Our sleep is made up of different phases that always take place in the same sequence and are repeated five to seven times per night. Whilst falling asleep, our pulse, respiratory rate and blood pressure stabilise and become more even as brain activity changes.
Light sleep: Light sleep is devided into two phases: The first phase is very short and only lasts just under 10 minutes. During this time the tension in your muscles reduces and they begin to twitch. In the second phase the muscles are relaxed. Light sleep accounts for around 50% of the healthy sleeping time. During this sleep phase, the sleeper responds particularly easily to external stimuli, i.e. they wake up more quickly.
Deep sleep: Breathing is now very even and the heart rate slows down. The muscles slacken. It is difficult to wake up the sleeper; they are recovering from physical activity.
REM sleep: REM ("Rapid Eye Movement") sleep kicks in every 60-90 minutes: Your eyes move rapidly, your brain is very active (you have many dreams), your breathing becomes irregular and your muscles are very relaxed. The time span of actions we perform in dreams corresponds roughly to real time. When an action we're dreaming about lasts for a longer period of time, the dreamer continues the action like a television series. Based on an average lifespan, we spend around six years dreaming. We have already forgotten 50% of our dreams just five minutes after waking up; after ten minutes this figure is 90%.
Source: Schlafen für Aufgeweckte, Dr. med. Michael Feld, 3. Auflage 2015