I wonder how many of these "surprising" results are markers of other life style decisions.
Regular sleeping hours as a marker for a less stressful life?
Also ran across this:
https://pmc.ncbi.nlm.nih.gov/articles/PMC5105607/
I doubt it's the actual reading. Maybe having a calmer life where you take the time to relax with a book.
Also, reading lowers stress. Which in turn improves sleep an overall mental / physical health. I always read before going to bed for at least 30 minutes, and anecdotally I have a much easier time to fall asleep that way.