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Why is most deep NREM sleep concentrated in the first third of the night?

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The concentration of deep NREM sleep (specifically stage 3, or slow-wave sleep) in the first third of the night is primarily driven by the body's homeostatic sleep pressure.12

This pressure builds up during periods of wakefulness, making an individual progressively sleepier as the day continues. When a person finally falls asleep, this accumulated homeostatic drive is at its peak, which facilitates the immediate onset and high proportion of deep, slow-wave sleep. As the night progresses and these homeostatic needs are met, the amount of deep NREM sleep in subsequent sleep cycles decreases, while the duration of REM sleep increases.312

This pattern is a fundamental aspect of the ultradian cycles that structure human sleep, where the body prioritizes recuperative slow-wave activity early in the sleep period following extended wakefulness.12