When alpha brain wave activity increases during NREM stage 3 sleep, people often report waking without feeling refreshed, regardless of how long they slept.

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Also known as deep sleep or slow-wave sleep, Stage 3 is a phase of NREM sleep defined by the presence of high-amplitude, low-frequency delta waves. During this stage, physiological functions such as heart rate and respiration decrease significantly, making it very difficult to awaken the individual.

Stage 3 Sleep (Slow-Wave Sleep)

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

Delta waves are low-frequency, high-amplitude brain waves that characterize NREM stage 3 sleep. Among sleeping brain wave patterns, they have the lowest frequency and highest amplitude, with a frequency below $$3$$ Hz.

Delta Waves

A polysomnograph, which records physiological changes during sleep, visually demonstrates the characteristics of delta waves. As seen in recordings from Stage 3 sleep, these waves are distinguished by their low frequency and high amplitude. Comparative charts of brainwave activity across different sleep stages often highlight Stage 3 to emphasize this unique, slow-wave pattern.

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