T Nation

During sleep

Does anyone know of the energy being burned during the 8-10 hours of sleep, how many percent is from muscle, glycogen and fat?

Is it the same as 8-10 hours of fasting or is it different because of the hormonal environment during sleep?

Also, how many percent of muscle glycogen is depleted during sleep - 30%, 50%, 100%?

It probably depends on what type of diet you normally eat. Obviously if you eat a high carb meal before bed you’ll be burning carbohydrate for a significant portion of your sleep time but for the most part your body uses fat for fuel during sleep. Muscle glycogen will be depleted very little as your liver should have more then enough glycogen to supply brain function etc. It’s a myth that muscle glycogen levels are used up during sleep, what’s depleted when you wake up if anything is liver glycogen which can easily be refilled with 30-50 grams carbohydrate. As far as protein use goes, the free amino acid pool might become depleted during this time but this doesn’t mean that your body is eating away at your muscle, rather it just means that no extra protein is floating around for growth.

Like Kelly so beautifully explained, there’s probably a mixture of fuel being burned during sleep, but the portion of carbs to fats is probably determined by your pre-bed feeding.

In addition, glucose needs will be met primarily by the liver, both through: 1. glycogenolysis, and 2. gluconeogenesis. The latter will probably be insignificant at this level of energy expenditure and will be a result of metabolic by-products (as opposed to amino acids). Glycogenolysis of liver glycogen will probably dominate the glucose substrate being utilized. At rest, the liver will supply about 5g/hr (i.e. 20kcal/hr). Therefore, you will probably deplete liver glycogen by 40-50g depending on your sleep length, which corresponds exactly to what Kelly stated. Note, though, that the rate of liver glycogen depletion is affected by: 1. physical activity, 2. exercise, and 3. incoming food (i.e. carbs).

As far as the amount of kcals burned during sleep, this equation should give you a pretty decent estimate:

Energy Expenditure (kcal per min) = 0.0175 kcal/kg/min x 0.9 x body weight (kg)

*Note: the 0.9 is a MET (i.e. metabolic equivalent) for sleeping and is a constant.

Finally, as Kelly illustrated, muscle glycogen is only used during activity involving the particular muscles. In addition, during very prolonged, strenuous exercise, muscle glycogen can be mobilized from inactive muscles, but only by first being converted into lactate, which then goes to the liver to be converted to glucose (i.e. gluconeogenesis).

Thanks Kelly and Timbo!

Timbo, once again you came through and then some! OK, using the formula you provided, I calculated my calorie expenditure to be 69 per hour (I’m 73 kg). Does that sound right? Now, is the equation the same when awake (not doing exercise) or does it only aply during sleep?

A7, you know I’d never let a brother down!

The same equation does not apply to waking hours. There are tons of METs (i.e. metabolic equivalents) for different activities.

Sitting quietly has a MET of 1.0
Standing quietly has a MET of 1.2
Those are just figures for inactivity. When you get into sitting and typing, it’s closer to 1.5. With housework, it can range from 2.5 to 4.5!

You’d just sub in any of those constants for the 0.9 in the equation above. You could theoretically do this for your whole day’s energy expenditure.

By the way, your calculations sound right.