Ever wondered why your flight lands at 6:05am instead of a far more civilised hour? At a surprising number of major airports, the answer is a noise curfew — a nightly window when scheduled flights simply cannot take off or land, or can only do so in sharply limited numbers.

London Heathrow has no blanket ban, but it comes close. No departures are scheduled between 10:45pm and 6:00am, and arrivals dry up between roughly 10:55pm and 4:50am, with a voluntary ban on flights landing before 4:30am. The UK government caps overnight movements at just 5,800 a year, plus noise quotas. For an airport running only two runways and already notorious for scarce, expensive slots, that makes launching or expanding service at Heathrow brutally hard.

Frankfurt, Lufthansa's main base, is stricter: the airfield is closed outright from 11:00pm to 5:00am, with heavily reduced capacity in the hour either side. Only emergencies, disaster relief and medical flights may operate. Sister hub Zurich is similar, shutting down from 11:30pm to 6:00am, though a 30-minute grace period absorbs late-running flights, and surcharges discourage airlines from squeezing in a last departure.

In Australia, Sydney Kingsford Smith blocks takeoffs and landings from 11:00pm to 6:00am, with limited shoulder-period flights at either end. Combined with slot restrictions, the curfew has long limited growth at the country's busiest airport — which is exactly why the new Western Sydney International Airport matters. It has no curfew at all, and Singapore Airlines is already planning flights that depart after Sydney's shutters have come down. Expect more overnight-oriented carriers to follow.

The strictest US example is Orange County's John Wayne Airport, which bans departures from 10:00pm (11:00pm for arrivals) until 7:00am, extended to 8:00am on Sundays. The airport is famous for its steep climb-out, forced by a short 5,700-foot runway surrounded by affluent neighbourhoods. Passengers notice: the takeoff feels like a rocket launch.

For travellers, these rules shape your itinerary more than you might think. Red-eye lovers will find overnight options thin at Heathrow, Frankfurt and Zurich, while long-haul arrivals into Sydney cluster just after 6:00am. Booking the first bank of morning flights usually means accepting that 5:30am alarm — and at curfew airports, there's no airline that can get around it.