Theory on why Eridians crave someone to watch while they sleep

This may have been obvious and common knowledge in the fandom but I just finished the book (loved it btw) and wanna get it out:

Humans see with their eyes. Humans sleep during the night because they can't see. Humans evolved all sleeping at the same time. Humans' ancient natural enemies likely slept at the same time as humans did.

Eridians' sight is not in any way reliant on the time of day - so, they never evolved any "objective best time to sleep". That's why their sleep time varies so wildly. This also means that their ancestors' natural predators' waking times must have often overlapped with eridians' sleeping times.

So which evolution path made sure they'd survive? "Release anxiety chemicals when sleeping without being watched by a buddy." And from that, it naturally ended up being customary.