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When it's cold out, it's time to switch up your night clothes. Swap out cotton for some flannel pajamas. You can buy flannel tops and bottoms, as well as night shirts. Flannel is a great insulator that will help you retain your body heat. Look for a fun or cute print to liven up your flannel wardrobe. Looser is generally better when it comes to sleepwear. You probably move around during your sleep, and it's best to wear clothing that will easily move with you. Look for loose clothing, but make sure you won't get tangled up in it if you toss and turn. If you wear pajama pants, make sure that the elastic is loose and breathable. Your feet might be one of the coldest parts of your body. To help keep your whole body toasty, keep your feet warm by sleeping in socks. Choose socks that feel comfortable to you. Make sure they're not too big or floppy. You don't want them coming off in the middle of the night! If flannel doesn't seem to be keeping you warm enough, consider adding additional layers of clothing. You could try a thermal t-shirt underneath your pajama top. You can also add a pair of form fitting leggings underneath your pants or night shirt. If you get cold during the night, add another layer. If you get too hot, just peel one off. A lot of your body heat can escape through your head. If you're really cold, consider wearing a hat to bed. You can wear a ski cap or even a hunter's style cap with ear flaps. Whatever feels good to you is the right hat. You could choose to wrap a scarf around your head (not your face) as an alternative to a hat.
Wear flannel pajamas. Pay attention to fit. Wear socks. Try layers. Cover your head.