We always heat Nalgene bottles and put them at the foot of our mummy bags when winter climbing. After heating the water on the MSR stove we then stick the hot bottles in the foam water bottle insulators made by OR so they actually stay warm all night long. Nothing like being toasty warm with 8" of 800 fill down and a hot water bottle at your feet while it is -10F, blowing 60 MPH outside with the tent singing and having your feet toasty warm. Ahh the joys of winter!!

Mt. Washington, NH