Also known as slow-wave sleep or NREM Stage 3, deep sleep is the most restorative phase of the sleep cycle, crucial for physical recovery, immune system function, and tissue repair. Brain waves during this stage are slow and synchronized, and it becomes much harder to wake someone. While some brief, non-vivid dreams can occur during deep sleep, this phase is not typically associated with rich, story-like dreams. Instead, that level of mental activity takes place during REM. Deep sleep helps your body heal, while REM sleep helps your mind dream. Understanding this difference is important for recognizing how each phase contributes to overall well-being.