Light Switches

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Yes, I have a thought... Perhaps it is meant to illustrate that Pierre and Franca are together a long time. You notice how they follow a pattern of switching off and on lights when they go to bed, they follow this pattern very smoothly, as if they've been doing it for years. So it is to show that they've fallen in a sort of routine; their marriage. Pierre breaks through this by starting a relation with Nicole, while Franca sort of sacrifices herself because she believes her man is doing business.
The first scene with the light switches, in Pierre's hotelroom in Lisbon, however doesn't really fit to this description. He turns his lights off, but when he has a date with Nicole, after she called him back, he switches all the light on. Haven't got a thought about this one...

"Kid, the next time I say, "Let's go someplace like Bolivia", let's go someplace like Bolivia."

I took one instance as Truffaut's playing with the convention of expressionist lighting. When Nicole calls Pierre back after their first encounter in the elevator he expresses his excitement by brightening the dim shadowy room (which reflected his initially disappointed disposition) by turning all the lights in his hotel room on. I viewed this as a somewhat ironic gesture, in tune with Truffauts preference for aesthetic realism.