This is in Iran, so presumably there’s less of that than in other places in the world?

This is Iran, so the researchers are often not great, or it's just straightup fraudulent. It's one of the epicenters of developing-world science fraud. So always worth keeping that in mind - there may not, in fact, be any facts that have to be explained.

Not to mention how difficult it can be to transition to sleeping with a bed partner, let alone one who is an insomniac.

"It can be argued that deriving a conclusive understanding from our preliminary findings is challenging without controlling for confounders (e.g., shared diet, stress exposure, and frequency of intimacy)." Seems like "stress exposure" is doing a lot of work there.