I’m diagnosed ADHD, however, I have a kid who is DEFINITELY not (she’s 11 and was many times more attentive and focused than her peers even as a young child). She never slept as much as other kids. She dropped naps early and then had the nerve to sleep 7-8 hours a night like an adult as a toddler. I remember people telling me she needs to get to bed earlier so I tried it. Yeah, she still only slept 7-8 hours a night, except with the early bedtimes, she started waking at 3 and 4 in the morning.
It’s hard for me to equate ADHD with dropping napping due to my experience, but I can relate to needing some sort of soothing mechanism to sleep and even then it can take awhile. Even as an adult, I shake my foot and/or listen to an audiobook. My non-ADHD, never-a-napper kid only needs darkness and silence. Sometimes she doesn’t even need silence. I’ll try to tell her some great idea 30 seconds after the lights are off and she will already be asleep.