During Stage 4 sleep you may be vaguely aware of your environment, and some of you may experience hypnogogia.

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Quizplus · Accepted Answer

Stage 4 sleep, also known as deep sleep or delta sleep, is a stage of non-REM sleep where the brain produces slow delta waves, and individuals are usually not aware of their environment. Hypnagogia, the transitional state from wakefulness to sleep, occurs during the onset of sleep, not in deep sleep stages.

During Stage 4 Sleep You May Be Vaguely Aware of Your