Deep sleep, also known as slow-wave sleep, is a crucial stage of the sleep cycle where the body undergoes significant restoration. During this phase, the brain produces delta waves, which are slow brain waves that help with healing and growth. This is when the body repairs tissues, builds bone and muscle, and strengthens the immune system, making it essential for overall health. In deep sleep, it's harder to wake someone up, and they may not respond to external stimuli. This stage typically occurs in the first half of the night and is vital for feeling refreshed and alert the next day. Without enough deep sleep, people may experience fatigue and difficulty concentrating.