Theoretically it can be any time of day. I don't think there is a particular time at which we are most productive hackers, it's whenever the mindset is right and the circumstances work.
Most of the time I hack in the night, that's between 11PM and 3 or 4AM. But I also have my moments in the train (30 minutes trip, starting at 17:34). And once even at school (1:30PM or so), that was the best coding experience I ever had at school and also the only time I hacked together with someone in real life.
There isn't really anything that keeps you from hacking at any particular time of day I think, to me there seem to be two prerequisites: You need to have a clear goal (which is something out of the ordinary and not like "get this css done for a client"), and you need to know that nobody will disturb you doing it.
Most of the time I hack in the night, that's between 11PM and 3 or 4AM. But I also have my moments in the train (30 minutes trip, starting at 17:34). And once even at school (1:30PM or so), that was the best coding experience I ever had at school and also the only time I hacked together with someone in real life.
There isn't really anything that keeps you from hacking at any particular time of day I think, to me there seem to be two prerequisites: You need to have a clear goal (which is something out of the ordinary and not like "get this css done for a client"), and you need to know that nobody will disturb you doing it.