I'm taking a probiotic called Omnibiotic SR-9 which is claimed to help for fatigue and stress resistence.
Additionally, I drink 1 small bottle of Actimel/Yacult during the day.
Of course I also drink 1-2 cups of coffee during the day, the number 1 supplement.
I take serotine influecing supplements (Ashwagandha, Rosea Rhodiola) only as acute medicine, not chronic daily usage.
^ This helped me a lot for general fatigue and energy levels. It also improved POIS a lot.