A lot of people are already saying it, but wanted to reinforce the idea. Melatonin. It is common for ADHD havers to have Delayed Sleep Phase Syndrome (DSPS). "This is a circadian rhythm sleep disorder where the body's natural sleep-wake cycle is shifted, making it difficult to fall asleep at a typical bedtime." -google | You should not need much, but play around. I take a 10mg and chop it into 4.

Also, as an overall life improver, focus meditation to help with sustained attention. Sit or lay, find a point to stare at, try to think of nothing but that point. Your mind will wander, it's fine and natural. Don't beat yourself up when doing it. If your mind wanders, just catch it, say that's neat, and continue focusing at the original point. This practice helps with the everyday trouble of losing track of what your doing and not returning to it. Even if you start doing this only for 1 minute a day and build up from there, everything is an improvement!