3. Nutrition and Diet
Improper nutrition and imbalanced diet may cause a lot of this unexplained tiredness and fatigue. If your diet contains lots of carbs, sugar, and processed foods, it is possible that this is the reason behind your lethargy.
Fix
Include complex carbs like whole grains, proteins and healthy fats in your meals and snacks. You can try having unprocessed food to avoid added sugar and have fruits, honey, etc. to satiate your sugar cravings.