I reached the same conclusion for myself - that a ~40 minute cardio workout actually results in less productivity rather than more. However, I switched to a 10

I reached the same conclusion for myself - that a ~40 minute cardio workout actually results in less productivity rather than more. However, I switched to a 10 minute cycling workout with high intensity intervals (4-5 intervals 30 seconds in length). I have only done this for a few weeks, but I find that it boosts daily energy rather than depleting it.