Quick answer
Usually caused by a fridge, freezer, or immersion heater switching on automatically. The element may be developing a fault that only appears under load.
Several appliances in your home switch on and off automatically at night — fridges, freezers, and immersion heaters are the most common. If one of these has a developing fault, it will trip the RCD every time it cycles on. Try turning off the immersion heater at its spur switch overnight. If the tripping stops, the immersion heater element is likely failing and needs replacing. For fridges and freezers, try plugging them into a different circuit temporarily. Never leave a repeatedly tripping RCD unattended — if the fault is in a fridge or freezer, the appliance itself may also be at risk of failure.
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