If you go to forums that cater to guinea pig owners, you will notice that the opinions are split between those who think that you should cover the cage at night and those who say ‘no’ to this practice. The thing is guinea pigs need a space to hide inside the cage, as they are afraid of predators. It doesn’t matter that they are safe in your home, as they cannot know that.
In case the cage you’re using has no hiding space, you should correct that, or cover the cage at night, to allow your cavies to have a quiet sleep. In case you have other pets in the house, such as cats and dogs, it is also a good idea to cover the cage, to prevent these animals from scaring the cavies.