Energy is a system, not a single feeling
A low-energy day can be caused by many different inputs, but useful self-observation starts by resisting the urge to explain everything at once. Sleep timing, recovery, activity, food, work intensity, stress and environment can all matter. The practical question is which pattern is visible enough to test.
Westmere treats energy as a longitudinal signal. One score is less informative than repeated observations taken in similar conditions. The aim is not to medicalize normal variation, but to make the pattern clearer before making the plan more complicated.