We spend about one third of every single day of our lives unconscious. Without fail, the dopaminergic pleasure of cuddling with the warmth and comfort of a big puffy blanket, among other things, convinces us each night that we should just close our eyes and go to sleep for seven hours. Why do we do such a silly thing? What biological benefit do we get from sleeping? And, most importantly, how does our brain accomplish such a task? These are a few of the many questions being asked, and answered, by the Siegel Lab at the Center for Sleep Research here at UCLA.
So what is actually going on upstairs while we blissfully neglect the stressful world around us every night?
There are two main types of sleep that our brain cycles through during the night: slow wave sleep and rapid eye movement (REM) sleep. As we groggily climb into bed and close our eyes, we quickly drift past the semi-coherent sleep stages one and two and into the first major type of sleep: slow wave, or non-REM, sleep. During slow wave sleep, which is comprised of sleep stages three and four, our brains consume the least amount of energy, we often are the most deeply asleep, exhibit sleep-walking behaviors, and have little or no dreams. Enormous numbers of neurons across the brain fire together to produce relatively slow, synchronous waves of action potentials while we are in slow wave sleep.
Guided by a small cluster of neurons in the brain stem, which is found at the base of the brain above the spinal cord, our brain cycles into the second major type of sleep called REM sleep.
REM sleep is perhaps one of the most intriguing stages of sleep because it shows brain activity that is almost identical to that of someone who is fully awake. Our breathing and heart rate become irregular, as when we are awake. Energy consumption and brain activity rise, and smaller groups of neurons fire frequently and independently of each other.
It is also during REM sleep that we have our most vivid and convoluted dreams, and when our motor cortex ceases to release its muscle-activating neurotransmitters so that we remain temporarily paralyzed. While we dream, bursts of activity arise from neurons associated with the people, places, and emotions we have experienced. The hippocampus, which helps us integrate our perceptions into a coherent plotline, is also highly active during REM sleep. According to the Activation-Synthesis hypothesis, the hippocampus tries to make sense of random bursts of sensory perception by quickly integrating them into the most realistic story it can create. What is not very active during this stage, however, is the frontal cortex which is responsible for correcting conceptual errors like “I am not capable of flying” or “There is no way that Hal Berry would actually go out with me.”
So why do we sleep? Despite the extensive amount of research being done to uncover the mysteries of how we sleep and what the purpose of it is, the exact functions of sleep remain somewhat of a mystery.
The UCLA Center for Sleep Research, headed by principle investigator Dr. Siegel, is doing its part to advance sleep research in a multifaceted, interdisciplinary way. Dr. Siegel and colleagues are answering many of the fundamental questions in the field by exploring the genetic, molecular, developmental, behavioral, and evolutionary aspects of sleep. The researchers there have published papers on a variety of sleep-related topics, including the neural mechanisms that make us sleep, the possible biological functions of REM and non-REM sleep, as well as the chemical and behavioral characteristics of narcoleptic patients, who fall asleep at random times of the day.
One interesting approach to looking at the functions of sleep taken by the Siegel lab is to compare the amounts and types of sleep that different species get. For example, you might assume that Homo sapiens have the most exhaustive mental activities, and therefore need the most sleep to recuperate. In fact, bats and opossums are the ones that spend more time asleep than any other mammal, getting as much as 20 hours of beauty sleep every day (God knows, they need it). Yet elephants, agreed to be one of the more intellectually inclined animals, spends a mere 3 hours a day sleeping.
An article published in 2005 in the journal Nature by the Siegel lab showed that, more than any other factor, an animal’s size and metabolic activity are the best things to predict its amount of daily sleep. This may be due to the fact that smaller animals tend to have a higher metabolic activity (they consume and spend more energy each day), which may increase the amount of oxidative stress that is put on the brain. More stress on the brain would require more nightly rest to recuperate from the fast-paced activities of the day. This is just one idea among many others that is being pursued by researchers.
According to Dr. Siegel, most of the data collected on sleep supports three general theories: 1) that sleep saves energy, 2) that it keeps animals from being active at inopportune times, and 3) that sleeping allows for the brain to recuperate from chemical changes occurring while awake.
One specific interest of the Center for Sleep Research is to find out why we have REM sleep, and thus possibly the reason why we have dreams. The subject has attracted much research, and has led to pointed disagreement among scientists.
Many articles both recently and historically have purported a strong correlation between REM sleep and memory consolidation, which is the process of solidifying important memories to make them more permanent. In fact, the belief that REM-sleep is necessary to retain information is fairly common. People say that you need to sleep for at least two hours before a test in order to ensure the onset of REM sleep, otherwise you wont remember anything you studied.
Yet Dr. Siegel and his collaborators have carefully but vigorously attacked these claims, concluding in a 2001 publication that “the existing literature does not indicate a major role for REM sleep in memory consolidation.” What about needing REM sleep before taking a test? In fact, poor performance on tests after not getting any sleep is more likely to be due to the fact that you are drowsy and not that you can’t remember learned information. Not getting any REM-sleep (or any other kind of sleep) may not disrupt your ability to retain information, but being half-awake during a test will.
A strong counterargument made by Dr. Siegel and colleagues to REM sleep-memory consolidation theories is that most antidepressants completely remove REM sleep from patients. Patients who regularly use anti-depressants, even those who have experienced an absence of REM sleep for over a year, do not show any significant memory or learning deficits. This, along with specific shortcomings of the studies that support the claim, show that perhaps REM-sleep does not act as a memory-forming catalyst.
Another interesting hypothesis discussed in the same article from 2005 was that REM sleep might allow us to awake up in a more alert, responsive state of mind. People are more easily awoken from REM sleep, and when awoken tend to be more responsive than if they were woken up during non-REM sleep. As the night progresses, the amount of time we spend in REM sleep slowly increases until we are spending most of our time asleep in the REM stage. Thus, by the time it starts to get light out and the need to react to a possibly dangerous environment grows, the chances that you are in the easily-awoken REM stage is very high.
REM sleep has also been suggested to play a role in the early development of our sensory systems. Before babies are born and their eyes, ears, and hands are not relaying sensory information to their brains, REM sleep may help the neuronal circuits fated to become our sensory systems develop by activating them sporadically.
There are currently many competing theories about the function and nature of the brain activity associated with sleep. Each hypothesis, on its own not able to explain all aspects of sleep, nevertheless brings another piece of the puzzle to the table.
Although the Siegel lab and the hundreds of other researchers working on sleep can not yet conclusively tell us why we sleep or dream, I’ll trust that the pleasure of letting my consciousness fall away from me each night is providing me with a significant biological advantage. Plus, do we really have the choice of turning down sleeping in until noon on a Saturday morning?
~Nate Jacobs
References:
-JM Siegel. (2005) “Clues to the Functions of Mammalian Sleep.” Nature. 437 (27): 1264-1271.
-JM Siegel. (2001) “The REM Sleep- Memory Consolidation Hypothesis.” Science. Nov. (294): 1058-1063.
-S Schwartz, P Maquet. (2002) “Sleep Imaging and the Neuropsychocological Assessment of Dreams.” Trends in Cog Sci. 6 (1): 23-30.
-JM Siegel. (2006) “The stuff dreams are made of: anatomical substrates of REM sleep.” Nature Neuroscience. 9 (6): 721-722.
-JM Siegel. (2003) “Why We Sleep.” Scientific American. Nov: 92-97.
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