I came to Berlin on December 27 and stayed until now. My 90 days are almost over but last week I went to Croatia (which is not part of Schengen) for one night and came back to Berlin. My friends have told me that once I'm out of Schengen area I can restart my 90 days again.
Can I stay in Schengen area for another 90 days after leaving it for one day or do I have to leave it for 3 months to restart the 90 days period?
Your friends are wrong.
Unless you have a long-stay visa or residence permit, you can only be in the Schengen area for 90 days out of any 180. Leaving and reentering does not change that.
(Another way to state this is that at any point on time you must be able to point to 90 different dates within the last 180 days when you were outside Schengen for that entire day, midnight to midnight).
Going to Croatia for a single night doesn't even affect the counting, because both the day you exit and the day you enter count as days in Schengen.
