Russian matryoshka nesting dolls contain many layers, but each layer underneath is smaller and contains less detail in the artwork. Likewise, each loop shows Nadia's world having less detail, especially as fish, mirrors, and even people start disappearing.
When Nadia first comes out of the bathroom, she mentions to Maxine that she turned 36 a couple of minutes ago. That means that every time she resets, it is at the exact moment she turns 36.
In one of Mikes early scenes during the second time skip, he quotes John Updike. This is what he and Beatrice were discussing for her dissertation, while also hooking up.
Alans theory that the time loop is because someone is a bad person is technically correct just not in the way he envisioned. Both he and Nadia have specific personality flaws that are self-destructive, they just dont know how to change them.
If Nadia and Alan die at the same time, then what exactly happened in his relatively uneventful loop reset when she died 4 times in a row falling down the stairs within a minute or two of the loop beginning?
He does mention when talking about his deaths "slipped in shower, bathtub electrocution..." so presumably during a couple of his loops he decided to clean up before going to confront Beatrice.