Understanding sleep stages

• Stage 1 : Light Sleep – is the first stage after being awake.  In this period, lasting only a few minutes, of relatively light sleep, your heartbeat, breathing, and eye movements slow, and your muscles relax. The brain waves begin to slow from their daytime wakefulness patterns. 
• Stage 2 : Light Sleep – is a period of light sleep before you enter deeper sleep.  Your heartbeat and breathing slow, and muscles relax even further.  Your body temperature drops and eye movements stop.  Brain wave activity slows but is marked by brief bursts of electrical activity.  You spend more of your repeated sleep cycles in stage 2 sleep than in other sleep stages.
• Stage 3&4 : Deep Sleep – are the phases that you need to feel refreshed in the morning.  They have longer durations in the first half of the night.  The heart beat and breathing slow to their lowest levels during sleep.  The muscles are relaxed and it may be difficult to awaken.  Brain waves become even slower. 
• REM sleep occurs about 90 minutes after falling asleep.  Eyes move rapidly from side to side behind closed eyelids.  Mixed frequency brain wave activity is similar to that seen in wakefulness.  Breathing becomes faster and irregular, and heart rate and blood pressure increase. Most of dreaming occurs during REM sleep, although some can also occur in non-REM sleep.  Usually arm and leg muscles become temporarily paralyzed, which prevents us from acting out our dreams.  As we age, we spend less time in REM sleep. 

Cycling through the sleep stages

 

Some interesting observations which I have personally experience most of the nights after I started using an Oura device.

Most of the deep sleep takes place in the first part of the night. REM duration is bigger in the second part of the night. Waking-up in the morning and remembering your dreams is probably a good sign. As we age both REM and Deep sleep tend to be shorter, but anything around the 20% mark should provide a quality sleep.

The resting heart rate lowers during the night and ideally should reach at its lowest around 2-4. Atfter that body starts preparing for waking you up by producing cortisol. Being more relaxed before you go to sleep, will drop your RHR allowing you to recharge your batteries and wake-up full of energy. This is one of the key indicator of a good night sleep.

A low body temperature gives your body the signal to start relaxing. This might be counter intuative, since most people like sleeping under warm blankets, however it is best to sleep in a cool environment and around 18-20 °C.