why do I get e48 only at night

Have you checked your electrical connections to your battery. Its possible that this is occurring because you have a drain on the electrical system buy having your lights and heater etc on, and its for some reason manifesting itself as an E48 EGR code. Trying turning on your lights during the day and all accessories you have on during you night drive, and test drive it for the same distance or more and see if you get the codes. If not, since the EGR is temperature sensitive it could be the difference in outside temperature that is causing the EGR issue during night driving and it should be replaced.