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1) Insulate inside. Sleeping bag liners (silk, synthetic, or wool) can add 5 to 25 degrees of warmth.
2) Insulate below. Use an inflatable pad to keep you off the cold ground, and layer foam on top to keep body heat close.
3) Stay dry. Lay raingear over your bag to shield it from frozen condensation. Avoid exhaling inside your coccoon—moist breath will leave it damp.
4) Shake it out. Clumpy insulation leaves cold spots. Let your bag fluff out a half hour before bed. Once in, shake your legs to evenly redistribute down.
5) Eat, eat, eat. Down a chocolate bar (and a hot beverage) before snuggling up; digesting carbohydrates and fats raises body temperature 30 minutes to an hour after consumption.
6) Maintain circulation. Warm body and cold toes? Remove tight socks or leggings to keep the blood flowing. Instead, wear loose clothing or—if you must—go bare.
7) Make a space heater. Fill a non-insulated bottle with boiling water, seal tightly, encase in a sock, and place in the bottom of your bag to keep feet warm.
8) Fill in gaps. Minimize the space you need to heat. Bag too long? Stuff the end with extra layers.
9) Exercise. Do sit-ups to warm up your bag pre-shut-eye. Add more reps whenever you get chilly.
10) Still cold? Insulate by piling dry leaves or pine branches under your tent.