Most people, at least outside of college, spend roughly a third of each day sleeping.  Though the physiological reasons for sleep are not fully understood, sleep’s pervasiveness across many higher life forms suggests a link between sleep and health maintenance in those types of life.  Research has shown sleep as positively correlated to health benefits.  Despite its importance, however, many people across the country are not getting enough sleep each night. While studying sleeping habits is certainly not glamorous work, finding the connection between sleep deprivation and the specific health issues implicated is of great interest to many scientists. Intensive research has been focused primarily on sleep deprivation in teenagers, as this period of time represents a critical developmental stage in human growth. The trend discovered in this research, however, has been noted across all stages of life.  Basch et al conducted studies that looked at nationally representative successive samples of sleep habits amongst high school students. The researchers studied groups of students from across the country, looking at their habits and how those habits changed over time. The research suggests that the quality, regularity, and duration of sleep are essential to maintenance of good health and that the large majority of high school students receive less than the Centers for Disease Control recommended nine to ten hours of sleep per day. The finding that sleep deprivation and health inversely correlate has been widely accepted by the scientific community, but the more interesting research centers on the adverse effects of sleep deprivation and the problems this poses to the brain plasticity of memory formation.

Though the physiological reasons for sleep are not fully understood, sleep’s pervasiveness across many higher life forms suggests a link between sleep and health maintenance in those types of life.

While most people consider sleep an all-or-nothing state, research conducted at Massachusetts Institute of Technology (MIT) suggests less of a dichotomy between wakefulness and slumber.  Sleep, when discussing the brain, is understood in two distinct forms—rapid eye movement (REM) sleep and non-REM sleep, often called slow wave sleep (SWS). Such waves are in reference to electroencephalographic readings taken to measure brain activity.  The work carried out by Jakob Voigts et al. at MIT sheds light onto the mysterious nature of local control of sleep. Until recently, most of the investigations done about sleep focused on the global control of sleep processes, which occurs when the entire brain is functioning in a paradigm of SWS. The MIT lab investigating this phenomenon discovered that a brain structure called the thalamic reticular nucleus (TRN) is responsible for relaying signals to the thalamus and the cortex of the brain, and in doing so produces what they described as “pockets of SWS activity” characteristic of deep sleep. If these pockets become pervasive enough, they engulf the entire brain in a state of sleep.  The TRN is able to create a circuit of sorts, which coordinates brain waves around the brain. This circuit serves as a key part of the memory consolidating process.  Sleep, developed though the TRN circuits, is thus implicated in forming lasting memories.  The team at MIT was able to stimulate the TRN of wakeful mice using optogenetics, a technique by which light is used to rouse areas of brain tissue. Using emitted light, activity in these neurons is stimulated by controlling ion-gated channels in the cells. During this procedure, small portions of the brain began to exhibit SWS activity, and with more intensive stimulation the entire cortex could display SWS activity.  One of the lead authors of the study, Laura Lewis, stated that inducing these slow waves across the cortex causes animals to act behaviorally as if they are drowsy.  These findings suggest that the feeling of zoning-out in sleep-deprived humans may be due to the phenomenon of local pockets of SWS activity in the cortex.

One of the lead authors of the study, Laura Lewis, stated that inducing these slow waves across the cortex causes animals to act behaviorally as if they are drowsy.  These findings suggest that the feeling of zoning-out in sleep-deprived humans may be due to the phenomenon of local pockets of SWS activity in the cortex.

Sleep, whether controlled locally as outlined above or globally as was previously understood, is essential to health maintenance. The basic process of sleep has been linked to memory formation in essential ways. Björn Rasch and colleagues at the University of Zurich describe memory formation as a three-step process: encoding, consolidation, and retrieval.  During the encoding stage, the perception of a stimulus causes the formation of a memory trace, which is very unstable and vulnerable to decay. Consolidation next occurs and integrates that trace into preexisting knowledge networks in the brain. This strengthens the memory into a less vulnerable form. The final step, or retrieval, is when the memory is actually accessed by conscious thought. All three steps in conjunction constitute the memory formation process.

Pierre Maquet of the University of Liège in Belgium has helped outline the importance of sleep in memory formation.  One of his studies focuses on the brain activity during sleep of test subjects who have just performed varying tasks. The brain activity during sleep, called use-dependent activity, is determined by the previous waking activity.  Use-dependent sleep activity represents an attempt to restore optimal neuronal function, through restoration of synaptic function.  The synapses in neurons use the time availed of them during sleep to return their function to optimal levels chemically and physiologically.  Synaptic restoration can be thought of as a chemical recovery on a molecular level that can only occur most effectively during sleep. When humans are put through various training exercises designed to test their brain, the general architecture of their sleep activity is altered.  Humans and rodents both exhibit increased REM sleep activity following a training effort.  The heightened level of REM sleep activity remains until the subject has mastered the task.  These finding suggest that REM sleep processes are implicated in solidifying learned information into long-term memory.

 

 “Zoning-out” may be the subconscious result of sleep deprivation as the TRN generates circuits that produce pockets of SWS.  In the lack of general brain slumber, the brain physically tries to generate local pockets of isolated brain activity to try to account for the missing REM sleep that is so critical for memory formation.

These results show the danger in depriving humans of sleep, especially REM sleep, which is now recognized as a critical period for memory formation.  The presence of pervasive sleep deprivation across the country represents a dangerous trend.  “Zoning-out” may be the subconscious result of sleep deprivation as the TRN generates circuits that produce pockets of SWS.  In the lack of general brain slumber, the brain physically tries to generate local pockets of isolated brain activity to try to account for the missing REM sleep that is so critical for memory formation.  The next time you find yourself struggling to remember something or having difficulty focusing, consider whether you have slept enough.  If you have not given your brain the needed time to consolidate memory traces, chances are your brain is trying to do it for you. Listen as your body processes speak for themselves.  Sleep on it.  

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REFERENCES

Basch, C. E., Basch, C. H., Juggles, K. V., & Rajan, S. (2014). Prevalence of sleep duration on an average school night among 4 nationally representative successive samples of american high school students, 2007–2013. Preventing Chronic Disease, 11

Maquet, P. (2001). The role of sleep in learning and memory. Science, 294, 10/15/2015.

Rasch, B., & Born, J. (2013). About sleep’s role in memory. Physiological Reviews, 93, 10/18/2015. doi:10.1152/physrev.00032.2012

Trafton, A. (2015, How the brain controls sleep: Brain structure generates pockets of sleep within the brain. MIT News

Voigts, J., Lewis, L. D., Flores, F. J., Schmitt, L. I., Wilson, M. A., Halassa, M. M., & Brown, E. N. (2015). Thalamic reticular nucleus induces fast and local modulation of arousal state. Elife, 10/15/2015.

 

 

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