The most likely cause is that the minimum room temperature has been set too high which means the boiler will keep coming on to maintain the set temperature, especially as the general ambient temperature falls at night. Our recommendation is to set a low temperature between 8°-12°C.
You can also check the obvious things like whether the programmer or time switch have been inadvertently left on.
If you are still having problems, it is advisable to call a qualified Gas Safe registered engineer to look into the issue further.