Greg provided some supplemental information to his original post. He said, in part, "When backpacking I get to sleep around 8:30 pm, but then get cold around midnight, to the point that I wake up shivering. This can occur in a 20 bag, on good pad, on a 35 night. If I then eat a couple hundred calories I can sleep for another hour or two before things repeat. Then, about 30 minutes before the 6 am alarm, it seems like my basal temperature plummets and very little can warm me up.
I just put these pieces together, after spending a year trying to figure out why quilts, sleeping bags, and pads were not keeping me warm. They were not at fault, I was.
I know I am running a calorie deficit though out the day 2200 calories for 20 miles over 10 hours, on average. I do not feel hungry or low on energy. I do notice an emotional "slump" around 2 pm, especially if the temperatures are above 70. I drink around a half liter of water an hour, depending time of day and temperatures.
I'm a 63 year old male, 5'9", 175# +/-, with managed hypothyroidism. I have good endurance, doing several 100+ mile trips a year.
My questions are: What can you tell me about metabolic cycles, sleep, and the optimal night-time mix of carbohydrates and fats to address this issue? Or what reading would be appropriate for a lay person to determine an approach? I'm as much, or more, interested in the "Why" than a prescriptive solution."
If the heat your body generates matches the heat your body looses through your insulation you will be thermo-neutral which provides thermal comfort for any time period. In your case either your body is generating less heat than the average male, as defined by the EN13537 standard at 46 W/m2, or your sleeping system is generating less insulation than what is required for a 20 degree bag (6.85 clo which equals 1.06 m2K/W) I strongly suspect that feeling cold when sleeping is caused by your basal metabolic rate being abnormally low as a result of your hypothyroidism. This is a condition in which the thyroid gland does not make enough thyroid hormone; one symptom of this condition is a reduced BMR. Using the thermic effect of food to compensate is only a two hour solution.
I suggest that you have your BMR measured at any local facility that does indirect calorimeter tests. Many gyms use the simple handheld BodyGem to provide this test for about $50 and up. Once you know your BMR, you can easily determine the additional insulation you require to be thermo-neutral. It = (K * DT * A) / H is the heat conduction formula. Since K, DT, and A are constants for this problem, the amount of insulation you require can easily be calculated from your actual BMR. Alternatively you can just determine this value by wearing progressively warmer sets of insulating clothing with your sleep system to determine your thermo-neutral point. You can then estimate the clo value of the supplemental clothing you used to order a higher insulated sleep system. Alternatively you can just continue to use the appropriate supplemental clothing with your sleep system.
Neither the EN13537 BMR nor your unique BMR includes the post 1-2 hour thermic effect of foods eaten. A meal will provide a maximum thermic effect for the period 60 to 120 minutes after consuming it and then rapidly drop off. Your two hour sleep / eat again cycle is generating temporary extra heat from the food’s thermic effect but this is only a 2 hour benefit regardless of what mix of protein (biggest benefit), carbohydrates, and fat that you eat.
Thermic effect of food is the increment in energy expenditure above resting metabolic rate due to the cost of processing food for storage and use. A common number used to estimate the magnitude of the thermic effect of food is about 10% of the caloric intake of a given time period, though the effect varies substantially for different food components. In general, the typical thermic effect of protein is 20%–35% of energy consumed; for carbohydrate, this number usually falls between 5% and 15%; and for fat you can assume it is about the same as carbohydrates although some studies have found that fat has a lower thermic effect compared to carbohydrate while others have found no difference between the two.
Your “30 minutes before the 6 am alarm” maximum coldness is a result of two factors. The primary factor is that this is normally the lowest temperature for each day. Secondarily you are no longer experiencing any residual thermic benefit to offset your insulation deficit.
In summary, the thermic effect of food provides a maximum benefit for only about 2 hours. For a full night’s sleep benefit, add the incremental insulation you need to be thermo-neutral with your lower than average BMR.