I mostly play alone, and honestly it's pretty fun. I have one other friend that I play Pokemon GO alongside of and we usually play GO when we get out of work or when we share days off. Though I will play it on my own during any hour of the morning, day, or night, and I have been out and about in the middle of my town by myself just wandering around playing GO for hours at a time.
Generally whenever I bump into GO players that are wandering around by themselves like I am, I will actually go out of my way to talk with them, share tips and tricks, and if they're on my team I'll ask them if they want to come gym raid with me.
Before Pokemon GO, I walk everywhere regardless due to my my job and my home location (and that I don't really enjoy driving anywhere), plus the town I live in is a resort town where everything is close together and designed to attract tourists in. Being in the middle of my town at 1-2 in the morning is not that abnormal for me to begin with, plus the town as a whole is incredibly quiet and safe. Crime is uncommon and the streets are either completely empty or filled with gentle bar hoppers.
Now that pokemon GO is around, I as well as many others have more reasons to safely travel the streets at the incredibly late hours of night. Playing it alone in the dark along unlit streets is just really not that intimidating or worrisome, and I often prefer it.