Hello,
Some folks get jet lag and other issues every time they fly. One reason is due to the air in the cabin of the plane. You see, it is constantly circulating, but, not fresh, and the longer you are in the air, the more "unfresh" if you excuse the word, the air becomes. In some, this can cause sinus issues, brain fog, and tiredness.
There are other things you can do to decrease the effects of jet lag.