I'd always just heard it stated as it helps him "keep his mind," so that would just suggest to me that he has nothing better to do, or perhaps even no desire to do anything but just sleep (since it is night time and that's when people usually do sleep). Whereas without the Wolfsbane, he doesn't keep his human mind and therefore the nocturnal instincts of the wolf demand he go out, hunt, etc.

That's how I see it.