If I get the same amount of sleep (8hours/night) and eat the same amount cals every day (about 2900 at the mo) then I've enough energy to do all the exercise I do in the day, and still feel energized to concentrate on TV/crosswords when relaxing, and put on about 1lb/week (weight-gain diet.) If I don't eat same amount of cals one day, I really notice the difference. Like today - even tho I did slightly less exercise than normal, didn't eat as much as normal and woah, do I ever feel light-headed right now. Seriously, my energy levels fall badly even after one day of different routines. I've conditioned my body to expect the same amount of exercise/food/rest, so when these get upset, all hell breaks loose and I feel dreadful. 'S not nice, I've got a mother of a headache