@sungw I find the concept of sleep regression useful (despite being skeptical at first), IF I can be very specific about what sleep regressions are.
I don't define sleep regression ISN'T when baby used to sleep well at night but no longer is. It's not a useful concept. I know my baby isn't sleeping well when he isn't sleeping well. I don't need to call it by another name just because.
Causes of a baby sleeping well previously not sleeping well previously can include:
-development of sleep association
-needing a schedule change (approaching a nap transition)
-developed a sleep debt (disruption in routine, illness, teething, travel, sleep environment, etc)
-going through developmental stuff
The last (going through developmental stuff) is what I call a regression, and it does feel VERY distinct, like baby is doing great on the same schedule for weeks, and then suddenly starts rolling or rocking or crying out of separation anxiety or loitering at naps while nothing else has changed.
The key is knowing that you just need to ride regressions out and catch baby up on sleep as much as you can. Some schedule tweaks here and there can help, but no drastic changes (like dropping a nap) are needed usually.
My LO is a pretty standard baby and hit every single regression in the books (4.5mo rolling, 8mo rocking/crawling, 9mo separation anxiety, 10mo nap strike, 12mo nap strike) and is very susceptible to overtiredness so hit all the nap transition run ups pretty hard too (3.5-4mo 4-3, 6.5-7.5mo 3-2, slowly entering 2-1 phase now at 12.5mo). It's a struggle but differentiation between a need for schedule changes vs just a temporary regression definitely helps me manage his sleep.