Copenhagen is a six hour time difference, and twelve hour flight away.
Going there on the midnight flight, I arrive at 6am in the morning.
I wake at 3 or 4am almost every night, because it's 9 or 10am in Singapore. Plus it's spring, so the sun's up before 5am. I get tired by 6pm because it's 12am in Singapore.
Before my body can adjust, I'm back on the noon flight and I arrive at 6am in the morning again, this time in Singapore. Hardly any sleep as it was a day flight.
And now I'm up wide awake at 5am, because it's still 11pm in Copenhagen.