Keep as cool as the other side of the pillow.
When you go to sleep at night, your body’s metabolic processes slow down. Without all that churning and flowing, your body temperature naturally drops, which is why people tend to find it more comfortable to sleep in a pleasantly cool room. If your room, bed, or body are too hot or cold, your body can’t reach that pleasant equilibrium (and in the case of heat, you have to put up with gross night sweats). Sleep is an absolute necessity, and if you’re having trouble reaching that ideal temperature, you’re not getting your best sleep.
If the problem is that your room or bed is too hot, the obvious solution is air conditioning. Unfortunately, not everyone has access to quality air conditioning. When I was in college, they wouldn’t even let you use AC until the summer, so spring was a sweaty nightmare. If AC isn’t an option, try more analog solutions. Open a window, plug in a tower fan, and aim it at your bed. Just having some extra air circulation can help you feel much cooler. If you’re a naturally hot sleeper, drink an ice-cold glass of water right before you go to bed for a pleasant, body-wide chill that should last you just long enough to get you to sleep. You might wake up sweaty, but at least you’ll actually get some sleep.
If you’ve got the opposite problem and your room is too cold, your weapon of choice should be layers. Wear pajama pants, keep your socks on, and wrap up in a big, fluffy blanket. Once again, though, sweat can complicate matters. A lot of folks can’t wear socks to sleep because their feet get sweaty, a problem that can be exasperated by certain kinds of blankets. Make sure the material in your blanket doesn’t disagree with your skin, and choose a sweat-wicking design. If you don’t have a single big blanket, try layering a few smaller blankets to get some proper insulation.
If you’re open to dropping some cash, there are lots of space age solutions for temperature regulation in bed. I just recently bought a blanket that’s one side cotton, other side space-age coolness absorbing fabric. It absorbs ambient cool air to keep the blanket an even temperature while keeping you insulated. It’s a wonderful thing.