I had the same problem, not just in the swamp but also in the evenings and when it was foggy. Getting old has its drawbacks. I ended up using Valheim Plus mod. It not only has a feature that removes the heavy fog from the game, but there is also a setting allowing one to have the same (or brighter) light levels during evenings as daytime. Even though my setting is set to a very low level, it makes evenings in the swamp brighter than it is during the daytime. The strain on my eyesight from playing Valheim made it impossible for me to play for any length of time. I complained to my son about it and he said, "Dad, I'm sure there's a mod for that". I love the swamp at night now in my full Fenris armour.