Sleep quality, duration, and timing play a major role in the release of certain hormones that impact appetite, blood sugar levels, insulin sensitivity, and even body composition. 


Your body runs on a “circadian rhythm," which is a 24-hour cycle that alternates between wake and sleep. This biological clock controls processes that occur every day, so when sleep is subpar, this can really throw your body off and negatively impact the nightly release of hormones in the following ways:


  • Cortisol is increased, which can contribute to higher blood sugar and muscle breakdown

  • Growth Hormone is reduced, potentially causing impaired fat breakdown (lipolysis)

  • Ghrelin is reduced and leptin is increased, causing an increase in appetite

  • Sensitivity to the hormone insulin is reduced, contributing to higher blood sugar 


These hormonal changes are a perfect storm for adding unwanted pounds since they unfavorably impact metabolism and increase hunger. 


New research also suggests that sleep deprivation causes physiological changes that mimic "metabolic adaptation”—the slowing of metabolism that makes fat loss more difficult for extreme dieters. This can be a double whammy for those in a calorie deficit already struggling to lose weight.


In a study on overweight adults dieting to lose weight, getting insufficient sleep was shown to interfere with fat loss and to lower sympathetic nervous system activity and resting metabolic rate.  Getting 5.5 hours of sleep as compared to 8.5 hours of sleep decreased fat loss by 55%, and increased muscle loss.


So, not only does subpar sleep sabotage fat loss and muscle building by altering hormones and their behavior, but it can also do this by impairing training performance and recovery. 


To prevent the negative effects of poor sleep, aim to get at least 7-9 hours of sleep (some people may need a little less, and some may need a little more).