My energy levels are not the same anymore

I just turned 40, and I find myself getting tired...quickly.

Like today, it's a nice saturday, and I want to stay awake and study my investment portfolio, but my body is telling me it's about time for a nap.

Over the work week, I start off strong, but around Tuesday evening at 3pm - I feel mentally exhausted, and that happens around the same time each following day of the week. And the thing is- I DO NOT MOVE DURING work.

I find that running on a treadmill seems to increase my energy reservoir, so I'm going to do that much more often.

Is anyone else experiencing this? And if so - what do you do to address it?