After learning about the brain waves in the previous post, let’s explore the stages of sleep and what happens in each stage. The neurological understanding of sleep cycle and brain waves will be useful in correlating the Yogasanas and Pranayam with their benefits in various sleep-related disorders. It will also help avoid habits that are detrimental to health.

This is the third part of the sleep series. In the first part, we covered the and the second one was about brain wave activity.

Based on the brain wave activity we can classify sleep into two basic types – Rapid Eye Movement (REM) and non-REM (NREM). At a high-level, one can think of REM stage as dreaming and processing of day’s experience/memory, and non-REM stage as restorative functioning. Non-REM can further be divided into three different stages – each linked to specific brain wave patterns and neural activity. You cycle through stages of Non-REM and REM sleep several times (normally five cycles) during a night – first comes non-REM sleep, followed by a shorter period of REM sleep, and then the cycle starts over again. Each cycle typically lasts for 90-110 minutes. After 2 to 3 cycles, the time you spend in REM sleep increases and non-REM decreases.

Let’s look at these stages in detail.

1. You are in stage I (light sleep) when you close your eyes and doze off. It is easy to wake you up in this stage. This is a changeover from wakefulness to sleep. Your heart rate, breathing, and metabolic rate decrease, and your muscles relax. The brain starts producing Alpha waves and then slowly transition to Theta waves as it moves to stage II. You might experience strange sensations known as hypnagogic hallucinations. This may include hearing or seeing things that are not there, or feeling the recurrence of something that you just did repetitive during the day (e.g. long-distance driving, or sailing on the boat).

2. You generally reach stage II (intermediate sleep) within thirty minutes of falling asleep. This 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, eye movement stops, and the brain wave activity slows. This stage is normally reached in 30 minutes. The actual time spent in this stage is about 20 minutes, but in this relatively brief period, your body physiology is preparing for going into the actual later stages of sleep which are restorative in nature. It is slightly difficult to wake up someone in this stage. The Theta waves, sleep spindles (a rapid burst of higher frequency brain waves that may be important for learning and memory) and K-complexes (a very high amplitude pattern of brain activity that may in some cases occur in response to environmental stimuli) brain waves are observed in this stage. Sleep spindles and K-complexes are believed to help inhibit certain cognitive processes and cortical arousals so as to maintain tranquil stage in the sleep (e.g. sleep through loud noises). They also help in memory consolidation – transferring from short-term to long-term memory.

3. Stage III, called deep or slow-wave sleep, is the deepest, most refreshing and restorative part of the entire sleep cycle. The brain produces low-frequency/high-amplitude Delta waves. This is reached around an hour after falling asleep. Your heartbeat and breathing slow down to their lowest levels during sleep. Your muscles are relaxed and it may be difficult to awaken you, and if someone wakes you up, you would feel disoriented for a few minutes. Brain waves become even slower. This stage is most apt to calm and reset the anxious brain as it helps in calming the prefrontal cortex (which normally helps keep our anxiety in check). This is the stage in which Human Growth Hormone (HGH) is released, especially in children, which is a powerful substance that plays a vital role in cellular repair. The body repairs and regrows tissues, builds bones and muscles, and strengthens the immune system. During deep sleep, the glymphatic system removes waste, such as neurotoxins and beta-amyloids, from the brain. This is also the stage where people talk or walk. Your longest periods of rejuvenating deep sleep occur in the first two sleep cycles. With each successive cycle, the deep sleep stage decreases and is replaced by REM stage sleep. After three sleep cycles, the deep sleep stage may disappear for the remainder of the night (while the period of REM stage increases in the last few cycles). Hence it is very important to get uninterrupted sleep for the first three sleep cycles for you to feel refreshed in the morning. The amount of deep sleep you get shortens as you age.

4. After stage III you move into REM sleep. It first occurs about 90 minutes after falling asleep. You dream at this stage of sleep. This is also called paradoxical sleep, because, as against the popular belief, during the REM phase the brain is very active but your body is prevented from doing anything. Your heart and respiratory rate may speed up a little during REM sleep. Your eyes move rapidly from side to side behind closed eyelids. You have a mixed frequency brain wave activity which is closer to that seen in wakefulness. Your breathing becomes faster and irregular, and your heart rate and blood pressure increase to near waking levels. Most of your dreaming occurs during REM sleep, although some can also occur in non-REM sleep. Your arm and leg muscles become temporarily paralyzed, which prevents you from acting out your dreams. With the exception of your eye muscles (external ocular muscles) and the diaphragm, you have no motor function during REM. This loss of muscle tone during REM sleep may conserve energy and protect you from acting out your dreams. EEG monitoring during REM sleep reveals rapid Alpha waves that are very similar to the brain waves present when you are actually awake. REM sleep plays an important role in learning and memory function. Your brain consolidates and processes information from the previous day so that it can be stored in your long-term memory. The period of REM typically is 10 minutes in the first cycle. Each of your later REM stages gets longer, and the final one may last up to an hour. That’s the reason as you get closer to morning, your chance of dreaming increases, and you are more likely to remember dreams. It is important to get sufficient REM sleep without which you face mental problems, including impaired memory, hallucinations, mood swings, and inability to concentrate. Alcohol though helps induce sleep, and may give a sense of sleep aid, it disrupts REM sleep (will cover this topic in a future post).

By this time, you may already have a question – which is the most important stage between stage III deep sleep or REM? In Stage III, the powerful HGH hormone is triggered -rejuvenating the body, while the REM stage repairs physiological functions, memory, and psyche. That means the deep sleep is geared towards physical overhauling, while REM is more of mental maintenance. So, both REM and stage III are very important but REM doing a higher-level function has an edge.

An average adult has five sleep cycles each night – alternating between several different levels of Non-REM and REM sleep – each cycle lasting for 90-110 minutes. You typically go through I->II->III->II->III->REM->I – and the entire cycle repeats. In case your sleep is interrupted in between, your will sleep cycle will get reset and you will restart the entire cycle from stage I. For example, if you are awakened in stage III before going to REM stage, you go back to stage I. Hence, it may so happen that with repeated interruptions, you may get into stage III and REM stage for insufficient time than normal – which may lead to short term issues like fatigue and irritation, and may result in a serious disorder if the situation persists. Interestingly, if you wake up during stage I, you feel a lot more rested than if you are disturbed in other stages – as stage I Alpha brain waves are closer to full consciousness.

So, for a healthy life, you need to have uninterrupted and complete sleep, which is possible through proper sleep hygiene, regular exercise, and Yoga practice. In future posts will cover various topics like sleep hygiene, how to improve sleep quality with Yoga, how to get back to sleep if you are interrupted in the middle of the night, and which Yoga poses can help you alleviate sleep-related problems.

Have you read our most popular post on neuroscientific explanation on “why Yoga inversions are so effective for relaxation and calmness of mind”