"mutated by radioactive fallout from the Fukushima nuclear power plant."

Highly unlikely, even though the wind current does travel that way.
I also don't know if you are buying the same strain.....maybe you can ask for a hybrid with a higher sativa % ....(as you prob. know Indica is known to put people to sleep.)
My guess would be, the chemicals in your body have built up, and your mind and body know and expect to sleep after smoking, so it does.