REM cycles are a key factor to the different stages of sleeping. Your body undergoes various stages where your brain waves, body temperature, dreaming, etc. These stages are important for how you sleep and what your body does.
Sleep cycles & a calculator that tells you the best times to fall asleep depending on what time you need to wake up in the morning. How interesting- if you need to be up at this says to go to sleep at either or due to different sleep cycles.