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What happens during the N1 stage of NREM sleep?

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The N1 stage of NREM sleep is a transitional state of drowsiness that occurs at sleep onset or following momentary arousals during the night. Typically lasting between one and seven minutes, this stage represents the brain's activity settling from wakefulness into sleep.12

Electrically, this transition is observed on an electroencephalography (EEG) tracing as a shift from the alpha waves (8–13 Hz) characteristic of wakefulness to a low-voltage, mixed-frequency pattern with a significant presence of theta-wave activity (4–7 Hz). While the distinctions between sleep stages are categorized for study, it is important to recognize that the physiological boundaries between them are often subtle and less clear than the terminology suggests.12