My question is can't a baby tell when he or she wants to sleep?
Excitement can easily keep little ones up up and more up, until they crash, and many babies and toddlers crash loudly and in a most upsetting way.
OP, I don't have a problem with the morning flight, b/c you can work around that.
But with the night flight, if baby has a freakout, if baby won't sleep, if baby is upset...you're keeping the whole plane-ful of passengers awake as well, when it's a normal time for people to take a snooze on the flight.
We've only done morning, afternoon, and later afternoon flights so far b/c of that. I would HATE to be the one with the baby screaming during nighttime. We've done that in a hotel and it was horrible and embarrassing.
I had an experience on a flight from JFK to Ireland once where a drunk woman going home was playing her guitar and singing and talking about the upcoming reunion with her boyfriend...she kept me, and others, up ALL night. It was very much NOT fun. That experience (and having to learn to drive car from the other side on the other side of the road with the stickshift under my left hand, at 7am, with no sleep, was just the cherry on top!) has really made me think about others when on planes, and is the reason we haven't done late evening flights yet.
something to think about!