> just interesting enough to keep your interest (and exhaust your brain) but not interesting or meaningful enough to keep you awake

Exactly. Back when I read more, I would have books to stimulate (daytime/commute reading) and 'bedtime books', which had to be interesting enough to read, but also effortful and ultimately tiring enough to promote sleep.

Now I do the same with TV shows in Plex, late at night. Unusually, it's the documentaries that stop me sleeping, and the classic US TV shows (that I like) that help me drift away.