Like all electronic devices, the sensor in your camera emits some heat while it's being used. As the sensor warms up, its noise level increases, leading to more speckles in your pictures. This effect is usually slight and, in my experience, often unnoticeable; it is counteracted by the fact that the temperature of the air is usually falling as the night wears on.
Nonetheless, for maximum quality in images of faint objects, it may be beneficial to let the sensor cool down for 30 to 60 seconds, or possibly even longer, between long exposures.2
2 I want to thank Ken Miller for drawing my attention to this fact.
Most of the heat is generated while the image is being read out, not while the photoelectrons are being accumulated. For this reason, live focusing, with continuous readout, will really heat up the sensor, and you should definitely allow at least 30 seconds for cool-down afterward, before taking the picture. The LCD display is also a heat source, so it may be advantageous to keep it turned off.
No matter how much heat it emits, the sensor will not keep getting warmer indefinitely. The warmer it becomes, the faster it will emit heat to the surrounding air. Even if used continuously, it will stabilize at a temperature only a few degrees higher than its surroundings.
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