Previous: The Chelyabinsk Meteor: What We Know
Next: Ice Quakes, Your Brain on Pot & the Body Language of Victory



View count:965,017
Last sync:2023-01-09 19:45
Why do we sleep at night instead of during the day? In this episode of SciShow Hank talks about circadian rhythms, how they work, and how they regulate different processes in our bodies.
Our President of Space for this episode:
Like SciShow? Want to help support us, and also get things to put on your walls, cover your torso and hold your liquids? Check out our awesome products over at DFTBA Records:

Or help support us by subscribing to our page on Subbable:
Looking for SciShow elsewhere on the internet?

Thanks Tank Tumblr:

Hank Green: Maybe you're a world traveler and you nothing of crossing a 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 co-hosted a 48 hour livestream 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.


Hank: Our internal time keeping 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 sleeping.

Researchers have found that as our body temperatures rise throughout the morning, our memory, alertness, and concentration also sharpens, 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 2am-4am, a close second is between 2pm and 3pm.  Now that may sound counterintuitive, unless you have ever had a class at 2pm 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  The 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 15% of our genes may be regulated by circadian rhythms.  So, when you get off that next trans-atlantic flight or finished pulling that all-nighter, do yourself a favor, listen to your body and take some more naps.  I'm going to go down for one right now, 'cause- it's- we're approaching the 2pm thing, so I'm not sure how this is going to look with the green-screen, you guys, but I don't care, I'm sleepy.  

Thank you for watching this SciShow Dose, and an extra special reverent thank you to President of Space Kurzgesagt.  You can check out his YouTube channel.  To find our how you can be decreed President of Space by me and a whole slew of other ways you can help keep us going here at SciShow, you can go to  If you have any questions or comments or ideas for us, we're on Facebook and Twitter and of course, down in the comments below, and if you want to keep getting smarter with us, you can go to and subscribe.