How does your baby wake up at night?

Got a 5 week old here. I just found out about active sleep. I let her sleep thru her little 3am grunting session and ended up getting 2.5 more hours of sleep. I always took the grunting session to mean that she's waking up to eat!! How often have I been waking up unnecessarily?!

I got up with her 2.5 hours later but maybe it was still too early? She looked like she was still asleep but was doing her grunting for a bit and let out a single, very short strong whimper/weak cry. So I took that to mean she's really getting up. Maybe I was wrong - she only ate an ounce eagerly and now I'm sitting here trying to get her to keep eating.

For those with an active sleeper, what are the cues you look for to get up at night? Do you wait until their eyes are open and they are clearly awake? Wait until they are stirring more than normal? Until they actually cry?

I don't remember my older ones having active sleep as babies. I think they all actually woke me up by crying. Could be forgetting something though.