The hormone leptin is intricately involved in the regulation of appetite, metabolism and calorie burning. Leptin is the chemical that tells your brain when you're full, when it should start burning up calories and, by extension, when it should create energy for your body to use. It triggers a series of messages and responses that starts in the hypothalamus and ends in the thyroid gland. The thyroid gland controls the way your body stores and uses energy.
During sleep, leptin levels increase, telling your brain you have plenty of energy for the time being and there's no need to trigger the feeling of hunger or the burning of calories. When you don't get enough sleep, you end up with too little leptin in your body, which, through a series of steps, makes your brain think you don't have enough energy for your needs. So your brain tells you you're hungry, even though you don't actually need food at that time, and it takes steps to store the calories you eat as fat so you'll have enough energy the next time you need it. The decrease in leptin brought on by sleep deprivation can result in a constant feeling of hunger and a general slow-down of your metabolism.
The other hormone found to be related to sleep and weight is ghrelin. The purpose of ghrelin is basically the exact opposite of leptin: It tells your brain when you need to eat, when it should stop burning calories and when it should store energy as fat. During sleep, levels of ghrelin decrease, because sleep requires far less energy than being awake does. People who don't sleep enough end up with too much ghrelin in their system, so the body thinks it's hungry and it needs more calories, and it stops burning those calories because it thinks there's a shortage.
Some scientists hypothesize that these hormonal changes that occur during sleep are the result of an evolutionary process that ended up with humans who could survive the food shortages of winters. Traditionally speaking, winters have long nights and little food, and summers have short nights and an abundance of food. With shorter nights comes less sleep, less leptin and more ghrelin, making the body eat as much as possible and save those calories for the long winter ahead. With winter comes more sleep, meaning more leptin and less ghrelin, both of which tell the body it's time to burn those calories it stored during the summer.
Sleep deprivation has also been found to increase levels of stress hormones and resistance to insulin, both of which also contribute to weight gain. Insulin resistance can also lead to type 2 diabetes.
The National Sleep Foundation offers the following tips to help make sure you get enough sleep for your body to function optimally:
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