Why does my ADHD show up as chronic mental fatigue?

Chronic mental fatigue in ADHD comes from constant cognitive overexertion. Even simple tasks require more executive effort, planning, and self monitoring. That effort is invisible but exhausting. Many adults with ADHD rely on anxiety, urgency, or perfectionism to function. Those strategies work temporarily but drain energy over time. The result is mental exhaustion rather than hyperactivity. Fatigue also reflects regulation inefficiency. The brain burns more fuel to achieve the same output. Clinically, this is not depression or laziness by default. It is cognitive overload. Treatment focuses on reducing load, increasing support, and allowing rest without guilt. When effort is matched to capacity, mental fatigue often improves significantly.