They talked about this during a press conference today. Theoretically it could operate at night, but the power draw to heat the motors and joints would be exces

They talked about this during a press conference today. Theoretically it could operate at night, but the power draw to heat the motors and joints would be excessive since it's so cold after sunset. So they don't and allow the MMRTG to charge the battery (and keep the heater for the electronics) while most of the machine is idle.