You cannot fly routes profitably in the middle of the night by now already.
Sami, no offense, but you need to spend a little more time playing your game if you actually think that's true.
Here's one on my full plane's route:
I can manage better LF's at 2am in AWS than most real airlines can manage to get at 2pm.
Sure, LF dropped 20% after midnight -- but I'm still making a ton of cash on the route. In real-life, with thin margins, a 20% drop in LF might be enough to make me think twice about operating that route. But, in AWS, with its massive profits, I could care less about a 20% drop in LF. Any well-ran airline in AWS can turn a profit with its planes half-empty. I literally give no thought to when a plane is scheduled aside from trying to back it to 23:55 if it's ever just a tiny bit over midnight because otherwise I'm giving away profit; but I do not worry one bit if a plane is flying between the hours of 12am and 5am and in some cases (see below) I actually go out of my way to schedule planes then.
Thanks to the way your game divides up demand with so much reliance on frequency, a popular strategy by players is to use your large aircraft during the day and then squeeze in little aircraft whenever you can, often in the middle of the night. The little aircraft at 2am ups your frequency which miraculously helps your large aircraft fly fuller when it departs at 9am. Your little aircraft fly fairly full even at 2am, your large aircraft gets to fly fuller because of frequency increases, and you make the most use of slots by saving the rarer/better slots for the larger aircraft during the day and use the ones at 2am where they're plentiful for your smaller planes.
I've become the market-share leader on hugely-competitive big routes (2000+/day) that only have slots available after midnight simply by flying 20 flights with F100s at 2am. I can find a route that's got 400% of its demand supplied by multiple players and hit it with a bunch of F100s at 2am and get each one of them 60-75% full, all while decimating
the marketshare and load factors of my competition flying at 8am with larger planes. And even with those load factors, flying that late, with that much competition, an F100 will still net a gross profit of about $11K/flight, less but that's still half of what it makes on a completely full flight operating in the middle of the day.