If you’ve ever wondered what sleep is, you’ll want to read Why We Sleep: The New Science of Sleep and Dreams. This book, written by a British scientist who is the director of the Center for Human Sleep Science at the University of California, Berkeley, explains why humans sleep at night.
NREM sleep
NREM sleep is crucial to the consolidation of new memories and optimisation of mental pathways for future learning. It associates with improvements in working memory, verbal fluency, motor learning and word retrieval. In addition, it believes to play a role in declarative memory, the ability to remember facts and details, and procedural memory, which helps us learn and perform new tasks.
Scientists have shown that this type of sleep is responsible for improving motor skills overnight, and the findings have important implications for society and medicine. Specifically, they found that increase in speed and accuracy were directly proportional to the amount of stage 2 NREM, which experience in the last two hours of an eight-hour sleep. The finding is particularly relevant to the development of new technologies and medical interventions. Further, it suggests that the amount of stage 2 NREM, or deep sleep, is crucial to motor-skill enhancement.
During this stage, the body releases growth hormone 13 and undertakes tissue, muscle and bone repair. It also regulates glucose metabolism, immune system function and memory. In addition, slow oscillations occur and may synchronize spindle waves. These slow oscillations often refers to as slow waves.
During NREM sleep, blood flow and ventilation also change. During NREM sleep, the blood is less oxygenated and the volume of carbon dioxide rises. This is a result of reduced pharyngeal muscle tone during sleep. Hypoventilation is also a common part of REM sleep.
Adaptation to protect from harm
Sleep is an evolutionary advantage that allows us to conserve energy and avoid danger. During the daytime, animals are active and are at risk from predators, so it makes sense that they would need to protect themselves by resting during the night. Sleeping also conserves energy and allows animals to stay hidden in the dark. Consequently, sleeping has become an essential part of our neurochemistry.
In addition to its benefits to our physical and mental well-being, sleep has a physiological role in protecting us from harm. Physiological changes during sleep force the organism to switch states. The physiological demands of the second niche force it to adapt. This means that sleeping animals can evolve into two distinct niches.
Memory consolidation
We know that our brains consolidate memories when we sleep, but how do we actually do it? This is a question that is still poorly understood. While the brain encodes vast amounts of information during daytime wakefulness, only a small fraction of this information is stored for long term use. Otherwise, the memory system would overload. Memory consolidation during sleep is an adaptive mechanism, allowing the brain to selectively strengthen memory traces rather than all of them.
Memory consolidation during sleep is selective and motivational, strengthening memories that relate to future goals. The mechanisms by which this process occurs are unclear, but they appear to involve an executive function of the prefrontal cortex. The executive functions of the prefrontal cortex links to anticipatory processing and regulate memory activation during anticipation of retrieval.
Sleep-associated memory consolidation influences by the effects of hormones. In the brain, the hormones norepinephrine and HPA are known to affect memory consolidation. However, these hormones are not released at the same levels during each sleep stage, and they are at their lowest levels during REM sleep. These phasic changes in hormone levels believe to represent favorable conditions for synaptic plastic processes.
Studies have shown that cortisol levels increase during late sleep, a stage of the sleep cycle dominated by REM, which supports the consolidation of procedural and emotional memories. A recent study found that cortisol deficiency during REM sleep reduced GR activity, and decreased the amount of time in REM sleep. This finding confirms previous findings that corticosteroids affect memory consolidation.
Immune system recovery
Recent research suggests that sleep is crucial for the immune system, and that short sleep duration can decrease immune function. Sleep deprivation associates with changes in leukocyte activity, IL-1 receptor expression, and antigen presentation. Furthermore, it has shown that sleep deprivation decreases the production of IL-2 and IL-12, two key molecules that play an important role in adaptive immunity. However, it is not entirely clear how sleep affects immune function.
The immune system produces proteins called cytokines during sleep, some of which promote sleep and others that protect the body from stress. However, sleep deprivation reduces the production of protective cytokines, and the number of cells and antibodies that fight infections are also decreased. Besides the fact that sleeping is important for restoring immune function, it’s also important for the body’s overall health and well-being. Insufficient sleep leads to an increased risk of heart disease, diabetes, and obesity.
A recent study in mice showed that sleep helps boost the immune system. It has also shown that it improves antibody responses to vaccinations. Consequently, sleeping for eight hours per day may prevent a recurrence of illnesses. In addition, sleep has shown to enhance the formation of immunological memory, which is essential for the immune system.
A lack of sleep decreases the ability of the immune system to fight viruses. People who sleep less often are more susceptible to infections like COVID-19. In addition to getting more sleep, you can also strengthen your immune system by getting at least 30 minutes of exercise daily.
