i believe in this "if your body wants something, you probably is in need of that thing". Listen to your body- if it doesnt feel like drinking, dont drink just because you always drink in the evening. try have a balanced diet - fresh vegetable juice is good - not the sugary drink you find in Y-class breaky trays. drink loads of water. think of having soup if you're not feeling very well. If you need to sleep, sleep.
Oh, and if you have flights at extreme hours, dress a little warmer to your liking. Chances are that you are at lower immune levels.