Final answer:
REM sleep is associated with virtually zero levels of Norepinephrine (NE), whereas sleep spindles and K-complexes are most commonly found in NREM stage 2 sleep.
Step-by-step explanation:
The sleep stage associated with virtually ZERO Norepinephrine (NE) levels is REM sleep. During REM sleep, there is a marked decrease in overall muscle tension and brain waves are very similar to those observed when a person is awake. This is the period of sleep in which dreaming occurs, and the body experiences paralysis of voluntary muscles with the exception of those that allow circulation and respiration.
Sleep spindles and K-complexes are most often associated with stage 2 of NREM sleep, as outlined in the provided reference material. These brain waves are active during this stage and serve as markers that differentiate it from other sleep stages.