Do you know why? It might be worth figuring out. For me is is mild ME/CFS but for you maybe not (a good tell for CFS is if doing any exercise at all e.g. 1km walk makes you feel rough later. Not delayed muscle soreness or a stitch but more like a flu say).

So many other things are possible like iron, thyroid, sleep apnea, ADHD, depression or burnout, long COVID, tropical illness. Maybe more.

Finally it is totally morally OK not to be a 100% productive worker bee. A "cure" is not necessary but might be worth rabbit holing if you desire and have the energy.