My mom has a story about heating up stones and wrapping them in cloth to put at the foot of the bed on cold nights during her childhood. I think the low-tech heat source sounds quaint and have come to do something similar in my home:
I heat a rice bag in the microwave and toss it under the blankets at the foot of the bed. It provides heat at the beginning of the night when you're trying to fall asleep. During this transitional season when there always seems to be a chill in the air, that additional heat is more than welcome.
Plus, it's safer than an electric blanket or heating pad (I've never liked the idea of plugging in my bedding). If your feet are also a little cold during these March nights, check out how to make a rice bag of your very own right here.
Image: Orna & Reid Willis' Worldly Live-Work Loft