Last couple hours of work steal my energy

I usually have good energy and feel well, have plans to get stuff done or go out after work, UNTIL the last 2hrs of work. I hit a slump, feel tired, and all I want to do is sit hunched over, and once work is over to roll on to my bed to rest.

How do you stay engaged in life, productivity, energy-wise, through out the day? More physical activity during work? Different food strategies?

Context: I get 8hrs of sleep, eat homemade food, I sit at my desk for work by a window, minimal meetings usually siloed work, I usually walk for half an hour at lunch, work 40hrs/wk, generally healthy, have a family w/ little kids