by Dr. Brooke Azie-Rentz

When it comes to sleep and fitness, intense exercise may not always be the answer. In a rested body (one that has slept at least 7 hours per night on a regular basis) high intensity exercise promotes fat loss, muscle gain, insulin receptor sensitivity, positive emotions and restful sleep. In a sleep deprived body, in contrast, high intensity exercise does almost exactly the opposite. When the body has slept fewer than five hours on any given night (or fewer than seven hours on a regular basis) resting cortisol levels become elevated,  pushing the body into an over-stressed state called catabolism. In this state, the body turns  to stored lean body mass for fuel, causing insulin receptors to down regulate and fat storing hormones to become elevated. Adding intense exercise to this mix only further upsets the apple cart.

The key to making sense of this is to remember that exercise itself is a mild stressor. Whenthe body’s resting cortisol levels are low, the small amount of cortisol stimulated in response to exercise is followed by a much larger wave of positive hormones such as the human growth hormone, our biggest muscle-builder and fat-fighter. When stress from exercise is applied to an already taxed, stressed-out body, however, cortisol levels only climb higher and our “super hero” hormones simply can’t keep up. The formula, therefore, is easy to remember: “Good sleep + intense exercise = good idea. Bad sleep + intense exercise = bad idea.” That said, a poor night’s sleep is no reason to skip the gym all together. When we haven’t slept, low intensity and restorative exercise modalities such as yoga can help lower resting cortisol levels, which will help to mitigate the            metabolically damaging consequences of catabolism.

Bottom line, if you’re well rested, go hard. If not, go for a walk — ideally outside. If you can’t get outdoors, consider an hour of  stretching or a very low intensity circuit. Your stressed out body will thank you. One final note on sleep and exercise? Beware the late night lift. Exercising within 2-3 hours of bedtime can be a stimulant and keep you from those precious zzz’s.