Ashwin Lewis
Sleep is a complex biological process that is essential to survival. Humans spend about a third of their lives sleeping, yet it remains one of the most mysterious body processes known to science. Sleep is a period of reduced activity, during which the body alternates between two distinct modes: rapid eye movement (REM) sleep and non-rapid eye movement (non-REM) sleep. While the brain remains highly active during sleep, the body experiences a decrease in muscle activity and interactions with the environment. Sleep is necessary for physical and mental health, as it allows the body and brain to rest, repair, and restore. It also plays a crucial role in brain functions, such as memory formation and learning, as well as maintaining overall well-being by regulating mood, metabolism, and immune function.
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What You'll Learn
Sleep is essential for health and survival
Sleep is a fundamental element of good health and even survival. It is a complex and dynamic process that affects how we function, and scientists are still working to fully understand it. Sleep is a state of reduced mental and physical activity, during which consciousness is altered and certain sensory activities are inhibited. While we sleep, our brain still exhibits plenty of detectable activity, and our body alternates between two distinct modes: rapid eye movement (REM) sleep and non-rapid eye movement (NREM or non-REM) sleep.
During REM sleep, we dream, and our eyes move rapidly from side to side behind closed eyelids. Our breathing becomes faster and irregular, and our heart rate and blood pressure increase to near-waking levels. Our muscles become temporarily paralysed, which prevents us from acting out our dreams. As we age, the proportion of time we spend in REM sleep decreases.
During non-REM sleep, our heartbeat and breathing slow to their lowest levels, and our muscles are relaxed. Brain waves also slow down, and the brain restores its supply of adenosine triphosphate (ATP), a molecule used for short-term energy storage and transport. Non-REM sleep consists of three stages, with the third being the deep sleep that we need to feel refreshed in the morning.
On average, the sleep cycle of alternate REM and non-REM sleep takes 90 minutes and occurs 4–6 times a night. We spend very little time in the waking state during sleep, with females spending 0–1% of their nightly sleep awake and males 0–2%.
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Sleep is a dynamic and complex process
Sleep is a state of reduced mental and physical activity, during which consciousness is altered and certain sensory activity is inhibited. It is characterised by decreased sensitivity to the external environment, such as closed eyelids or functional blindness. Sleep differs from wakefulness in terms of responsiveness to external stimuli, with a marked decrease in muscle activity and interactions with the surrounding environment.
The sleep cycle alternates between two distinct modes: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. During REM sleep, the eyes move rapidly behind closed eyelids, brain wave activity increases, and the body experiences virtual paralysis. Dreaming predominantly occurs during this stage, with reports of vivid and hallucinatory dreams. In contrast, NREM sleep is characterised by slower brain waves and deeper relaxation. There are three stages of NREM sleep, with the third stage being the deepest and most restorative sleep.
The brain exhibits complex and predictable patterns of activity during sleep. Brain waves slow down, and the brain restores its supply of adenosine triphosphate (ATP), an energy molecule. Sleep also plays a crucial role in brain function, facilitating the formation and maintenance of neural pathways for learning and memory creation. Recent studies suggest that sleep helps remove toxins and waste products from the brain, a process known as "brain housekeeping".
The duration and quality of sleep are essential for healthy sleep patterns. While the typical sleep schedule in Western societies consists of a consolidated block of about eight hours, daytime napping is common in many other cultures and can enhance energy levels and alertness. However, the use of artificial light, particularly from digital screens, has disrupted natural sleep patterns for many people.
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Sleep is a period of reduced responsiveness
Sleep is a normal, reversible, and recurrent state of reduced responsiveness to external stimulation. It is characterised by complex and predictable changes in physiology, including decreased muscle activity and interactions with the surrounding environment. Sleep is a highly conserved behaviour across animal evolution, likely going back hundreds of millions of years.
During sleep, the brain exhibits active patterns, including coordinated, spontaneous, and internally generated brain activity. While the brain remains responsive to salient sensory events, such as loud noises, it becomes less sensitive to external stimuli. This is reflected in the typical closed eyelids or functional blindness associated with sleep. Presleep activities often involve seeking environments with reduced or monotonous sensory stimulation.
The internal circadian clock promotes sleep daily at night, when it is dark. Sleep is regulated by the interaction of two systems: the internal biological clock and the sleep-wake homeostat. These systems determine the timing of transitions from wakefulness to sleep and vice versa, explaining why we typically stay awake during the day and sleep at night.
The sleep cycle alternates between two distinct modes: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. During REM sleep, the eyes move rapidly behind closed eyelids, and the body experiences virtual paralysis to prevent acting out dreams. Dreaming is more frequent and vivid during REM sleep. NREM sleep is divided into three stages (N1, N2, and N3), with the final stage being the deepest and most refreshing sleep.
Sleep is essential for maintaining proper function and health. It allows the body and mind to restore and repair themselves. Lack of sleep is linked to various health issues and impairs cognitive functions such as concentration and response speed. Sleep is as vital to survival as food and water.
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Sleep is a time of brain activity and repair
Sleep is a natural, reversible, and periodic state marked by the absence of wakefulness and a loss of consciousness of one's surroundings. It is a time when the body and brain undergo various processes that are essential for maintaining health and well-being.
During sleep, the brain remains remarkably active, exhibiting predictable patterns of brain activity that experts have organized into stages. These stages broadly fall into two categories: rapid eye movement (REM) sleep and non-rapid eye movement (non-REM) sleep. The sleep cycle alternates between these two types of sleep, with each cycle typically lasting about 90 minutes.
Non-REM sleep can be further divided into three stages. The first stage, N1, is a transition period between wakefulness and sleep. As you progress into N2 sleep, your heart rate and breathing slow down, and your body temperature drops. The third stage, N3 or delta sleep, is the period of deep sleep, where it may be difficult to wake the sleeper. This deep sleep is crucial for feeling refreshed the next day.
REM sleep, on the other hand, is characterized by rapid eye movements, increased brain activity, and dreaming. Your arm and leg muscles become temporarily paralyzed during this stage, preventing you from acting out your dreams. The proportion of REM sleep increases towards the end of your sleep, just before natural awakening.
While you sleep, your brain and body engage in repair and restoration processes. The brain removes toxins and waste products that have accumulated during wakefulness, such as amyloid, which is now considered a core purpose of sleep. Additionally, the brain restores its supply of adenosine triphosphate (ATP), an essential molecule for energy storage and transport.
Sleep is not just a passive activity but an active process that involves complex brain activity and dynamic physiological changes. It is a fundamental element of good health, affecting our cognitive, emotional, and physical well-being.
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Sleep patterns vary across cultures
Sleep is a biological necessity, but it is also deeply intertwined with cultural practices and traditions. Sleep patterns are shaped by local environments and the ways in which groups of people live. For example, the sleep patterns of the Malagasy and Hadza people are influenced by their exposure to natural sunlight and reduced exposure to blue light from screens and artificial lighting. In contrast, the sleep patterns of present-day Westerners are influenced by their work schedules and the use of technology.
Historical events, social norms, and the environment also play a significant role in shaping sleep traditions. For instance, the Spanish siesta and Japanese inemuri traditions reflect different cultural approaches to balancing work and rest. In Japan, public napping is widely accepted and even encouraged in some workplaces, with employees taking short power naps at their desks or during meetings, signalling dedication and hard work. On the other hand, in the Western world, there is often a priority placed on work over rest, with less emphasis on sleep as a necessary part of daily life.
Sleep environments and arrangements also vary across cultures. While it is common in Western industrialised societies to sleep alone in a quiet, dark, climate-controlled space with soft bedding, this is a relatively modern development. In Japan, for instance, many people sleep on futons, which are thin mattresses placed directly on the floor, believed to promote better posture and circulation. In Latin America and the Caribbean, hammocks are integral to daily life, used for sleeping, relaxation, and socialising, with the gentle rocking motion believed to promote deeper sleep.
Research has found that sleep duration varies significantly across cultures, with people from some East Asian countries sleeping about an hour less than those in Western nations. Interestingly, those from cultures with shorter sleep durations do not necessarily suffer from more negative health outcomes. Instead, the optimal sleep duration seems to be relative to cultural norms, as people whose sleep duration aligns with their cultural ideals tend to enjoy better health.
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Frequently asked questions
To sleep in means to get up later than usual in the morning. It does not necessarily mean that you slept longer than usual, but that you got out of bed later than you normally would.
When you sleep in, you deliberately choose to sleep late. When you oversleep, it's because you didn't hear your alarm or it didn't go off.
This depends on the individual. A good indicator is if you wake up naturally without an alarm. Feeling refreshed and energised when you wake up is also a sign that you've had sufficient rest.
