The only time I have had trouble falling asleep is if I had caffeine after 6 pm (I go to bed around 10 pm).
I have to ask is alcohol or caffeine some sort of factor in your troubles?
I don't drink a lot of alcohol as of late, mainly as a money-and-waist saving expenditure.
There are nights where I'll hit the pillow and be asleep in minutes, and there are nights where I wont. My sleeping can be very erratic. If you were to ask my S/O, she'd tell you i'm out like a light in 2 seconds, but that's only because her issues are worse than mine, so to her it seems like I'm an amazing sleeper.
She sleeps like a log on her days off, however, so to each their own. Woe be to the foolish man who dares try to wake her.
But I'm usually in that half awake / half asleep mode most nights, for a non-trivial amount of time. I've gotten used to it; The trick is to never open your eyes. Keep "playing" asleep, and eventually it'll happen. Once you open your eyes and let your brain turn on more fully, you're screwed.
Or perhaps it's a combination of habits and fitness?
Or could be a symptom of something bad, like exhaustion?
It does seem to have a downside though; he requires much more sleep to function than I do.
Wait, you let your dog out AFTER she's done her business? You're doing it wrong.