The "paradoxical sleep" takes its name from the paradox that is observed between the important neuronal activity of the brain (very close to the awakening) and the total absence of muscle tone outside the heart, respiratory and ocular muscles. This stage of sleep is also called "REM sleep" for "Rapid Eye Movement sleep" because, during this phase, the eyes of the sleepers make rapid jerks under his closed eyelid.
It is this stage that we usually associate dreams.
Please note that more and more studies seem to show that we also dream outside REM sleep phases. However, paradoxically, the dreams reported seem more frequent and vivid.
In the adult, paradoxical sleep occupies about 20-25% of the total sleep time and is more present at the end of the night. In fact, when you do not sleep enough, it is this type of sleep that is most often amputated. Moreover, by accumulating a sleep debt, this proportion also decreases since the deep sleep tends to increase.
It is therefore normal to see how much REM sleep varies from one sleep cycle to the next during the same night, or from one night to the next.