It is always a good idea to turn your air conditioning unit down at night to save energy, but also to help maintain the unit. The unit should also be serviced regularly by a trained refrigeration engineer.
Restricted airflow and a dirty unit could be contributing to the problem. Most units will have an air filter under the front indoor plastic cover and grill, these can be removed, cleaned and replaced easily. The rest of the unit can be cleaned with a toothbrush, but it is important to be extremely careful and gentle. If you are in any doubt or you do not feel comfortable doing so, call a refrigeration engineer.
The most common reason for overnight freezing is having the thermostat set too low. If it is a digital unit set the thermostat no lower than 22 degrees C. If it is an analogue unit it should be about two notches below maximum, note that you should hear your box a/c cycle on and off as it reaches its desired temperature, if it is not then you have it set too low drop the temp until you hear the compressor turn off .The fan will continue to run.
If the problem persists then you should get a professional refrigeration engineer to have a look at it.