Just on a whim I deep-dived into understanding Sleep. What is Sleep? Why do we sleep? What happens when we sleep? What is adequate sleep? How to sleep better? And many more such questions related to sleep. I found a lot of stuff that I never knew before and it was a joy to know something utterly new.
Here is a wall of links, literally portals, that lead to fascinating places on the planet of sleep.
Khan Academy has a good, short video explaining Circadian Rhythms and effect on Sleep.