How much sleep is the bare minimum you need to function?

My friend A had a baby 8 days ago, and was ecstatic today because she'd gotten a three-hour stretch of sleep last night.

Send yourself back in time to those early days and think about how much sleep you could function decently on (and by function decently I mean go through normal daily activities without walking into walls or bursting into tears for no reason). How many hours was that? How many hours do you need now, and how old are your kids?

Three hours did not feel like enough for me then, and it wouldn't now. Four was my number, and if I got five I was golden. Now, I'm a lightweight who has older children, and I need five, for sure. (I've also noticed that when I'm studying or doing schoolwork I just physically max out and can't keep reading--let alone processing--past a certain hour of the night. In undergrad I could go for 48 hours and keep on reading and understanding literary criticism. No more...)

If you'd like to pass along something to A that you wish someone had told you when your baby was 8 days old, please do.