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Go to Brilliant.org/SciShow to check  out their Applied Probability course and for 20% off an annual premium subscription. [♪ INTRO] For something so relaxing, sleep is  actually surprisingly complicated. We spend a third of our lives sleeping, but  there’s a lot we still don’t know about it.

Like, how do we know when our bodies get tired? How do I sense that sensation? Scientists have pondered this  question for a long time, and they might just have gotten  close to solving the mystery.

Sleep is driven by two factors. One is the circadian rhythm, which our body systems go  through over a 24-hour day. And the second one is homeostatic sleep  pressure, which is when your need for sleep increases the longer you stay  awake and decreases the longer you sleep.

So, in essence, circadian  rhythms determine when you sleep, while your drive for sleep is more  influenced by homeostatic sleep pressure. But scientists aren’t totally sure  what drives homeostatic sleep pressure. So they have looked to other  processes that follow the same pattern as homeostatic sleep pressure, like DNA  damage in cells, which also increases while we’re awake or sleep deprived  and decreases during sleep.

The DNA in your cells accumulates  damage throughout the day: for instance, UV light can damage  the DNA in your skin cells. If the DNA isn’t repaired, it could lead  to things like mutations or cell death. Most of those DNA errors are fixed as they happen, but neurons can’t fix DNA  damage until you’re asleep.

That means that DNA damage in neurons builds up during the day when we’re awake. So, scientists in Israel hypothesized  that this buildup in the brain is what increases sleep pressure,  which makes us feel tired. And they tested this hypothesis using  zebrafish larvae, because they have similar nervous systems to humans and  regulate their sleep in a similar way.

When researchers damaged the DNA in  neurons of zebrafish larvae, sure enough, they fell asleep more quickly and got  more sleep than they normally did. And the more damaged their  DNA was, the more they slept. Once their DNA was back to normal,  so were their sleep patterns.

But the scientists wanted to know  what it was about damaged neuronal DNA that told the fish to sleep. So, they looked at a protein called  PARP-1, which detects DNA damage and flags it to start the repair response. And they found that levels of this protein  shoot up in the zebrafish’s neurons during the day alongside  the buildup of DNA damage, then drop back down throughout the  night as the damage is repaired.

Scientists also deprived the fish  of sleep and found that there was even more PARP-1 present  than normal, exactly the same as in homeostatic sleep pressure,  which increases as you stay awake. They also forced fish cells to  have more PARP-1 than normal. And when they did that, the zebrafish slept more.

And when they blocked PARP-1 from  flagging DNA damage, the fish slept less. Specifically, they slept less near the  end of daylight and throughout the night, or the exact times you’d expect  sleep pressure to be at its highest. It was like they didn’t know they were tired.

Now, we can’t say for sure that zebrafish  feel tiredness in the same way we do, because zebrafish can’t  tell us how they’re feeling. But this is still a great starting point  to better understand the role of DNA damage and sleep in all sorts of things, including  neurodegenerative diseases and aging. And at the very least, it’s  a great excuse to take a nap.

And if you'd like to calculate the probability  that you’ll sleep more than you intend to during your next nap you should  check out today’s sponsor Brilliant. They’re an online learning  platform with courses about science, engineering, computer science, and math. And they have a whole interactive  course on applied probability where they guide you on cultivating  a more quantitative worldview.

If you’d like to give Brilliant a try,  you can sign up at Brilliant.org/SciShow to save 20% off an annual premium subscription. Checking them out supports us too, so thanks! [♪ OUTRO]