The memoirs of a nurse in the Scottish Women's Hospitals - an all-female unit that served with the Serbian Army in WWII and took part in the terrible retreat through the Albanian mountains in the winter of 1915 - relates how every member of the unit had her rubber hot water bottle, and amazed and amused their Serbian comrades by always filling them at night before lying down in their blankets (there were no tents) , and even more by using the not-quite-cold water to wash every morning before starting the day's march. The macho Serbs thought this was quite bizarre and evidence of the well-known madness of the British people. But 200,000 Serbs died of cold, hunger and disease on that march, and none of the women of the SWH did.
Obviously, cuddling a hot bottle would mean you would sleep better, and thus keep up your strength. Washing daily would protect you from infection, possibly not much given that I suspect it was only face and hands, maybe feet, but would certainly be good for morale. And, not having to expend energy keeping one's body from freezing all night would significantly conserve calories. But how *many* would it conserve? How many slices of bread a day, say, was each nurse better off for using her bottle?
I realise the variables are far too many for a definitive answer, but can anybody give at least some idea?