Last year sometime I read an interesting article or blog post, maybe this one or this one, mentioning a book called Evening's Empire: A History of the Night in Early Modern Europe, to the effect that until the relatively recent past humans had two sleep cycles: a "first sleep" and a "second sleep", punctuated by some time awake in the middle of the night. I've also read that there are cycles within cycles, because 90 minutes is a kind of unit of sleep for humans. For instance, in olden days before electric lights and such, people might have fallen asleep at 8:30 PM, slept for 3 90-minute cycles until 1 AM, been awake for 90 minutes until 2:30, then slept for another 2 90-minute cycles until 5:30 AM.
(Now that I've become aware of it, these 90-minute cycles do ring true for me. In fact, one could imagine dividing the day into 16 90-minute segments instead of 24 60-minute segments!)
Sleep seems trivial -- that unimportant form of inactivity between the times when you actually get stuff done -- but it's one of those natural phenomena that is fundamental to one's quality of life. It's better to do a good job of being inactive than a poor job of being active.