There should be a poll to gauge how many people read this as a horror scenario and how many read this as 'sexy times' :D
Could "pojawia się" also translate as "show himself" in English (ie, "he showed himself only at night")?
Hmmm... seems to me that it's very similar, but I would use the more literal translation of "pokazuje się".
Okay, after a second thought - added.
Well, the only emphasis is on 'the night being the only time he appears', so it looks the same as in English to me.