Are you familiar with a study where merely changing something improved outcomes for a while? They made lights brighter and workers were more productive for a bit, then they dim them back down, workers are again productive but for a bit. First they though oh, let's just make lights brighter everywhere eh? but it turned out nah. Probably it's just workers feeling like you are doing something for their benefit, so once you stop it's back to the baseline.

It seems important to eliminate that possibility, because otherwise sleeping mask or not literally doesn't matter and it's just about doing something and the study is simplified to "caring about sleep improves sleep". Which can be totally true even if bollocks per western medicine tradition to date, but then the study should be about that and not about sleeping masks eh?

Regression to the mean, which apparently (per Wikipedia article on placebo) is what really happened in those old studies that made people think "placebo effect" is a big deal.