I wouldn't worry about it, I'm just over the 40 day milestone too, and without alcohol my energy dies out shortly after coming home from work, only on a day off like tomorrow, being Sunday, do I generally feel like having energy to do more activities.
I wonder if this is what normal people feel like? some of my workmates say they go to bed at 10pm every night, and I used to think that was madness, the night was still young at that time!! . . . but not now, a warm mug of Green Tea and bed!!