Delta waves are most clearly associated with sleep.

NREM-3

Explanation:

NREM-3 is most commonly known as 'Deep Sleep' .In this stage of sleep, you're at the hardest point to be waken up, and you will feel groggy /disoriented if you're forced to wake up in this state.

During this state, your brain consist of long and slow delta waves (which is not exist in other stage)  , indicating that there are very little activity in your brain and your brain is expriencing a restorative mode.

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