It's a bit more complicated than a strict curfew, at least at LHR and I expect at AMS too. A limited number of flights can land during curfew hours, for example for diversions. There are limits on numbers until 6 am and there are limits/penalties on noisier aircraft types until 6 am and 7 am. It makes sense for ATC, airport operations and the operating airlines to have a flights depart late enough and have a planned air speed to not be circling, for fuel efficiency as well as simplifying queuing. LHR give quite a lot of detail: https://www.heathrow.com/noise/heath.../night-flights