There are a limited number of LHR "nightslots".
In short, if you are scheduled in prior to about 0615 (to allow for Taxi in) then your flight is probably planned for one of these nightslots. If you are not scheduled in prior to this, but the aircraft touches down prior to 0600, you "lose" a slot. Also some unscheduled late departures from LHR may use nightslots...
You can end up where the airline "runs out", and then flights scheduled in prior to 0600 don't have one, and have to wait...
HTH