It may just be taking you a while to get adjusted to flying different hours. Be sure you're eating properly and not filling up on junk food. Vitamin supplements could be a good idea. I use a vitamin C and B12 drink at the start of my flying day. That and a dose of Ging Seng really helps keeping the energy level up.
If you can, try to get out a walk a bit. May sound a little crazy, but getting 20 minutes of sustained cardio can make a big difference, especially if you can do so in ambient light.
Hang in there!
Dea