Tina: Hey Steph, why are you up?

Roomate: where are you up?

It's 1am Tina: I'm always up at this time.

It's when I'm most productive.

In the flow.

Roomate: I told you when we became roommates that I'm an early bird.

Tina: And I told you that I was a night owl Roomate: Why are you like this?

I don't get it.

We're pretty much the same person, but why are sleep cycles so drastically different?

Tina: Well, it has to do with— Roomate: Save it.

You can tell me the science in the morning.

I'm gonna go back to bed.

Tina: Technically!

Every 24 hours our bodies go through a cycle of activity and rest.

This cycle called our circadian rhythm influences our appetite, body temperature, heart rate, hormone secretion, and more.

The whole 24 hour cycle runs on an internal clock that keeps ticking all day long.

Key to this daily routine are clock genes.

There are a number of genes that work together to regulate our circadian rhythm.

But we do have a gene that's literally called the CLOCK gene, where CLOCK stands for circadian locomotor output cycles kaput.

While we generally talk about our circadian rhythm as a 24 hour clock.

That's not exactly correct, but it's pretty close.

One study shows that the average human runs on a 24 hour and 11 minute clock, with about a 29 minute gap between the fastest and slowest cycles.

Genetic Variation affects many aspects of our circadian rhythm, including the time we spend awake but also the production of various hormones, including cortisol and melatonin.

Most of this variation is minimal.

But there are some mutations that cause circadian rhythms that are so misaligned with our 24 hour cycle that they're diagnosed as medical conditions.

With so much variation in our internal clocks.

You might think it's a miracle that humans are even remotely in sync, but that's no miracle.

Hey, I thought you went back sleep.

Roomate: I did, but somebody interrupted my sleep.

Tina: You know, it's okay to be a bit nocturnal.

Roomate: No, we're not built to be up this late.

You're not normal.

Tina: What is normal?

Our internal clocks are not the only clocks are our circadian rhythms run on.

The ultimate circadian rhythm is the sun.

The word circadian comes from Latin words "circa" meaning around and "dies" meaning day.

See if we relied on our internal clocks alone, we would be in trouble.

There are about 37.2 trillion clocks in our body: one in each cell.

And despite having the same clock genes, they aren't synced with each other.

In fact, if you grow individual cells in a dish, they'll develop their own unique circadian rhythms.

While that's fine for the lab, it's very important that all the cellular clocks in your body stay synchronized.

After all, it wouldn't be very good if your liver was running on a different sleep schedule than your heart.

So there's a master controller keeping all 37.2 trillion cells in sync.

Every person has a master clock that keeps everything synched—our SCN.

Our suprachiasmatic nuclei: two clusters of nerve cells in our hypothalamus.

The SCN coordinate the body as a whole making sure that everything happens at the exact time that it's meant to.

Unlike the rest of your body, the SCN don't use our genes to tell time, they use the environment.

The SCN receive information directly from the environment.

Their main source of information comes from our eyes.

The SCN use the information on how light or dark it is to help calibrate our internal clock so that they don't drift too far from the sun's daily cycle.

But there's still one big question.

Why are humans awake during the day at all?

Couldn't our clocks just as easily have evolved to work in reverse?

Roomate: Obviously, you're working in reverse.

Tina: Maybe I'm just an evolutionary innovator.

Roomate: Okay, Tina.

There's nothing innovative about being up at 2am with a onesie on.

Tina: Looking at our evolutionary relatives across the order of primates, we see that being diurnal (awake during the day and asleep at night) is indeed the most common setting.

However, Primates can also be nocturnal (awake at night), or corpuscular (most active at dusk and dawn), or even cathemeral (sporadically awake during hours of daylight and darkness).

When you look across different primate species at their hours of wakefulness, primates that deviate from diurnality do so because they're occupying a particular ecological niche.

Specifically, they experience some kind of advantage to being awake beyond daylight hours only.

Whether it's because it helps them avoid predators or because it helps them be better predators.

With significant selective pressure, animals may deviate from their inherited sleep cycle.

Without it, there would be no need to adapt and change from the state of our direct evolutionary ancestors.

So to summarize, humans are diurnal because our most recent evolutionary ancestors were probably diurnal, and we have not experienced enough pressure from selections to change that pattern.

Guess I'll check socials.

Influencer: I'm sure you know somebody that sleep, right now.

Successful people like me, we say "who needs sleep."

You know they say "the early bird gets the worm," but those are just the worms I decide to leave behind.

If you need to sleep longer than the time it takes to blink your eyes, then you'll never be successful.

I'm offering you the privilege of buying my program "More Success by Sleeping Less."

I can help you virtually eliminate sleep from your life.

Tina: Wait.

What?

—Trust me, it works.

Tina: So we know how circadian rhythms work, where they come from evolutionarily.

And why humans are a diurnal species.

But with all of that in mind, can we change our sleep patterns?

Could I go from being a late night person to naturally waking up at 6am?

Could my roommate go to bed at 2am like a normal person?

We can learn from jet lag, which is a form of circadian desynchrony.

A sleep disorder where the body's natural rhythms do not match the external environment.

While travelers can experience symptoms for up to a week, their circadian rhythms will usually reset to a local time.

So, they have technically changed their rhythm pushing their natural cycle forward or back by several hours.

However, travelers have the advantage of aligning their internal clocks with the new day-night cycle of the place that they traveled to.

They have the sun on their side.

Without it, the process is a lot harder.

In studies of people who are completely blind or kept in isolation where they're not allowed to see light or clocks, scientists found that their circadian rhythms do start to ignore the day-night cycle and instead operate solely on their internal clock.

So what happens when your internal clock and external clock are giving you conflicting information?

Well, this is something that's experienced by people who live in regions of inconsistent hours of daylight due to latitude.

To avoid insomnia and circadian rhythm disorders in these settings, they need to actively regulate external time cues for their body.

This includes artificially creating light during daytime hours using lamps that mimics sunlight, and some parts of the year artificially blocking out daylight to create darkness.

Multiple factors give your body an indication of time.

Those factors called zeitgebers include light but also atmospheric conditions, temperature, exercise, eating, and more.

Some of these factors are out of our hands, but others we can control.

As long as people get the rest they need at some point during the day.

There is some flexibility in when that rest occurs.

So get your Z's when it works for you.

And on that note, I'm gonna go take a nap.

Sleepy time Roomate: Good morning!

Time to get up!

[music and blender blarring] Tina: Before you go we're looking for your feedback.

Each year PBS Digital Studios conducts an audience survey.

It helps us understand what you like and what you want to see more of.

You also get to help PBS pick new shows.

It only takes a few minutes but your feedback is extremely valuable to us.

There's a link in the description.

Thanks in advance.

Our internal clocks are not the only clocks that our circadian *stumbles* Tina.

Oh [flails around] Hey, Steph.

Tina: Oh, hey, Steph!

What are you doing up?

Roommate: What am I doing up?

[giggles] Tina: Just DO IT!

Producers: Did you hear me, Tina?

Tina!

Director: hey, T. We need you.

Tina: Oh, sorry.

I actually fell asleep