Hi!
There are 2 distinct stages of sleep: Non-REM sleep and REM sleep. The difference between them is the absence or presence of Rapid Eye Movements.
The Non-REM sleep (or slow sleep) is the stage of sleep where the brain activity is at its lowest point, there is a predominance of delta waves, and the muscular tone decreases.
The REM sleep (also known as paradoxical sleep) is the stage of sleep where dreams appear, the brain is at a high and fast activity level, but there is a total relaxation of the muscles.
So to answer your question: Brianna is in a quiet period of sleep where her heart rate and respiration are slow, and her brain activity is at its lowest point. sleep researchers would say she is in NON-rem sleep.stage 5 sleep.