Which sleep stage is the deepest?

The deepest stage of sleep is generally considered to be slow-wave sleep (SWS), also known as deep sleep, which occurs during stages three and four of non-REM sleep. During these stages, the brain exhibits very slow brain waves called delta waves, making it difficult to wake a person from this state 1 2.

However, it is worth noting that REM (Rapid Eye Movement) sleep, which occurs later in the night, also has unique attributes. While it is not traditionally classified as the deepest sleep, it is a vital stage characterized by vivid dreaming and significant brain activity. REM sleep becomes deeper in older adults as the amount of slow-wave sleep decreases 3.