Tip for Turndown service

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I've always tipped in the morning for housekeeping service in a hotel, but in those instances where the hotel has turndown service, I never have - usually because I don't know about it until I come back to the room afterwards. When I am in the room, I turn the service down. What is the protocol for tipping the turndown service at night? Am I supposed to tip, and if so, how much?