for Awesome is starting Friday, Devember 7th. And you can totally sleep when you need to. Just not too much. 

Yep! I did say too much. Because it turns out, getting too much sleep is also bad for you. 

Olivia:
When people talk about having too much of a good thing, they usually mean over-indulging in something like cake or fast food--things you probably enjoy even if they aren't very good for you. 

But what about something that's objectively important to your health and wellbeing? Like sleep.

Well, it turns out that you can get too much of that, too. Most people need seven to nine hours of sleep, and oversleeping is connected to health problems like depression, heart disease, and diabetes. 

A 2014 study of 894 pairs of twins, for example, showed that the genetic risk of depression was higher in subjects who got less than seven hours of sleep or nine or more hours of sleep every night. Meaning: people who overslept or underslept were more likely to be depressed because of genetic factors, as opposed to environmental ones. 

And according to a study on the sleep habits of 400,000 Taiwanese adults, the risk of coronary heart disease is about the same in people who sleep less than four hours a night as it is in those who sleep more than eight hours a night. Subjects who underslept had a 35 percent higher risk of heart disease, and people who overslept had a 34 percent increase. 

Another study, published in 2009, followed 276 subjects for six years, and found that people who slept either less than seven hours or more than eight hours were at least twice as likely to develop Type 2 diabetes or trouble tolerating glucose. 

And there's more! A 2013 study of about 54,000 adults over the age of 44 found links between too much sleep and increased rates of heart disease, diabetes, obesity, stroke, and mental health issues. In fact, the rates of coronary heart disease, diabetes, and stroke were even higher in people who overslept than in those who slept too little. 

So, the links are there. Sleep correlates with all kinds of health problems. But it's hard to say whether too much sleep actually causes these issues. It's totally possible that oversleeping is actually a symptom of things like depression or heart disease. Or that there's some other connection. Either way, consistently sleeping too much might be a bad sign.

Michael:
I guess there can be too much of a good thing. I'm looking at you, pumpkin pie.

But it's definitely the season to cozy up and start those good sleeping habits. If you're not tired of hearing about your brain and sleep, check out our episode on SciShow Psych, where Brit tells you what you can learn from your dreams. 

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