The author presents a vast amount of scientific evidence amount pretty much every corner of why we sleep, from its evolutionary roots thousands of years ago, to the importance of dreams and REM sleep for your memory, reasons and impact of insomnia, to what happens in the neurochemistry in your brain when you drink coffee, alcohol or sleep pills, and much, much more.
If read only one book this year, let me strongly recommend you consider this one.
ps: if you prefer audio, the narrated version from Audible is also very good. There are just a few non-essential graphs, that you can download from their side in PDF.
[1] Why We Sleep, by Matthew Walker - https://smile.amazon.com/Why-We-Sleep-Unlocking-Dreams-ebook... [non affiliate link]
[2] Quora - How do CEOs who sleep for only 4-5 hours daily manage to function and run multi-million dollar companies for years? http://qr.ae/TUpOrd