SLEEP 
Sleeping is divided into 5 stages that have different mental and physiological characteristics:

• Stage 1: When we fall asleep, sleeping Stage 1 begins. During this stage, the transition from wakefulness to sleeping occurs. We are half aware of our surroundings and we may not realize that we have begun to fall asleep. Muscle activity remains similar to the awake but relaxed state. During the first sleep cycle we spend about 10 minutes to Stage 1 before moving to the deeper Stage 2.
• Stage 2: After falling asleep the body enters the sleeping Stage 2. During this stage, the heart rate, breathing and temperature all reduce. Muscles are becoming more relaxed. During the first sleep cycle we spend about 20 minutes to Stage 2 before moving to the even deeper Stages 3 and 4. Almost 50% of our total sleep is spent in Stage 2.
• Stages 3 and 4: When the body enters these stages, deep sleep occurs. The main regeneration process and physical activity recovery are accomplished in sleeping Stages 3 and 4. A  person waking up from these phases may feel groggy and suffer sleep inertia. It is not advisable to fly within 30 min of waking up from such a deep sleep. The greater the physical activity during the day, the longer the deep sleep experienced. During the first sleep cycle we spend about 30 minutes to Stages 3 and 4.
• REM Sleep (Paradoxical Sleep): The fifth sleeping stage is called 'REM (Rapid Eye Movement) sleep. During this stage great mental activity occurs. Rapid eye movement takes place as well while the muscles remain relaxed. This sleeping stage is responsible for memory, emotional equilibrium, good humor. The greater the mental activity during the day, the longer the REM sleep experienced. During the first sleep cycle we spend about 10 minutes to REM sleep stage.

The first cycle that includes all of the 5 sleeping stages lasts for about 90 minutes. After the first 90 minutes of sleeping, the cycles are repeated over and over until we wake up getting progressively longer.