4 Answers

Possible reasons:

1. A fault in the thermostat.

2. Heat leaking out of house through draughts ( drafts)

3. Poor insulation

If this is sudden, I would rule out the windows and insulation and think of a mechanical failure in the system, either the thermostat or a sensor unit on the heating unit.  Is it a furnace (hot air) or a boiler (steam or hot water)?  If the boiler, maybe you need water in the feed tube (steam).  These are the things that the lay person can check and correct, otherwise time to call the heating repair contractor. 

Perhaps your clothing is too thick.

That shouldn't be happening. Something is wrong with your heater. I would call a heating repair company to take a look at it. It will also save you a lot of money once your heater is working properly.

Shelly Slader |  http://airtigers.com/heating.html