According to the National Sleep Foundation, sleep happens in cycles that repeat themselves about every 90 minutes. The first stage, which is called N1, includes falling asleep and light sleep. N2 is the onset of sleep when your breathing becomes regular, you detach from your surroundings, and your body temperature drops. N3 is the deepest and more restorative phase of sleep when growth hormone is released, blood pressure drops, and your breathing slows. REM starts about 90 minutes after falling asleep and recurs throughout the night. During this phase, your eyes dart back and forth, your brain is engaged, and you dream.