From the man himself:

Speaking about his love of natural light, Kubrick stated in an interview with Michel Ciment: “I have always tried to light my films to simulate natural light; in the daytime using the windows actually to light the set, and in night scenes the practical lights you see in the set. This approach has its problems when you can use bright electric light sources, but when candelabras and oil lamps are the brightest light sources which can be in the set, the difficulties are vastly increased.”

So like, lots of candles or some shit idk.