This article points to some sources, looks like the same ones referred to in 'Why We Sleep': https://www.theatlantic.com/health/archive/2017/12/even-smal...

Studies cited: https://www.ncbi.nlm.nih.gov/pubmed/12683478 https://www.bmj.com/content/357/bmj.j2353

It is one of the most recommended books in book recommendation threads.

I've seen it mentioned in a dozen threads here since I ordered it (on an HN recommendation) a month ago.