I'm one of those people, if I get under cold covers, my feet just clam up, and don't warm up, so I won't fall asleep till about 2 or 3. When I lived at our school with only solar electricity for 20+ years, I used a hot water bottle or two. Now that I'm in a house that has mains power, I've used an electric mattress pad for two winters.
My tips for hot water bottles: Yes, fill it with boiling water! In fact in the coldest part of winter, I preheat it like a teacup: fill it with boiling water, wait a few seconds, then pour the water back into the pot and reboil it! I wear socks to bed in winter anyway, so it doesn't burn my feet. I lay the rubber bag-like part flat on the counter and bend the neck upright to fill it, to reduce "burping" up of boiling water, air pockets, and spillage. Then push it gently to get the air out before closing. Don't overfill: two floppy hot water bags are more comfortable than one overfilled one. One for the knees, one for the feet, in January. Mmmmm....
My tips for electric mattress warmers (called electric blankets in India): The instructions on the ones sold here say put it under your bottom sheet, and I like that. If you're concerned about synthetic materials next to your body, that helps. If you're concerned about an electromagnetic field around your body, then use it to preheat the bed and then turn it off when you get in it. If you have good enough covers (and good enough circulation to your feet) that should be enough to keep you warm all night. This winter, I'm planning to put a foam camping mattress under my mattress, because in previous winters I felt like heat leaked away downwards overnight. (BTW it's not "heat rises" but only "hot air rises" -- Heat can conduct away equally up or down).
The electric mattress pad uses only about 150W (adjustable for high, medium, low heat, but I use mine on high, 140W for the coldest part of winter). If you use it to preheat the bed before getting in, it's a very small amount of electricity each night. Less than 1/10 of what any kind of stand alone electric heater would use, such as a blower or a parabolic thingy, especially if those have to be left on for longer than the mattress pad.