You know I was wondering about this. I'd read a study on breast tumours in humans that links lack of darkness as a cause of reduced restful sleep possibly linking to tumour growth. I've always covered the upstairs cage with a dark fleece as the window is really bright up there, but I've started covering the condo with a sheet in the daytime. The small window in the hall is really bright and I figure if they are sleeping they don't need the light. Rats in the wild would be sleeping in dark tunnels underground. Not sure if it will help, but I can't see it would hurt.