Sorry if that's inappropriate as an advice, or whatever. But what helped me personally (I'm currenlty unemployed which bad in an of itself also has this side effect of feeling both bored and tired at the same time) is instead of having this idea of "5 (6, 7, whatever) hour work day" to replace it with 1h-30min work interval (not more) and 20 minute intermission (not less) after each interval. Obviously only works if you set your own schedule. But, I found it to be helpful to me to clear my mind after productivity drops (after every one-and-half hour). You decide whether to implement this advice or not, but I just wanted to risk and share it. :-)