First, the ferry is out of sight of land except for when it leaves Copenhagen in the afternoon, and when it sails up the Oslofjord in the morning, so you don't necessarily miss much during the hours of darkness. Secondly, depending on what time of the year you are actually travelling, it may not get dark at all.
I think we decided to drive both ways, going up in one day, and coming back stopping in Gothenburg...sounds like about an 8 hour drive each way? The trip is next week, so I figgure not a ton of sun, but not a small amount...