What is the EASA definition of night time?
According to this handy document from EASA, night is defined as:


  …the period between the end of evening civil twilight and the beginning of morning civil twilight, or such other period between sunset and sunrise as may be prescribed by the appropriate authority, as defined by the Member State.


(The FAA and ICAO use a similar definition.)



You can get the Civil Twilight times from a convenient astronomical calendar, but ignoring the specifics, the same rule of thumb the FAA uses for purposes of night currency will likely work for you too:


Night starts one hour after sunset
(Sunset being when the last of the visible sun disc drops below the horizon.)
Night ends one hour before sunrise
(Sunrise being when the first of the visible sun disk breaks the horizon.)


This actually cheats you out of a few minutes of "night" on either side of the calculation, but it's conservative guidance if you're conservative in what you log you won't accidentally mess up and give yourself more night time than you're really entitled too (i.e. you won't make yourself "night current" when you really aren't).



Note that I'm a US pilot -- since all my flying and training is based on FAA regs in the US if I'm completely off the mark and any European pilots or CFIs want to set me right it would be much appreciated :-)