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How do the amounts of deep NREM sleep and REM sleep change over a night?

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In a typical night of sleep, the distribution of deep non-REM (NREM) sleep and REM sleep shifts significantly as the night progresses. Sleep is organized into ultradian cycles, each lasting approximately 90 to 110 minutes, which repeat four to six times throughout the night.12

The progression of these stages changes over the course of the night:

First Third of the Night: This period is dominated by deep NREM sleep (specifically stage 3). The body prioritizes this restorative sleep early on, resulting in the highest percentage of deep NREM sleep during these initial hours.1
Middle Third of the Night: As the night continues, the duration of deep NREM sleep decreases, and the NREM portions of the cycles become increasingly composed of lighter stage 2 sleep.1
Last Third of the Night: The composition of sleep shifts toward REM sleep. The REM portions of the cycles lengthen as the night progresses, and the majority of REM sleep occurs during this final period.1

Overall, while NREM sleep accounts for approximately 75 percent of total sleep time, the specific balance between deep NREM and REM sleep is dynamic, favoring deep sleep early and REM sleep later.1