why does my heating switch itself on in the morning?

i have a valiant combi and the last two mornings i have noted the heating has come on very early in the morning. my boiler is sited in the garage is this normal is it due to cold weather i note there is a thermostat in the garage could this have soemthing to do with it. i usually just switch my heating on when i need it this seems such a waste as i am in bed when the heating is coming on please advise thankyou