The Four Stages of Sleep Cycle


Did you know that we spend almost a third of our lives sleeping? Adequate sleep is crucial for proper brain functions to occur – without sleep, we would not be able to form or maintain the brain pathways that allow us to learn, concentrate, or create new memories.

Sleep affects almost all the tissues and systems in our body such as the heart, brain, lungs, immunity, and metabolism. In fact, research has shown that sleep aids in the removal of toxins in our brains that build up when we are awake. Keep reading to find out more about our sleep cycle and brain wave activities. 

How Does Sleep Work? 

Our sleep cycle is regulated by an internal body clock. This controls when we feel tired and are ready to go to bed, or when we feel refreshed and alert. This internal clock runs on a 24-hour cycle, also known as the circadian rhythm. When we wake up, as the day passes we become increasingly tired, which peaks in the evening leading up to bedtime. This is also known as sleep-wake homeostasis and is linked to adenosine, an organic compound produced in the brain. Adenosine levels increase throughout the day as we become more tired and when we sleep, the body breaks down this compound. 

What Is The Sleep Cycle?

The total duration of our sleep is made up of multiple rounds of the sleep cycle comprising 4 individual stages. An individual might go through four to six sleep cycles in one night with each lasting about 90 minutes on average. The first sleep cycle is often the shortest, ranging from 70 to 100 minutes while subsequent cycles may last between 90 to 120 minutes. As the night passes, the time spent in each sleep cycle will change. Sleep cycles vary amongst individuals and from night to night, and can be influenced by factors such as age, regular sleep patterns, or alcohol consumption. 

What Are The Different Sleep Stages? 

Over the course of a good night’s sleep, four sleep stages will occur and these cycles are determined based on analysis of brain activity during sleep, showing distinct patterns that differentiate each stage. The breakdown of an individual’s sleep into various cycles and stages is referred to as sleep architecture. 


Sleep Stage  Type of Sleep  Normal Length 
Stage 1  Non-REM (Rapid Eye Movement) 1-5 minutes 
Stage 2  Non-REM  10-60 minutes 
Stage 3  Non-REM  20-40 minutes 
Stage 4  REM  10-60 minutes 


There are 2 basic types of sleep: REM sleep and non-REM sleep (which also has three different stages). Each type of sleep is linked to specific brain waves and neuronal activity.  As we sleep, we cycle through all stages of non-REM and REM sleep many times a night, with increasingly longer and deeper REM periods occurring towards the morning. 

Non-REM Sleep Stage 

Stage 1 non-REM sleep – this is the transition from wakefulness to sleep. During this period of light sleep, our heartbeat, breathing and eye movements slow down. Our muscles start to relax with occasional twitches and our brain waves start to slow down from their daytime wakefulness patterns. 

Stage 2 non-REM sleep – this is a period of light sleep before we enter even deeper sleep. Heart rate and breathing slow down even further, with muscles relaxing deeper. Our body temperature drops and eye movements finally come to a stop. Brain wave activity slows down but is marked by brief bursts of electrical activity. We spend more of our repeated sleep cycles in stage 2 sleep than in any other sleep stage. 

Stage 3 non-REM sleep – this is an important period of deep sleep needed for us to feel completely refreshed in the morning. It usually occurs for longer periods in the first half of the night, with the heart rate and breathing slowing down to their lowest levels. At this stage, it may be difficult to wake someone. 

REM Sleep Stage 

REM sleep – this usually occurs about 90 minutes after falling asleep. While our eyelids are closed, our eyes move rapidly, breathing becomes faster and irregular, and the heart rate and blood pressure increase to near-waking levels. Most of our dreams occur during this stage, although some do occur in non-REM sleep. Our arm and leg muscles become temporarily paralyzed, which prevents us from acting out our dreams. 

To consolidate our memories, a mix of non-REM and REM sleep is required. What does this mean for our lifestyle and sleep habits? What other factors affect our sleep? Find out more in the second part of this sleep series.

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