My T's office has big windows, but I like when it's overcast outside when I see her. It feels cozier. She will sometimes light a candle (she has asked before if it's okay with me), and she uses lamps instead of overhead florescent lighting. I think it helps with intimacy and opening up. I could totally see my T continuing with a session in a power outage. It takes A LOT to make her cancel. We have had sessions during tornado warnings before.