Two. Broken sleep, which is not the same as short sleep

Everybody warns you that you will not get enough sleep. Almost nobody tells you that the damage is done by the breaking rather than by the total.

Sleep runs in stages, and the useful repair work happens deep in a stretch. Wake somebody every ninety minutes and they can spend eight hours in bed and get very little of it. Their capacity to hold a feeling steady the next day drops anyway.

That is why the maths of hours in bed never seems to add up, and why you can be told you are getting plenty of rest and know perfectly well that you are not.