Sleep is an active state. Although we are sleeping, our brain oscillates between different stages. The latter are detected in real time by Dreem headband, which then reports them each morning on the hypnogram.
When you are falling asleep, we are indeed going from a wake state to stages N1 then N2. We progressively observe less movements under your eyelids and the electrical waves emitted by your brain get slower and less intense. You are in a state of light sleep.
About twenty minutes later, there is no eye movement and the EEG (electroencephalogram - which measures the electrical activity of the brain) essentially detects intense slow wave. You are in a state of deep sleep, during which it is hard to wake up.
Another thirty minutes later, the brain might switch back to a light sleep N2 stage. At this point, it can, for instance, wake up, or enter Rapid Eye Movement Sleep (REMS). The latter is also known as paradoxical sleep because of physiological similarities to waking states. It is characterized by intense movements of the eyes, high frequency EEG with sawtooth waves, and is tightly linked with dreams.
Then, you can fall back non rapid eye movement sleep (NREMS) or wake up. Actually, the sleep stages cycle is far from being unchanging. For instance, Slow Wave Sleep is more present at the beginning than at the end of the night.
For more informations on sleep stages, go to our dedicated article "Understanding my Sleep".