I ride before work, and loved it when I had boarders who were early birds and would come ride with me. I’m a small barn, so it also provided me a bit of help with the admittedly quick, morning routine.
In the winter there is light and in the summer it is cooler.
The horses get used to someone leaving, the only time it requires extra from the person riding is if a horse would be alone on that side of the farm due to a paired turnout issue. In that instance, they have to move the other horse to keep peace. We’re a small group, so everyone knows if that is the case and where they would need to move another horse. You put your horse back where you found it, and you’re expected to leave the barn as clean as, or cleaner, than you found it. As the owner, I’m the only one allowed to leave a mess behind for me to clean up later.
My dressage trainer, I haul in, often has a super-early client who is leaving when I show up at 7:15am.