It’s normal to feel sore after a workout, though there are multiple other reasons why you may be feeling exhausted the day after working out. You may have overexerted yourself during your workout by exercising too hard, too long, or too frequently without resting. Other potential causes of exhaustion the day after working out include sleeping poorly during the night, as sleep is vital for muscle recovery, or not eating properly or drinking enough water after working out. Both nutrition and hydration are vital for peak athletic performance and energy levels.