Phase 5 is the REM sleep with fast waves. It is called like tis because of the quick eye movement there is. The muscular tone decreases, and physical activity increases, decreases the deep sleep. It is also hard to wake up in this phase even if it is one of the last phases of sleep. In REM sleep it is when you have the most nightmares, and it is the last step of sleeping. The most recent when waking up, therefore, when you wake up it is possible that the dream you remember most is the most recent (it could be a nightmare) and you could have dreamed it just before you wake up. Source: Mabel Ovando (neuropsychologist and clinical psychologist).