Reading Dr. Matthew P. Walker's Why We Sleep: Unlocking the Power of Sleep and Dreams (2017). Fascinating, well written book on a subject that I (we) deal with every night.

Starts out with evolutionary sleep, and the animal kingdom. Did not know that birds can sleep with one half their brain hemisphere asleep, and the other awake, with that eye keeping a look out. So one species line up on a branch, with all the birds in between with both side of their brains asleep, and the ones on the ends with one side awake, with that eye keeping watch. They then awake, turn around, and the other side wakes up to keep watch.

from ch 1 2nd para:

Routinely sleeping less than six or seven hours a night demolishes your immune system, more than doubling your risk of cancer. Insufficient sleep is a key lifestyle factor in determining whether or not you will develop Alzheimer's disease. Inadequate sleep - even moderate reductions for just one week - disrupts blood sugar levels so profoundly that you will be classified as prediabetic.