Assuming the RCD isn't faulty, then basically you have leakage to earth somewhere. The normal culprits are 'wet' appliances (e.g. washing machine, dishwasher etc), another common one is outside lights (e.g. security floodlights), the seals perish over time, and then water gets in - if you have any, take a look and see what the seals are like, and if there's any water inside.
Have you bought any new electrical equipment recently, as if so it might be something to suspect, also, is anything set to come on on a timer overnight, as that would be an obvious thing to check...