We are constantly busy. Our calendars are full of colorful blocks of activities and our to-do lists are destined to be unfinished. We set up not one, not two, but three alarms to remember all the things we need to do. We try our best to have a healthy balance of work, education, family, social life, and alone-time but we end up complaining that we just don’t have enough time in a day to do all of this. Wouldn’t it be great if we just had more time every day to do all of this? Wouldn’t it be great if we had the entire 24 hours and did not have to sleep at all? Why do we have to sleep anyways?
Sleep is a strange adaptation. We are most vulnerable when we are sleeping. In sleep mode, most of the brain shuts down especially dorsolateral pre-frontal cortex, which is the area important for higher-level functions such as executive control, working memory and reasoning, i.e. thinking and decision making. Out in the wild, animals can easily get hurt or killed when they are sleeping; they might have their food stolen, their nest destroyed or their offsprings killed. It seems strange that such a risky adaptation has been preserved over the course of evolution. But the fact that it has been preserved shows that it is very important.
One of the most established theories of the importance of sleep has to do with overnight learning and memory consolidation. Memory consolidation is the creation of stable and efficient representations of initially unstable and redundant forms of information, through a series of elimination and reorganization cascades from intercellular to intra-regional level activities. Bursts of activity in deep-brain structures in different sleep stages enhance specific information transfer between neocortical and hippocampal neurons. It has been shown that rapid eye movement (REM) stage of sleep is important in consolidation of procedural and spatial memory, and slow wave stage of sleep (non-REM sleep) is important in consolidation of declarative memory.
Not surprisingly all of the major psychiatric disorders list sleep disruption as one of their characteristics. For example schizophrenia patients have an almost 40% reduction in sleep spindles, the sigma waves seen in EEG at slow wave sleep, causing and absence of sleep-dependent memory consolidation. Although the correlation is surely present, some even believe that sleep disruption could have a causal role in many of these diseases. Could the increase in ADD patients in the last decade be due to decreased quality of sleep?
More recently another crucial function of sleep is being investigated. Our lymphatic system is crucial for waste clearance in all of the peripheral structures of our body, however it does not have access to central nervous system (CNS), which includes brain and spinal cord. The way CNS performs the waste clearance is through the glymphatic system that largely depends on the glial cells abundantly found in the CNS. When the brain is sleeping, it swells a considerably amount and form pores through which glial cells help clear the toxins and byproducts using the cerebrospinal fluid (CSF). This makes sleep and glymphatic system very crucial to have a healthy brain, as they are responsible for removing toxins such as amyloid-beta, which is one of the proteins that aggregates and leads to Alzheimer’s Disease.
Okay, enough with this blog post. I need to sleep.
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