Welcome. My child, like me, does not seem to feel fatigue until it is at a breaking point. He was getting 5 hours of sleep a night from a very young age on. His pediatrician suggested 1mg melatonin 1 hour before bed, and that has helped a lot. The increased sleep seems to have calmed down a lot of his hyperactivity. You might want to ask your pediatrician about that.
I recall at one stage dopamine and melatonin became buzz words; mostly I personally don't suggest this for children, but doctors do what pays, sadly. If child is 5 pls try holistic approaches first.
Obviously sugar is not good, some food trigger ADHD as in hyperactivity!! I left afternoon naps, (and really did try) and they slept through night. With sleep, routine is tops. So wake him up early, then he may be bit drowsy in day (my eyes are worst with inadequate sleep) but should be tired by night, and continue waking early until establish a routine. (Unsure about naps, sometimes it can be overload stress naps - so then we leave em to nap)
Also set mood for sleep, calm, storytime, dim lights, and repeat into routine.
You can try introducing healthy choices (test for nut allergies) and despite musli bars seeming healthy, full of sugar, same with many health snacks (misleading) so let child go hungry, (not starve but) and continue going hungry until eat chicken soup, no more argument!! Less sweets chips chocolate, the easier mom's job.
Exciting: platter with fruit variations, and stimulate sensses with new exciting health options, make healthy smoothies.
And rewards that fun based, let's go to farmyard to see animals, we doing this on weekend when you finish XYZ