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Sleep & Metabolism


Last week I had a conversation with an athlete about the accuracy of the calorie burn stats on their smartwatch. Some people take this number very seriously, sharing this on their training logs and with other people in their training group (that is an issue for a soapbox blog), but does your watch really know how many calories your body has burned?

According to my Garmin, 2000 calories in minus 500 burned means I need 500 calories to refuel. The math is so logical and pretty, but does your watch care about what type of calories you are burning? Was it a piece of chicken or a chocolate chip cookie? Does it matter? Well, do wood and plastic burn at the same rate? Does your watch know about your medical and dietary history? Does your watch regulate your hormones and know what your metabolic efficiency is? All of these are factors that go into your calorie-burning ability.

Cortisol is a major player in this game. Cortisol is the hormone that is released when your body is under stress. Good stress, bad stress, controllable stress, and non-controllable stress, cortisol does it all. Cortisol’s partner in crime is the glucocorticoid hormone, which is vital in metabolizing glucose. When cortisol is around, glucose doesn’t want to leave the party, so your metabolism is less efficient in breaking down glucose. Cortisol also encourages triglycerides to hang out. Cortisol is a rebel that throws off the balance of your metabolism and unfortunately, your metabolism doesn’t always bounce back as efficiently. Your metabolism is dependent on how many hormones interact together, and how they have acted together in the past will change their relationship permanently.

There is a hormone that is an enemy of cortisol, melatonin. The two are basically exact opposites, meaning that when your melatonin levels are at their highest, your cortisol levels should be at their lowest. Your sleep habits can seriously and permanently affect how your metabolism works. If you aren’t getting enough sleep, your cortisol will be high and you are deregulating your hypothalamic-pituitary axis, the system that triggers all of your metabolic hormones to process glucose, fats, and proteins. Research shows that 6 consecutive nights of restricted sleep (defined as 4 hours of sleep or less) can decrease the effectiveness of your metabolism by 60%. It takes time for your metabolism to get back on track, and in some cases, these episodes permanently change the efficiency of your metabolism.

Sleep and stress are obviously major players in how your metabolism burns calories, and this is the theory behind new technologies that track your sleep and strain. Keep in mind that all of these technologies do not know your past history that may have affected your metabolism. Your best bet is, to be honest with yourself about what your body is asking you for: fuel, sleep, recovery, and exercise. Pay attention!

Source: Hirotsu C., Tufik s., Andersen M. Interactions between sleep, stress, and metabolism: From physiological to pathological conditions. Sleep Science. September 2015:143-152

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