Miya Pearson
Sleep is essential for our health and well-being, and we spend about a third of our lives doing it. While we sleep, our brain remains active, and our body still requires energy to function. Our brain uses energy to repair and restore our muscles, joints, and vital organs, which is why we feel refreshed in the morning. Energy use is particularly high during REM (rapid eye movement) sleep, when our brain is highly active, burning the most glucose, and our heart rate and blood pressure rise. A healthy person weighing 125 pounds burns approximately 38 calories per hour of sleep.
| Characteristics | Values |
|---|---|
| Energy use during sleep | High during REM sleep |
| Calories burned during sleep | Depends on basal metabolic rate (BMR) |
| Muscle repair and growth | Occurs mostly or only during sleep |
| Energy conservation | Sleep reduces caloric needs |
| Brain activity during sleep | Brain is highly active during REM sleep |
| Memory consolidation | Sleep strengthens memories |
| Metabolism and sleep | Metabolic rate drops during sleep |
| Sleep and weight | Lack of sleep increases risk of weight gain |
| Sleep duration | Recommended: 7 to 9 hours for adults |
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What You'll Learn
Energy conservation theory
The energy conservation theory of sleep suggests that the primary purpose of sleep is to conserve energy. By sleeping, humans are able to function at a lower metabolism, which lowers the number of calories needed. This would have been especially important for early humans, who may have needed to conserve as much energy as possible for survival.
During sleep, the brain is highly active, especially during REM (rapid-eye movement) sleep, when a person dreams. The brain's activity during sleep is directed towards repairing and restoring the body, as well as consolidating memories and knowledge. This process requires energy, and studies have shown that the brain consumes around 20% of the body's total energy consumption during sleep.
The amount of energy spent during sleep depends on various factors, including basal metabolic rate (BMR), which is influenced by weight, age, sex, lifestyle, body size, and composition. On average, a person burns around 38 calories per hour of sleep.
While the energy conservation theory provides a possible explanation for the function of sleep, it is not universally accepted. Some experts argue that the brain is extremely active during REM sleep, which goes against the idea of energy conservation. Additionally, other theories, such as the restorative theory, have been proposed to explain why we sleep.
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Sleep and brain plasticity
Sleep is known to play a role in brain plasticity, with studies showing that both REM and non-REM sleep states are important for brain development and learning.
Brain Development
Large amounts of sleep in early life suggest that sleep may play a role in brain maturation. In particular, the influence of sleep in developing the visual system has been highlighted. The current data suggest that both REM and non-REM sleep states are important for brain development.
Learning and Memory
The relationship between sleep and learning or memory formation has been the focus of much research. It is known for sure that sleep is crucial for learning, but which stage of sleep is more important? Does learning occur in the light REM sleep stage or the deep, non-REM phase of sleep?
Two studies have helped to shed light on these questions. The first study found that a restless deep sleep resulted in visibly reduced learning efficiency. The researchers explained that their results hinged on the brain’s synapses and their roles in learning. During the day, synapses switch on in response to the stimuli that the brain receives from the environment, but during sleep, the activity of these synapses goes back to normal. Without this restorative period, they stay excited at their peak activity for too long, which interferes with the brain’s neuroplasticity.
The second study showed that sleep does not just enable the brain to learn new things but also unlearn. The findings suggest that deep non-REM sleep is not so much to learn new things as it is to suppress information. Furthermore, not only was it hard for the participants to recognize the sounds that the researchers had played to them in their deep non-REM sleep, but they also found it more difficult to (re)learn these sounds, compared with entirely new sounds. The findings add to the evidence that deep sleep helps maintain neuroplasticity. Specifically, light non-REM sleep (stage 2) may help excite synapses, while deep non-REM sleep may help them relax, or ‘downscale’.
Unifying Sleep Theories
These findings are significant because they help unify two previously contradictory schools of thought. One sees sleep’s primary function as learning and consolidating new information, while the other sees it as discarding useless information so as not to overwhelm the brain. As scientists gather more and more neuroscientific evidence about how sleep works, it becomes apparent that overall, such divisions and dichotomies are perhaps not the most useful way of looking at sleep or the role sleep has in learning.
For example, a study published last month shows that REM and non-REM sleep work together to boost learning. Namely, non-REM sleep boosts the performance of newly acquired skills by restoring flexibility and neuroplasticity, while REM sleep stabilizes these improvements and prevents new learning from erasing them.
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Sleep and memory
Sleep is an active process that plays a vital role in memory formation and consolidation. During sleep, the brain has the opportunity to decide which recent memories are important enough to keep. Experts believe that one of the key reasons we sleep is to form long-term memories. While getting a good night's sleep can help strengthen memories, not getting enough sleep can impair your ability to recall information.
Sleep is defined as a natural and reversible state of reduced responsiveness to external stimuli and relative inactivity, accompanied by a loss of consciousness. Sleep occurs in regular intervals and is homeostatically regulated, i.e., a loss or delay of sleep results in subsequently prolonged sleep.
During sleep, the brain is very active and uses energy to repair and restore itself. The brain also consolidates memories made throughout the day. The capability to form memory is critical to the strategic adaptation of an organism to changing environmental demands.
There are three main mental processes involved with memory:
- Encoding: This is the first step in memory formation in which new information enters the brain. Memories can be fleeting at this point when they’re still new.
- Consolidation: After the brain acquires a memory, it engages in a series of processes to strengthen and stabilize the memory.
- Retrieval: Retrieval is any time a person accesses a memory.
While encoding and retrieval occur more frequently when a person is awake, sleep is thought to have a powerful role in memory consolidation, which promotes long-term memory. Experts believe that memory consolidation may be effective during sleep because the brain doesn’t face as many external distractions as it does when awake. Moreover, the processing of information and experiences during sleep not only enhances each memory but is linked to improvements in creativity, problem-solving abilities, and emotional stability.
The amount of energy spent during sleep depends on many factors, including basal metabolism, metabolic response to food, physical activity, growth, pregnancy, and lactation.
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Sleep and metabolism
Sleep is intricately connected to various hormonal and metabolic processes in the body and is important in maintaining metabolic homeostasis. Sleep deprivation and sleep disorders may have profound metabolic and cardiovascular implications. Sleep deprivation, sleep disordered breathing, and circadian misalignment are believed to cause metabolic dysregulation through myriad pathways involving sympathetic overstimulation, hormonal imbalance, and subclinical inflammation.
Metabolism in Normal Sleep
Human sleep consists of non-rapid eye movement sleep (NREM) and REM sleep. NREM is further divided into three stages (N1, N2, and N3). N3, also referred to as slow-wave sleep, is considered deep sleep, with the body being least metabolically active during this period. During this time, the metabolic rate reduces by around 15% and reaches a minimum in the morning in a standard circadian pattern.
Consequences of Sleep Deprivation
The impact of sleep on glucose regulation has been studied for some time, but metabolic dysregulation with sleep loss has only recently been understood. Studies have shown that a week of sleep deprivation can result in a significant alteration in metabolic and endocrine function. Sleep deprivation can also affect the body's ability to process insulin, which is vital in converting sugar and other foods into usable energy.
Sleep Loss and Appetite
Sleep deprivation may affect eating behaviour, favouring non-homeostatic food intake (food intake driven by emotional/psychological need rather than caloric need). Acute sleep deprivation increases ghrelin levels, the "hunger hormone", which can mean an increased appetite for energy-dense foods. Sleep deprivation also reduces leptin levels, an appetite-suppressant hormone.
Sleep Deprivation and Weight
Epidemiological studies have shown an association between decreased sleep and an increase in body mass index (BMI). Data suggests weight gain with sleep deprivation, though a few studies have also noted weight gain with prolonged sleep.
Obstructive Sleep Apnea and Type II Diabetes
Obstructive sleep apnea (OSA) is a highly prevalent disorder affecting 2-4% of the population. It is characterised by intermittent but repetitive cessation of breathing accompanied by hypoxemia or reduced levels of oxygen in the blood. More than 50% of patients with type II diabetes have OSA. Cross-sectional studies have confirmed OSA and have similarly shown increased insulin resistance, glucose intolerance, and an increase in HgA1C.
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Sleep and weight
Sleep plays a crucial role in weight management, and getting enough shut-eye is as important for your health, well-being, and weight as diet and exercise. Here are some ways in which sleep affects your weight:
- Hormone Regulation: Sleep is essential for regulating hormones that control hunger and appetite. Sleep deprivation can disrupt the balance of ghrelin and leptin, the hormones that signal hunger and fullness, respectively. This dysregulation leads to increased appetite and cravings for energy-dense, high-carbohydrate foods, resulting in higher calorie intake.
- Metabolism: Poor sleep can lower your metabolism, making it harder to burn calories. Sleep deprivation can lead to metabolic dysregulation, increased oxidative stress, glucose intolerance, and insulin resistance. Insulin sensitivity dropped by over 30% in one study after just 4 days of insufficient sleep.
- Physical Activity: Lack of sleep can decrease your motivation to exercise and make you more likely to be sedentary. Regular physical activity, on the other hand, can improve sleep quality and duration.
- Food Choices: Sleep deprivation alters your brain function and impairs decision-making. This makes it harder to make healthy food choices and increases the reward response to high-calorie foods.
- Late-Night Snacking: Staying up late increases the likelihood of late-night snacking and creates a larger window for eating. Sleep-deprived individuals often opt for less nutritious, high-calorie options.
- Weight Gain: Short sleep, typically defined as less than 6-7 hours, has been consistently linked to a higher body mass index (BMI) and weight gain. Studies have found a significant association between short sleep duration and greater waist circumference, indicating increased belly fat accumulation.
- Cortisol Spike: Insufficient sleep triggers a spike in cortisol, the stress hormone. This signals your body to conserve energy, leading to increased fat storage.
- Muscle Synthesis: Poor sleep quality may decrease muscle synthesis, which can lower your resting metabolic rate (RMR) and contribute to muscle breakdown.
In summary, getting adequate, quality sleep is crucial for weight management. It helps regulate hormones, metabolism, and food choices while also providing the energy needed for physical activity. Lack of sleep can lead to increased appetite, cravings for unhealthy foods, and higher calorie intake, ultimately contributing to weight gain.
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Frequently asked questions
Yes, your body still requires energy to function while you sleep. Your brain is active during sleep, and your body needs energy to repair and restore itself. Energy use is particularly high during REM (rapid eye movement) sleep when your brain is highly active and you burn the most glucose.
The amount of energy used during sleep depends on several factors, including your basal metabolic rate (BMR), weight, sex, metabolism, and fitness level. On average, a 125-pound person burns 38 calories per hour of sleep.
Sleep helps restore your body's energy levels. During sleep, your body repairs cells, restores energy, and releases molecules like hormones and proteins. Sleep also affects your weight by controlling hunger hormones, and a lack of sleep can disrupt this balance, leading to increased hunger and potential weight gain.
