Age, weight, fitness level, too much alcohol but are you on any sort of daily medication?
A couple of years ago I went through several months where I felt so tired, everything was an effort & I would fall asleep during the day if I sat down. I thought I had something seriously wrong with me but then began to think about what had changed over the few months leading up to the problem. The only thing I could think of which was different was the daily medication I have to take for hypertension which my GP had changed to something “more up to date & better” (he said). As an experiment, I immediately stopped taking it but as a precaution took ½ aspirin a day & after 2 days, it was as if I’d got my life back. All medication has side effects (read the small print!) which will affect some more than others; I’m now on a different medication which agrees with me.