There are two basic types of sleep: rapid eye movement (REM) sleep and non-REM sleep (which has three different stages), each is linked to specific brain waves and neuronal activity. You cycle through all stages of non-REM and REM sleep several times during a typical night, with increasingly longer, deeper REM periods occurring toward morning.
So, once we fall asleep, our bodies follow a sleep cycle divided into four stages.
Stage 1 NREM: This first stage marks the transition between wakefulness and sleep, and consists of LIGHT SLEEP. Muscles relax and your heart rate, breathing, and eye movements begin to slow down, as do your brain waves, which are more active when you are awake. Stage 1 typically LASTS SEVERAL MINUTES. Your brain waves begin to slow from their daytime wakefulness patterns.
Stage 2 NREM: This second NREM sleep stage is characterized by DEEPER SLEEP as your heart rate and breathing rates continue slowing down and the muscles become more relaxed. Eye movements will cease and your body temperature will decrease. Apart from some brief moments of higher frequency electrical activity, brain waves also remain slow. You spend more of your repeated sleep cycles in stage 2 sleep than in other sleep stages as this stage is typically THE LONGEST OF THE FOUR SLEEP STAGES.
Stage 3 NREM: This stage plays an IMPORTANT ROLE IN MAKING YOU FEEL REFRESHED AND ALERT THE NEXT DAY. Heartbeat, breathing, and brain wave activity all reach their lowest levels, and the muscles are as relaxed as they will be. In this stage it may be difficult to awaken you. This stage will be longer at first and DECREASE IN DURATION THROUGHOUT THE NIGHT.
REM: The first REM stage will OCCUR ABOUT 90 MINUTES AFTER YOU FALL ASLEEP. As the name suggests, your eyes will move back and forth rather quickly under your eyelids. Breathing rate, heart rate, and blood pressure will begin to increase. DREAMING will typically occur during REM sleep, and your arms and legs will become paralyzed – it’s believed this is intended to prevent you from physically acting out on your dreams. The duration of each REM sleep cycle INCREASES AS THE NIGHT PROGRESSES. Numerous studies have also linked REM sleep to MEMORY CONSOLIDATION , the process of converting recently learned experiences into long-term memories. The duration of the REM stage will decrease as you age, causing you to spend more time in the NREM stages.
These four stages will repeat cyclically throughout the night until you wake up. For most people, the duration of each cycle will last about 90-120 minutes . NREM sleep constitutes about 75% to 80% of each cycle. You may also wake up briefly during the night but not remember the next day. These episodes are known as “W” stages.