Final answer:
Delta waves are the large, slow brain waves associated with the deepest stages of sleep, specifically during stage 3 NREM sleep, and denote when the brain is in deep relaxation.
Step-by-step explanation:
The large, slow brain waves associated with sleep are called delta waves. During stage 3 of non-REM (NREM) sleep, which is often referred to as deep sleep or slow-wave sleep, delta waves are the predominant pattern of brain activity.
These waves are characterized by their low frequency (less than 3 Hz) and high amplitude, marking the deepest stages of sleep when it is most difficult to wake someone. In contrast, sleep spindles, which are short bursts of rapid, rhythmic brain activity, occur during stage 2 of NREM sleep and are thought to be important for learning and memory.