Hope I'm not coming out too hard on you like you think Betty did. But yes, you will sometimes have to stay up all (or at least very late) night if you wan't to win the megalopolis league.
To win megalopolis league 20 000 points in 24 hours is what you have to do five days in a row. I've done it several weeks (without buying simCash) so I would say it's not impossible but actually just normal. Here is a guide, hope it helps:
http://answers.ea.com/t5/SimCity-BuildIt-Strategies/Being-competetive-in-Mayors-League-without-spending-real-money/m-p/5389133#U5389133
I agree on your point that about that the starting time matters. If you are asleep when the competition starts then those hours are lost. Compared to someone that can start a long mission just before going to bed. Same thing with when it ends; if you are asleep when it then you will loose hours. I am lucky since it starts in the morning and ends at noon so I have always been able to make full use of all hours.
But the same thing is true regarding your working or study hours so it is almost impossible to make the contest fair for everyone. Maybe haviing a timeframe of say 12 hours when you can start and then every player has exactly 125 hours to do complete it?