Honestly I might not have ADHD if I managed to sleep 6.5-8 hours a night. But that's absolutely not how it is, no matter what I do. My weekly average is 4.5-5 hours, most weeks, a little more some weeks. And this is even though I rarely use an alarm these days and no one expects me to do anything in the morning. Around 5 hours on average is the best I can possibly manage, right now.

You could conclude that this must be my natural setpoint for sleep and how much I need, but that isn't true. After enough sleep deprivation, mammals sleep less and never rebound to the amount they used to sleep. That persists even though they show various signs of sleep deficit. So that's true of me as well. When I do manage to sleep 5.5+ hours, I feel much better, sharper, more energized, more normal. But I can't get that to happen more than a few times a month. And even then, it's because I slept much less than average in the preceding few days.

Depression and insomnia can both look just like ADHD. But ADHD can also cause depression and insomnia. Most people with ADHD have irregular sleep patterns by nature and their impulsivity worsens this further, and most people with ADHD are first diagnosed with depression that sets in as a consequence of their ADHD. It's just that when the depression goes away, they still have the ADHD tendencies. That's how I was diagnosed also.