The stimulus is a change, which can provoke the change in the body of the living organism. Generating a response to the stimulus is the characteristics of the living beings. The drop in the body temperature is a sort of stimulus response in the body of the organism, which generates a response for resting (slowing down metabolism) and trapping heat from the sunlight, in case of lizards.
The stimulus in this case is the drop down in body temperature and the response is slowing down the body activity and trapping of heat energy from sunlight.