Growth hormone (GH) is produced by the pituitary gland and plays a central role in muscle maintenance, fat metabolism, bone density, and tissue repair. Unlike some hormones that respond mainly to food or stress, GH's biggest natural trigger is deep sleep — the majority of your daily GH release happens in the first few hours of quality sleep.
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​High-intensity exercise and fasting also raise GH, while poor sleep, high blood sugar (and the insulin spikes that come with it), and excess body fat all suppress it.
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​GH naturally declines with age, and that decline is part of why body composition tends to shift toward more fat and less muscle over the years, even without major changes in diet. It's also a good example of why sleep quality deserves as much attention as diet and exercise in any conversation about weight and body composition — you can't out-train a consistent lack of deep sleep.