If your sheets or feet feel cold on a winter's night, we highly recommend the use of a hot water bottle, sometimes called a hottie in the UK. This small rubber water bottle contains about 1-2 quarts of hot water heated on the stove, and we covered it with a flannel cover (cost about $10 for the bottle and a few more $ for a cover). This will give you instant warm sheets by your toes! Just tuck it in a few minutes before bed. It radiates the heat all night. Before we installed our wood burning furnace, we allowed the winter temperatures in our home to drop down to 60 F during sleeping hours. We used a hot water bottle nearly every evening. It allows us to save energy, yet remain comfortable all night. Our programmable thermostat kicks on the heat around 5:30am and gets the house warmed up again before we get out of bed. This is truly green technology that works, and the rubber bottle will last at least several generations. It isn't new, but it is a time tested way of staying warm all night while your furnace remains off.
Go one more step and cover your bed with a feather-down comforter. For cool spring or late fall such a comforter alone will keep you toasty. Add the hot water bottle once winter kicks in. You can't beat this combination (unless you factor in skin-skin contact). You'll save money and fossil fuels all winter and your feet will thank you.