In our last session the PC's prepared an attack on drow. One PC had caster level 13 and intended to cast darkvision before resting with a duration of 13 hours. So after 9 hours resting and preparing spells for the next day there would still be 4 hours of darkvision left.
Is that legal? It does not conflict with the recent casting limit because the spell is cast more than 8 hours before preparing new spells. So, technically it is ok, I suppose, but it feels like an "extra spell" and it opens the door for preparing a fight with spells cast the other day.