From the viewpoint of your student he/she can only log the time as Pu/T up until sunset. Legally you can fly until official night time even without a night rating but unless you have a night instructor rating (min 2 hours training in the air + 1 on the ground) your lesson legally ends at sunset in terms of your student logging the time as Pu/T.
Why is there this disparity with a PPL student only being able to log time until sunset, even though it is still official day (unless with a night instructor in which case I presume all time can be logged with the relevant night hours going towards the night qual)?