I agree. I used to just use free weights for upper body, and only calf raises, and machines for leg presses and quads, but when I started squatting with just a barbell (non-guided), my lower back pain went away, and standing up off the mattress I had on the floor became a breeze. I jump rope and run as well (barefoot or with FiveFingers by Vibram). I find jumping rope to be a great way to get in 20 min. intense cardio to music. My last job was diving and fixing underwater hydraulics and electrical systems, and always walking with about maybe 3 hours sitting at a desk per day. Jumping rope, or the run, made me feel sharp many hours after. Sometimes weights would make me focus, and be very clear, but I tired and became groggy afterwards. I suspect your body takes a hit, and wants you to sleep to repair all the muscle you tore down.