First question: is the 8-am lawn mowing a typical behaviour or just occasionally heard? He may have a full day planned that requires an early start. Bottom line, sucks to be you, but I get the "pleasure" every week from my condo corp, courtesy the lawn maintenance company the board hired. 8 am til noon, at least once a week. Not continuous, just enough that I can't get any sleep during those four hours, no matter what time I might have been working 'til the night before.
Might try to have a pleasant conversation with him later in the week: try to be as positive as possible. Focus on how busy he seems, that he was mowing his lawn so early. Try to understand his motivation and lifestyle; this will take a lot of relaxation on your part. Only mention in passing that you prefer to party later into the night, and would be more likely to be sleeping than working on your lawn at 8am. Leave it to him to figure it out.
If it persists, let him know, again in a pleasant tone, that the night before was one of those nights you were partying, and that when he chose to mow his lawn you had chosen to sleep.
If he doesn't care, then you're out of luck: he's within his rights. But you can give him the "stink eye" before a friendly wave "Hello".
At the end of it all, it's not worth losing a neighbour-friend over a few hours sleep, no matter how pissed off you might be. But he'll know that his choices aren't helping neighbourly relations.