Sleepiness predicted by lack of sleep

In what I hesitate to describe as a surprising outcome, a recent paper in Sleep Research says:
It was concluded that the main determinants of daytime sleepiness in a real-life day-to-day context were short sleep, poor sleep and early rising, and that days with high sleepiness ended with ratings of poorer health.
Jokes aside, I suspect it is extremely important that studies of these sort are conducted in order to get a measure of ground truth in the matter. Imagine if they'd discovered that something in here was not as expected!