[ELI5] Why does the body twitch when trying to fall asleep if you are really tired?

Question

[ELI5] Why does the body twitch when trying to fall asleep if you are really tired?

In: 3

Answer 1

There’s no hard confirmed reason, but the leading hypothesis is that it’s a leftover instinct reaction, developed when our early ancestors would sleep in trees.

Those that started to fall out and awoke in a startle lived longer and reproduced more than those that stayed asleep while they fell out of the tree.