No. First of all there are users from all over the world. That means different time zones. Majority of users are probably in the USA and near evenings and nights is when most users have their greatest amount of free time. So that's when most users are active.
I'm in a different time zone which means when this site is most active seemingly at night I'm not on here as I'm sleeping. I still see plenty of activity on here most days when I am able to come on. I don't believe this site is only active at night.