Wow.

Added to my reading list. Thanks.

A classic.

This is my assumption. I think we would have to have some sort of resting time. It's interesting to look at how certain organisms have adapted to the regions of the world where there is not night/day cycle for extended periods. Cannabis Sativa, for instance, requires longer nights of about 12 hours in order for the plant to begin flowering. However, Cannabis Ruderalis, which was originally discovered in siberia, automatically flowers after around 30 days because as you can imagine, there is no break in light in the region for many months.

People haven't evolved sleep in Alaska, though. Sleep evolved waaaaayyy before humans even existed, let alone started living that close to the poles. And we've not been around for long enough for evolution to change anything about something so deeply integrated into the way our biology works as sleep.

Harder to guess because Alaskans still have circadian rhythms - that's a basic biological function built into your brain, that a lot of life on earth evolved with. (I assume circadian rhythm predates humans, because other animals have it, too.)

In a tidally locked world, there would be no circadian rhythm, at least not one driven by the day/night light cycle. I mean yeah they'd probably still need to sleep but the underlying biological mechanisms would be completely different

Long answer that I'll try to make short, but basically... Because tidal forces.

The tide on earth comes from the spinning, and the gravity from the moon interacting with the earth's spin. The ocean's water sloshes about relatively easily, but the tidal forces (the gravity) is actually affecting everything, all the crust and mantle. The entire planet actually flexes a tiny, tiny bit with each spin.

That flexing carries a cost; energy has to come from somewhere and it has to go somewhere. So the flexing turns into thermal energy; heat. Only very slight on earth, but still there.

Jupiter's moon Io is a giant pile of volcanoes because of this effect. It's super close to Jupiter and still spins pretty quickly and so flexes a huge amount, generates a lot of heat, so lots of magma flying about.

But that heat radiates into space slowly. Which over billions of years means the spinning planet loses energy. Which means it spins slower and slower.

Eventually it stops spinning and will always have the same face pointing towards the bigger object. Just like our moon does now. Eventually, far far in the future, the earth will also stop spinning (because of the tidal forces) and one side will always face the moon. That's why we believe the days were much shorter when the planet was still newly formed; it spun faster, but has lost roughly half it's spinning speed over the last 4.5 billion years. Because of the moon.

That has happened with a lot of moons in the solar system (because moons are small and don't have store much energy) and it can happen with planets too. Usually, the closer the little thing is to the big thing (e.g. moon near planet, or planet near star) the faster the energy comes out of it, the sooner it stops spinning.

And the thing that causes that to happen... Tidal forces. Same forces that create the tides on Earth. Thus; locked into place by tidal forces; tidally locked.

We're just fortunate that we have the oceans which slosh about so easily and obviously, otherwise it would have taken us a lot longer to figure it out.