Some airports, especially the newer giant ones such as Dulles, Denver International, and Dallas/Ft. Worth, ARE built way outside of town where there are few people affected by the noise.
Some smaller airports are right in the middle of populated areas but can get away with having late night curfews because they don’t have enough demand to require all-night services. For example, San Jose, Burbank, Orange County, Washington National.
Lastly are the big airports that are in the middle of cities yet are busy enough to require 24-hour service. Mainly they’ve been there for many years, before the communities grew up around them, so the people must have known there was an airport there before deciding to live there. For example, JFK, Chicago O’Hare.
San Francisco and Los Angeles airports are odd cases, both being old and busy. They happen to be located such that takeoffs are generally over water. At SFO landings are also over water most of the time. At LAX landings are over water during certain hours late at night.
Noise regulations tend to limit the type of aircraft that can fly out of a given airport over residential areas, but do not prohibit all take offs and landings. The regulations are established by the FAA/Transport Canada/CAA equivalent and are part of the Aeronautics Act and the Canadian Aviation Regulations in Canada.
For Montreal specifically, the airport is open 24hours a day to jet and propeller aircraft weighing less than 45000kg - heavier planes are not allowed to fly between midnight and 7am (barring an emergency need, of course).
So an A319 or A320 could land, but it looks like an A321 couldn’t. B747s, 777, A330/340 are right out, as is the B727, etc. Dash-8 100, 200, 300 can land, but certain Q400s can’t.
I’ll try and dig up the actual regulations if I have time.
I used to take a red-eye from Denver to NYC, and frankly, I had a hell of a time booking it online. It left at 12:30 a.m. When I booked it online, it always moved it back a day–or it looked like it moved it back a day–so I had to call a live person (always a fun thing) and make sure I had the right date. I don’t know why they had a problem with this. At 2 a.m. they don’t have a problem, but at half-past midnight I guess they think you meant to leave the day before?
If other people had trouble with the date thing, maybe that’s a reason for lowered popularity.
(I took it once with my kid, who was about 8 at the time. I thought he would sleep on the plane. I was wrong. We were surrounded by people who were in party mode, and the kid is always ready to party.)
Anyway this flight was always full, but also contained people who had been bumped from earlier flights. I think it’s still going, although I haven’t flown it since '08.
Transport Canada seems to have simply agreed to the conditions set inICAO Annex 16 for noise emission standards. I don’t really have time to read it all and summarize, but there it is if anyone is interested!