You went to bed at a reasonable time. You didn't run a marathon yesterday. Your schedule may be busy, but it isn't dramatically different from a few years ago. And yet by mid-afternoon, you feel as though your battery is almost empty. If this sounds familiar, you may have wondered: Why am I so tired after 40? It is tempting to blame age. Or hormones. Or a “slower metabolism.” But fatigue is not a diagnosis, and turning 40 does not create a single biological reason for low ene