I was diagnosed with Chronic Fatigue Syndrome (more properly known as Myalgic Encephalomyelitis, sometimes also called Systemic Exercise Intolerance Disease) before meeting my husband who has Inattentive ADD. There is a constellation of symptoms required for ME/CFS diagnosis you can find at solvecfs.org. It is basically an immune dysfunction with brain inflammation, although that is still under research. Yes, fatigue is one of the main symptoms, but as Penny pointed out there are many things that make us feel tired. Many with ME are so depleted they become bed-bound for periods of time, as I was before I met my husband.
For my ADD husband, I’ve observed that when he is motivated and excited about something, he seems to have plenty of energy. But when he overdoes it or when feels “lost”/not focusing, he can easily take a nap at almost any time of the day or night. He falls asleep easily if his brain isn’t stimulated. And he tends to sleep long hours overnight, too. ME/CFS has disordered sleep/insomnia as a primary symptom. So there are definitely some differences.