Why do I feel mentally exhausted even on low-demand days?

This is very common in adults with ADHD. Mental fatigue is often about effort rather than activity level. Even on quieter days, your brain may still be working hard to plan, regulate attention, manage emotions, and stay organized. Those processes are more effortful in ADHD and can be draining even without obvious stressors. Another factor is background mental noise. If your mind is constantly scanning, thinking, or trying to self regulate, it never fully rests. That leads to a sense of exhaustion that does not match how much you did. Clinicians recognize this as executive fatigue, not lack of resilience or motivation.