Besides food, sleep is the most important requirement of every living being, including humans. Sleep refreshes and rejuvenates us, helping our bodies repair and recharge. Sleep stages and sleep cycles play a very crucial role in this process. Whether we are tired, sick, or stressed, sleep heals and re-energizes us.
The process of sleep can be complicated to understand. It isn’t as simple as closing our eyes and dozing off. Several complex processes take place in the brain, bringing about sleep. Our brains work as hard while we are sleeping as when we are awake. Sleep is divided into different stages and cycles. Controlled by the production of hormone regulation, muscle recovery, and memory formation, the primary sleep hormone, the hypothalamus signals the rest of the body if it is time to sleep or to stay awake. Disruption in melatonin production affects the normal sleep-wake cycle.
To understand sleep, we need to understand the different sleep stages and sleep cycles in detail.
What Are Different Sleep Stages?
When we sleep, our brains pass through five different stages, ranging from light to deep sleep. The stages are classified into REM (rapid eye movement) and NREM (non-rapid eye movement) sleep. During the night, our bodies go through the five stages four to five times. Each cycle consists of the five stages. This means, while we sleep, our bodies go through four or five sleep cycles. Each cycle of sleep is roughly 90 minutes long. In an 8-hour sleep, there are four to five 90 minute cycles. In infants and children, each stage is about 50 to 60 minutes long.
Each stage of sleep has an important role in the sleep cycle. All five stages play their own function in rejuvenating and recharging the body for the next day. This includes hormone regulation, muscle recovery, and memory formation. Without enough time to sleep, most of these functions remain incomplete, resulting in fatigue and sleepiness. We might think that only deep sleep is the important stage, but all the stages are equally essential. Without a full night of sleep, the sleep cycle isn’t complete, and the restoration work remains unfulfilled.
Stage 1 of Sleep
Depending on how tired you are, it takes only a few seconds to enter the first stage of sleep, called the transitional phase. This is a non-REM stage, which means the sleep isn’t deep at all. The sleep in this phase is the lightest. People usually flit in and out of consciousness in this stage. If you are exhausted, it will take you only a few seconds to enter the transitional stage, but if you suffer from insomnia or have delayed sleep onset, it can take you an hour or more. Usually, you can still hear everything that’s going on around you and even the slightest noise can wake you up. It is also normal to toss and turn and try to get comfortable. Waking up from this stage is also quick and easy.
In this stage, sometimes your muscles will jerk, jolting you awake. This is called hypnic myoclonia, sleep twitch, or hypnic jerk. These jerks happen because of involuntary muscular contractions. It has been named after the hypnagogic state which is the transitional phase between wakefulness and sleep. Although not all people experience this, they are very common while a person is drifting off. You might even wake up with a start feeling like you were falling. This stage lasts between one and seven minutes. The first stage leads to the second stage of the sleep cycle.
Stage 2 of Sleep
The second stage of sleep is also a non-REM phase, but it is still deeper than the first stage. This phase is when muscles relax, heartbeats begin to slow down, core body temperature drops, eye movements stop, and the person completely drifts off to sleep. Waking up from this stage isn’t as quick as the first stage because the person is making his way to deep sleep. Although this is also a light sleep stage, a person spends more than 50 percent of his sleep in this phase, making it the longest stage in the cycle.
Stage two of the sleep cycle is the precursor to deep sleep. Electrical activity in the brain occurs at a lower frequency compared to the first stage. Brain waves become smaller, with occasional spurts of rapid waves. This phase lasts between 10 and 20 minutes, leading to the third stage.
Stage 3 of Sleep
The third phase can be called the most important stage of the sleep cycle. Lasting between 10 and 40 minutes, this is the stage when sleep is the deepest. During this phase, the heart rate and breathing are at the lowest, and the person has no consciousness of what is happening around him. Waking up from this stage is difficult; if you wake someone up from this stage of sleep, they will remain groggy and disoriented for some time before awakening fully. This stage is also called slow wave sleep when growth and restorative hormones are released.
These hormones have an important role in the development and nourishment of the body. In infants and children, a significant portion of brain and body development takes places at this sleep stage. Besides the growth, these hormones also aid in appetite control. The growth hormones aid in replenishing muscles and tissues that were used during that day, while the hormone leptin suppresses appetite and limits the feeling of excessive hunger. When a person doesn’t get sufficient sleep, leptin doesn’t function well, leading to a rise in the appetite-inducing hormone ghrelin. This is why lack of sleep has a connection with weight gain and obesity
Stage 4 of Sleep
This is also a non-REM phase, but the sleep is almost like a coma. It takes a long time to wake up from this stage. This stage of the sleep cycle is often compared to a comatose state because the heart rate and the body temperature are at the lowest, resembling a person in a coma. Muscle movements don’t occur in this stage, while the breathing is rhythmic. A person usually doesn’t turn or change sides in this stage because the body is completely immobile. This stage lasts around 30 minutes, around when the delta waves are produced.
The delta waves in the brain are associated with relaxation and sleep. Delta waves also enhance the body’s healing abilities. These slow deep waves are responsible for deep sleep. The higher the delta waves, the deeper the sleep. Delta waves are most commonly found in infants because they sleep the deepest. As a person ages, delta waves are produced less. This is also the stage when bedwetting, night terrors, and sleepwalking occurs. Although the causes are unclear, when these instances happen, the person has no memory of them. Even if a person wakes up briefly during this sleep phase, they soon fall back asleep and have no memory of waking up.
Stage 5 of Sleep or REM Stage
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