.svg){kind=icon}
The importance and functions of sleep
Sleep serves two primary functions: it allows the body to regenerate and to conserve energy. While these concepts are not mutually exclusive, the scientific community is divided between two main theories: the restorative theory and the evolutionary theory.
The restorative theory posits that the fundamental function of sleep is to allow the body, and especially the brain, to properly recover after a full day of exertion. During sleep, the brain undergoes a process of rebuilding protein stores and replenishing glycogen and energy reserves.
According to the second theory—the evolutionary theory—the primary, and at the same time original function of sleep is energy conservation. From this perspective, sleep can be compared to hibernation in animals. Hibernation is not a rest period following intense activity in warmer months, but rather a survival strategy—a method for reducing energy consumption during food shortages. In animals that hibernate, body temperature drops to match the ambient temperature (just enough to avoid excessive cooling). In the next stage, the heart rate drops to a minimum, breathing frequency decreases, brain activity slows, and metabolism decelerates. As a result, the organism uses less energy and increases its chances of survival in harsh conditions.
How long should a healthy sleep last?
According to data provided by the National Sleep Foundation, most people need 7-9 hours of sleep. Some individuals, however, require 10 hours of sleep, though these are special cases—professional athletes exposed to exhausting physical exertion, people suffering from depression, or those with sleep apnea (which involves waking up 100-200 times a night to catch one's breath). There are also people who only need 6 hours of sleep without any negative impact on their health or physical condition. The amount of sleep required is, therefore, an individual matter.
An interesting case (described in the books "Two cases of healthy insomnia" and "Biological Psychology") concerns two men who spent an average of only 3 hours a day sleeping and woke up feeling refreshed. Another case that generated significant interest is that of a 70-year-old woman, described in the 1970s, who spent an average of about one hour a day sleeping and often felt no need for sleep at all.
The effects of sleep deprivation
Sleep is of immense importance for the body's functioning—it allows for the maintenance of mental and physical balance. Studies on the restorative functions of sleep have shown that people who went without sleep for a week or longer (as part of experiments or to gain notoriety) experienced a range of unpleasant symptoms and a decline in well-being. The most common symptoms included dizziness, difficulty concentrating, irritability, hand tremors, and hallucinations. Short-term sleep deprivation may stimulate certain immune responses, but prolonged insomnia causes disruptions in the functioning of the immune system.
Sleep is a time of intense regeneration for nerve cells and for organizing and consolidating all the information gathered throughout the day. It is essential for maintaining productivity, creativity, and the proper functioning of bodily processes, such as:
- neurotransmitter production,
- production of new neurons (neurogenesis),
- growth hormone activity,
- memory consolidation,
- nervous system regeneration,
- glymphatic system activity (brain cleansing when norepinephrine levels are low),
- reaction time, memory, logical reasoning, and decision-making.
Why are sleep stages important?
In an electroencephalogram (EEG), which records the brain's electrical activity using surface electrodes placed on the scalp, one can observe low-amplitude, high-frequency waves. Based on EEG and other physiological measurements, we can distinguish two types of sleep: NREM sleep and REM sleep.
NREM sleep is characterized by high-amplitude, low-frequency EEG patterns. During this sleep stage, muscle tone is maintained, and body movements sometimes occur to change position, such as turning from side to side. Breathing frequency decreases, heart rate slows, and intestinal motility slows down. It is during this phase that the most significant release of growth hormone occurs.
As you fall asleep, your EEG readings show a transition from wakefulness through four stages of sleep. Each stage (1-4) is characterized by a gradual decrease in EEG wave frequency. The deeper the NREM sleep stage, the more difficult it is to wake the person up. It turns out that if you wake someone during NREM sleep, they are often disoriented, unaware of their surroundings, and can quickly fall back asleep.
REM sleep, sometimes referred to as paradoxical sleep, is distinguished by a different EEG pattern that resembles that of wakefulness. During this phase, muscle tone is absent, though there may be twitches in the external eye muscles, resulting in rapid eye movements, as well as contractions of the middle ear muscles and limbs. This phase causes irregular breathing patterns, heart rate, mean arterial pressure, and fluctuating body temperature. People awakened during REM sleep very often remember their dreams and are able to recount them. If they do not, they usually forget them within 1-2 minutes.
REM sleep accounts for about 8 hours a day in newborns, but averages only 2 hours in adults. Because the sleep of a young child differs from that of an adult, the criteria for defining this phase vary accordingly.
According to scientists, REM sleep plays a crucial role in memory. It helps consolidate memory traces by removing unnecessary connections formed and accumulated by chance throughout the day. The process of eliminating these redundant connections serves an important purpose: it is essential for clearing the memory, ensuring that only the most important and valuable information remains in our minds.
What factors affect sleep quality?
Modern scientific research on sleep has identified a number of factors that influence it. Find out what to pay attention to in order to get a good night's sleep.
- Avoid alcohol and overeating before bed
One common myth is the belief that consuming alcoholic beverages in the evening has a positive effect on sleep.
While alcoholic beverages do affect individual sleep stages, they do so only negatively—they disrupt deep sleep and lower its overall quality. Experts advise avoiding alcohol before bed and limiting all stimulants, such as coffee and energy drinks. The effects of caffeine can last for several hours (depending on your metabolism), so it is worth paying attention to what time you have your last cup of coffee. It is generally accepted that for an average, healthy man, caffeine remains active for about 4.5 hours, while for a woman, it lasts about 5 hours.
Healthy sleep is influenced not only by a balanced diet, eating healthy products, meals rich in vegetables and fruits, or limiting highly processed foods—other eating habits are also important. It is best to avoid overeating in the evening. According to research, eating meals late at night—especially when bedtime follows shortly after—increases the risk of developing type 2 diabetes.This is caused by high levels of endogenous melatonin, which is linked to reduced insulin production (decreased glucose tolerance).
Melatonin, the hormone that regulates the sleep-wake cycle, is typically secreted around 9:00 PM, with its peak concentration occurring between 2:00 AM and 4:00 AM. Therefore, the optimal choice is to eat your last meal 2-3 hours before going to bed.
- Schedule your workout during the day
It might seem like an intense workout before bed is a great idea, but that is another myth. After an exhausting workout, we may feel muscle fatigue, but the body is highly stimulated. For those who cannot find time to exercise during the day, a warm bath, relaxation and breathing exercises, or meditation are recommended after an evening workout.
Meditation and breathing exercises will also help those exposed to high levels of stress, which not only makes it difficult to fall asleep but also disrupts sleep phases, causing frequent waking. In many cases, stress is the very cause of insomnia.
- Avoid using screens
Another factor that disrupts sleep is exposure to blue light, which can come from phone, tablet, or laptop screens. Blue light inhibits melatonin production, so it is better to avoid using these devices 2-3 hours before sleep.
- Ventilate your bedroom
An overly high temperature in the bedroom and a lack of fresh air also negatively affect sleep quality. The optimal temperature for sleep is 18-21 degrees Celsius. It is therefore worth airing out the room thoroughly before going to bed.
- Establish a proper routine
Many studies have shown a correlation between a lack of consistent sleep duration and metabolic parameters. It turns out that this irregularity negatively affects appetite. There are theories suggesting that people who do not get continuous, restorative sleep at night and instead take naps during the day are more prone to developing neurodegenerative diseases. It is suggested that this may be linked to the cyclical nature of the brain's "cleansing" system.
A good practice is therefore establish a regular sleep and wake schedule. It is recommended to aim for a consistent number of hours of sleep. Maintaining this cycle, even on weekends, is essential for synchronizing your body and improving sleep quality.
If we want to properly take care of our physical health and enjoy a well-functioning brain, we must always remember that sleep quality is vital to our lives and overall well-being.
Bibliography:
- Podgórska A., "Tak działa mózg. Jak mądrze dbać o jego funkcjonowanie", Grupa Wydawnicza Foksal, Warsaw 2023.
- Longstaff A., "Neuroscience: Instant Notes", PWN Scientific Publishers, Warsaw 2012.
- Kalat J. W., "Biological Psychology", PWN Scientific Publishers, Warsaw 2006.
- Meddis R., Pearson A. J., Langford G., "An extreme case of healthy insomnia", Elsevier Ireland Ltd., 1973.
- Crick F., Mitchison G., "The function of dream sleep", Nature, 30, 1983.
