During Autumn/Winter, I have mine set to come on during the night if it drops below 14 in the house, that way, when it's set to reach 18 at 5am onwards it doesn't use to much energy to reach that temp. That's my thought process anyway!
Eg if the house dropped to 10 overnight, it would have to use quite a bit of energy to bump it up to 18 in the morning so that the house is warm when we get up.
What temp to you have yours set to?