Like Fire and Water

I've been making a point to take a 5-mile walk at least twice a week for a little while now. My current routes have me going through a small park or two and on a nature trail. And as you might expect, there are plenty of walkers, joggers and bikers who are out exercising as well. Having walked in the morning, day and night, there's been something very interesting that I've learned: As the day goes on, the people get grumpier.

In the morning, all the exercisers are really nice and happy. They will wave and say "hey" as you pass by. But at night? The other person won't respond at all. If you attempt any form of niceness, they will look at you like you have violated their personal space and JUST BACK AWAY, WILL YOU?

One theory I've heard is that people are just plain tired at night and don't have energy to be nice. Do you think that's it? The difference is, well, night and day.