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Your body does its heaviest repair before midnight-ish — not at dawn
💡 Deep sleep and its repair-driving growth-hormone surge are packed into the early hours of the night, while REM builds up toward morning.
Sleep isn't one uniform state — it moves through cycles, and the deepest stage, called slow-wave sleep (or deep sleep), is heavily concentrated in the first half of the night. This is when your body does most of its physical maintenance. The largest daily pulse of growth hormone — which drives tissue repair, muscle recovery, and bone maintenance — is released during these early deep-sleep periods. Skimp on the first few hours and you blunt that repair window.
As the night goes on, the balance shifts. Deep sleep shrinks with each cycle, while REM sleep — linked to dreaming and emotional and memory processing — grows longer, dominating the hours closer to morning.
This front-loading has a practical consequence: when you sleep and how solid those early hours are matters, not just the total count. A consistent, undisrupted opening stretch protects the repair-heavy deep sleep, while the emotional and memory benefits of REM lean on your final hours.