I'm a fit, 34 M and lately realizing that feeling tired and having low energy happens to me more often than I think. I started tracking how many days of a month I actually feel good (1-5 star rating) with an app. For each day, I enter the rating and tags on what happened that day like good/bad weather, work stress, good/bad sleep, exercise, drinking etc, so i can analyze the pattern later.
The result is I have about a week of feeling good. Rest of the month is mostly meh, with some terrible days. Biggest contributors to my mood are still good night sleep and weather.
Anyway I'm just sharing the results. I'm actively working on improving my overall mood. Beyond the physical fundamentals (sleep, drinking water, eating healthy etc), I've noticed the mental aspects of life matter just as much. Spending quality time with people I care about, having things to look forward to, and feeling excited about life make a huge difference too.