Are you really asking if you should reboot at night? Usually the question is either whether shut down at night or to reboot at all, not specifically rebooting at night. I'll address both.
Shutting down at night is useful and recommended for most users. Unless you are running servers on your system, then there is no practical reason to leave it on at night. Not only does it waste electricity, but the lights interfere with biorhythms which provide for lesser quality sleep. Further, it adds extra wear on the system in a few ways, for example, the hard-drive records the number of hours that it has been running. Also, if something bad happens (e.g., for some reason the system comes under load, leading to it heating up; a virus attack, OS problem, rain, etc.), it will not be attended to in a timely manner since you are asleep which can damage it unnecessarily.
Rebooting is completely different. Some people recommend doing it now and then while others think it is normally unnecessary. The truth is somewhere in between. Obviously rebooting is inconvenient, especially when you have programs running. On the other hand, a nice fresh boot is indeed good because numerous processes will end up starting during the course of usage that are not run at startup. These extra processes are not always necessary and trying to end them manually can be a hassle. Worse, like it or not, memory leaks do exist, and there is no practical way to flush everything to get that just-booted new-system smell, uh, I mean feeling. In the end, it is up to you. Unless rebooting is required (such as after installing programs or Windows updates), then you can manual do so if you feel the system is getting sluggish or after you finish using large, resource-hungry programs.
There is no need to reboot at night, and can even be avoided while using a system if you shut down at night. That way, each day you start up, any required changes from the night before are automatically done, and you start with a fresh system that should usually last until you shut down at night.