I second parent's comment that for me this was one of those perspective-altering books in recent years. Even now, months after I finished, I still have several thoughts going through my head: why schools start so early, the societal impact of sleep deprivation, why doctors complete ignore the importance of sleep for your health [5], what happens when you drink alcohol (particularly at night), how Alzheimer's works, lucid dreams, and more.
I had already read about most of it in piecemeal over the years, but having everything pieced together for me, in a cohesive and fluid narrative, really helped for things to "click" for me.
Of course, if you want just the TL;DR, the gist is "Yeah, you really should sleep 7 to 8h per night, no joke". But if you want to understand the WHY, then it's a great read.
[1] https://news.ycombinator.com/item?id=17381548
[2] https://news.ycombinator.com/item?id=17446932
[3] https://news.ycombinator.com/item?id=17520658