Learn English – someone who tries to be funny but they’re not

I think you can use a night owl, or a night-person.

I think a sleepless man is fine. Another similar word is an insomniac (a person who suffers from insomnia), but I believe that you know it already.

Actually, what you describe sounds pretty much like me myself. :)

Sometimes, people called me a vampire, a batman, or a dark knight, which is not quite to my taste. In casual chatting, I personally prefer a night dweller or a man of the night more.

Here is an excerpt from Wikipedia's page Night owl:

A night owl or evening person is a person who tends to stay up until late at night. The opposite of a night owl is an early bird, a lark as opposed to owl, someone who tends to begin sleeping at a time that is considered early and also wakes early.

Gullible is good IMO.

From http://www.learnersdictionary.com/definition/gullible: easily fooled or cheated, especially : quick to believe something that is not true. I'm not gullible enough to believe something that outrageous. They sell overpriced souvenirs to gullible tourists.

Credulous also fits, but from my experience/cultural-exposure, many people have told me they were (or someone else was) gullible, but I don't recall anyone describing someone as credulous. Strange.

Wikipedia has a comparison under gullibility:

• Gullibility is a failure of social intelligence in which a person is easily tricked or manipulated into an ill-advised course of action. It is closely related to credulity, which is the tendency to believe unlikely propositions that are unsupported by evidence.

There is also some comparison (and interesting discussion) of the terms in wikipedia's credulity though (IMO) the definitions given are not as precise as the various references suggest.

This ngram is interesting, showing that credulous has declined in the 20th century. This COCA report shows gullible is more common now.