Sleep in one of the most fascinating things about human (and animal) behavior. It is an involuntary function of our body in which we spend hours lying, unresponsive to our environment. The fascination with sleep has been around for thousands of years. Psychology has been exploring the wonders of sleep and dreams for centuries, and what we know about this incredible, day-to-day occurrence is still scarce. Because sleep is all about brain activity, it is still an area of interest for many researchers and doctors.
However, what we do know about sleep is that is consists of different cycles. The brain activity during these cycles is different, which helps us understand and characterize brain activity and the effects sleep has on our mental and physical being and health. That is why we’ve decided to take a closer look at the different stages of sleep, how are they characterized and how our brain and body respond to each.
The Anatomy of Sleep – How do we fall asleep?
In order to understand the stages of sleep, it is important to understand the brain structures involved with sleep;
- Hypothalamus – a small region of the brain responsible for numerous bodily functions. Hypothalamus consists of nerve cell groups that serve as the main control center of body temperature, arousal, and sleep. These nerve cells receive necessary information from our eyes, when they are directly exposed to light, and when they’re not. This way, the behavior of sleep is controlled, as well as the circadian rhythms and sleep cycles. To put it simply, when we close our eyes for sleeping, the hypothalamus receives this as an order to control the sleep initiation and cycle.
- Thalamus – a small structure within the brain responsible for motor and sensory signals and functions. Thalamus transfers information we receive through sense to the cerebral cortex. Then, the brain processes the information from short- to long-term memory. In regards to sleep, thalamus doesn’t process senses and the information when we sleep. That is why you’re able to dissociate from the real world and enter the world of dreams. When we dream, the thalamus becomes active, responding to the sensations we experience in the dreams.
- Brain stem – the base of the brain responsible for the flow of messages between the brain and the rest of the body. The brain stem is also responsible for numerous bodily functions, including swallowing, breathing, heart rate, blood pressure, consciousness and of course, being awake and being asleep. The brain stem, together with the hypothalamus, reduces brain activity and promotes sleep. Furthermore, brain stem plays an important role in the REM stage, as it relaxes our muscles and limbs during sleep.
- Pineal gland – a small, pea-shaped gland responsible form hormone regulation. The pineal gland is especially famous for its production of melatonin, a hormone responsible for the regulation of sleep patterns. The pineal gland also responds to light exposure and understands that the lack of light (eyes closed) should initiate melatonin production.
- Basal forebrain – the front area of the brain responsible for wakefulness and promotion of sleep by the release of adenosine (a chemical that supports sleep drive).
- Amygdala – a small set of neurons responsible for processing emotions. The amygdala is especially active during REM sleep, as it helps process memory in regards to previous emotional experiences.
Stage of Sleep – Light Sleep (non-REM sleep)
Light sleep occurs at the very beginning of the first stage of sleep. It is considered to be a preparatory stage, where the body is starting to calm down, breathing and heart rate slows down and the muscles are starting to relax. Light sleep is also known as the non-REM sleep, and is known to have three stages;
- Stage 1 – the brain nerve control center becomes active and recognizes the changes in behavior, especially between wakefulness and sleepiness. During this stage, the brain focuses on the transition between being awake into being sleep. This stage lasts shortly, taking approximately 3% of all of the sleep cycles your body goes through. During this stage, your body is in the so-called wakeful relaxation mode. It is very easy to wake up from this stage and to have your sleep cycle completely interrupted.
- Stage 2 – the sleep mode is fully initiated at this point. The brain activity slows down and there is no rapid eye movement. During this stage, the brain activity tends to burst from time to time; this means that the memory is being processed from short-term to long-term memory. This also means that the body has moved from the phase of wakeful relaxation into the phase of sleep. The body is completely relaxed, the limbs and muscles can react to certain external stimuli, in the form of muscle spasm or sudden jerks.
- Stage 3 – at this point, you’ve entered sleep and are about to move into a deep sleep. This occurs around 45 minutes into the sleeping phase. The breathing is at this stage stable and regular; however, the blood pressure falls and the pulse is 30% slowed down when compared to the waking rate. The body stops responding to external stimuli and it is difficult to wake up.
Stage of Sleep – Deep Sleep
Deep sleep occurs 45 to 90 minutes into the sleeping cycle. It is considered to be the most important part of the sleeping cycle. The reason for that lies in the fact that the body recovers completely during the deep sleep. The body is relaxed and ready to accumulate energy for the next day, heal injuries and illnesses and is overall focused on dealing with any damage to the body.
Deep sleep is often referred to as slow-wave sleep or delta sleep. Deep sleep may last anywhere between 45 and 90 minutes and often occurs several times during the sleep cycle. The first phase of deep sleep lasts the longest, while each new phase is shorter. Deep sleep intertwines with light sleep in healthy individuals. Usually, deep sleep occurs more in the first half of the night, while light sleep occurs more in the second half of the night. In some individuals, deep sleep doesn’t take place, or it occurs only in its first phase. Lack of deep sleep often leads to sleeping conditions, like insomnia, sleep interruption, sleep apnea, anxiety, depression and overall can affect one’s health negatively.
Stage of Sleep – REM (rapid eye movement)
Following the light and the deep sleep, REM is the stage of sleep where we go deeper into the state of sleep; so much deeper that we eventually start dreaming. REM is the only stage of sleep when we can dream. During this stage, our eyes move rapidly, and brain activity is incredibly high. The whole body is dissociated from reality, and the experiences/emotions in the dreams now feel real. The body only reacts to the events in the dream and processes them as memory and information. REM is one of the most famous and most interesting stages of sleep. Here are some other interesting characteristics of REM sleep;
- The first REM cycle lasts approximately 10 minutes, however, each new REM cycle last progressively longer.
- REM sleep is characterized by rapid eye movement; the eyes move quickly from left to right under the closed eyelids. It is believed that the eye movement is directly linked to dreaming.
- During REM sleep, people experience some of the most vivid dreams. If we wake up directly from the REM sleep, we are able to recall the dreams in great detail.
- Compared to the light sleep, during the REM stage our heart rate and blood pressure increase. However, the body temperature falls drastically, and our body is at the state of being almost completely immobile. The legs and arms seem to be paralyzed; this is believed to be a neurological barrier that prevents us from reacting to or acting out or dreams.
- During REM sleep our breathing becomes faster and more shallow than it was in the previous sleep stages.
- Everything we’ve learned, read, seen, heard or felt is being processed into long-term memory during REM sleep.
- Some animals, like giraffes, horses or elephants spend 1 hour daily in REM sleep. Other animals, like cats or ferrets, can spend up to 8 hours daily in REM sleep. Humans, on the other hand, usually get between 1 and 4 hours of REM sleep, if healthy and not suffering from sleeping disorders.
- Certain brain regions that control muscle movement are only active during REM sleep. That is why we experience distinct twitches and spasms that can be easily singled out.
- When waking up in the middle of REM sleep, it is easy to go back to sleep again. The reason for that lies in the fact that our brain recognizes the need for more sleep, and that the wake-up effect of the REM sleep hasn’t yet been reached.
Read More: How Much Deep Sleep Do You Need?
How do we go through stages of sleep?
We’ve explained the process of falling asleep. However, one cannot help but wonder how is it that we go through a sleep cycle of several hours. How does our body hold such a function for such a long time? Here’s how;
- Circadian rhythms – also known as the sleeping pattern, circadian rhythms are the physical, mental and behavioral changes our bodies go through every day. These changes usually refer to light and darkness, or wakefulness and sleep. Because of these changes, our body feels awake or sleepy. Circadian rhythms control the time when you’ll feel sleepy, and when you will feel like waking up, as they synchronize with your daily activities and level of tiredness or restfulness.
- Sleep-wake homeostasis – also known as a natural regulation of the balance between sleep and wakefulness. The sleep homeostasis is a reminder to the body that it needs to sleep. The more hours you’re awake, the stronger the sleep homeostasis becomes. This means that, when you fall asleep, you will sleep longer and you’ll be more likely to experience deep sleep phases several times.
The importance and beneficial functions of sleep are undeniable. Sleep is surely one of the most interesting, and most mysterious aspect of human and animal life. Just like phones, laptops, and other gadgets, humans and animals also need to recharge in order to perform fully in regards to daily activities and obligations. Our bodies are truly incredible; no wonder scientists have been fascinated by sleeping for hundreds of years now. From Sigmund Freud who was fascinated by sleep in regards to dreaming, to Eugen Aserinsky, who discovered REM sleep, our need to know more about the state of sleep will probably never cease.