I had not heard the term momsomnia before, but now that I look into it a bit, I'm fascinated — thanks for bringing it up. It seems quite similar to other forms of stress and anxiety that keep some people up at night, related to not being able to wind down. Specifically with momsomnia, all the stress of being a mom is compounded by, well, as sleep expert Shelby Harris told the reporter Jessica Grose in the NYT:

"Even though my baby started sleeping through the night, I stopped sleeping.” A lot of moms — some dads, but mostly moms — almost become hard-wired to listening for what I call “sleep threats.” For example, if their kid is going to cry or come and get them. Even after the threat is gone, they continue to have trouble with sleep because they’re so conditioned to listening."

More: https://www.nytimes.com/2021/09/29/parenting/insomnia-parents-advice.html