[tremon]

In my mind, all of your other examples have a singular answer, even if the answer isn't the full story.

one reason why we respirate

To replenish the oxygen in our blood.

one reason why we digest

To extract energy from our food.

one reason why we circulate

To distribute nutrients around the body.

Simplified as those answers may be, they're not wrong FAFAIK, they're incomplete. Do we have such a single not-completely-wrong answer for why we sleep?

[vacri]

Well, the one thing you've missed in all the items is waste removal, which is just as important as nutrient distribution. Also, we absorb more from our food than just energy. And even then, you caveat your answers with "but these are simplified and incomplete".

Then compare to the article, a bit of long-form journalism rather than throw-away summary lines, which opens with the statement that there's one crucial reason for sleep and we don't know it.