Nocturnal means active at night, and diurnal is just the opposite. It is really hard to define Harry Potter as one of these types because we know nothing about his schedule. But according to the time at which the most significant events occurred, I would say that he is nocturnal human. Most of the fights took place in the evening or at night time: first trip to Forbidden Forest, first visit to the secret section, first fight with Lord Voldemort and many other plot events.