The Obesity Code: Unlocking the Secrets of Weight Loss (Why Intermittent Fasting Is the Key to Controlling Your Weight)

by Jason Fung

A single night of sleep deprivation increases cortisol levels by more than 100 percent.27 By the next evening, cortisol is still 37 percent to 45 percent higher.28 Restriction of sleep to four hours in healthy volunteers resulted in a 40 percent decrease in insulin sensitivity,29 even after a single sleep-deprived night.30 After five days of sleep restriction, insulin secretion increased 20 percent and insulin sensitivity decreased by 25 percent. Cortisol increased by 20 percent.31 In another study, shortened sleep duration increased the risk of type 2 diabetes.32 Both leptin and ghrelin, key hormones in the control of body fatness and appetite, show a daily rhythm and are disrupted by sleep disturbance. Both the Wisconsin Sleep Cohort Study and the Quebec Family study demonstrated that short sleep duration33 is associated with higher body weight, decreased leptin and increased ghrelin. Sleep deprivation clearly will undermine weight loss efforts.34 Interestingly, sleep deprivation under low-stress conditions does not decrease leptin or increase hunger,35 which suggests that it is not the sleep loss per se that is harmful, but the activation of the stress hormones and hunger mechanisms. Getting enough good sleep is essential to any weight loss plan.