Our sleep cycle and the different stages of sleep.

So what happens to we're asleep? Despite the fact that we spend a good portion of our lives fast asleep (around a third), most of us aren’t really aware of the fact that we experience different stages of sleep and different times of the night. Sleep is a vastly complicated science, and a typical night of sleep consists of just five sleep stages. Though sleep can also be divided into two broader stages, non-rapid eye movement (NREM), and rapid eye movement (REM) sleep. 

The vast majority of our sleep (around 75 to 80 per cent) is NREM; during NREM sleep, dreams tend to be more abstract and quite vague, whereas during REM sleep dreams are more detailed and emotionally charged. During a nights sleep, every 90 to 110 minutes you cycle through five different stages of sleep, often experiencing anything between three to five dreams each night. 



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Stage One: Within a few minutes of falling asleep your breathing gradually becomes more steady and the heart rate begins to slow down. Your brain produces what are called alpha and theta waves (alpha waves are involved in relaxing you while theta waves are involved in your emotional experience) and your eye movements slow down. This stage of sleep is fairly brief, lasting up to seven minutes. This is where you are in light sleep stage, meaning that you’re somewhat conscious and can be woken easily. 

Stage Two: During this stage your muscle activity decreases further and your awareness of the outside world begins to fade away. As a sleeper you would not likely be conscious enough to notice any outside stimuli. During this stage, which is still also fairly light, the brain produces sudden increases in brain wave frequency known as sleep spindles (sleep spindles are bursts of brain activity visible on an EEG monitor). Shortly after your brain waves begin to slow down.

Stage Three: Deep, slow brain waves known as Delta Waves begin to emerge during this stage (Delta waves are associated with relaxation and healing). This stage transitions you from a light sleep to a deep sleep.

Stage Four: This is a deep sleep that lasts for roughly 30 minutes producing more delta waves. Your body begins to perform restorative functions such as tissue and muscle growth, energy restoration and memory consolidation where your brain files away any new information.

Stage Five: Most dreaming occurs during this stage known as REM. REM sleep is defined by the eye movement, increased respiration rate and an in increase brain activity. REM sleep is also referred to as paradoxical sleep because, while the brain and other systems in your become more active, your muscles become more relaxed to the point of paralysis. Dreaming occurs due to an increase in brain activity, though your voluntary muscles become paralyzed. Voluntary muscles are the muscles you can consciously move such as moving your arms and legs or tensing your abdominals. Involuntary muscles are the muscles you do not have conscious control over such as your heart or your inner gut. Muscle paralysis during this stage of sleep is a function to prevent you acting out your dreams whilst you're sleeping.