Updated: May 28, 2019
Sleep. It’s something we all feel the need to do and hopefully do every night.
So why is it, then, that the majority of the population is misinformed when it comes to sleep? I believe that if we’re doing something for ⅓ of our lives we should, at the very least, know why we’re doing it.
In today’s post, we delve into the science of sleep including circadian rhythms, sleep cycles and stages, the health benefits of sleep and the detrimental effects of a lack of good sleep. We’ll also be giving you a data-backed answer to the universal question: “How many hours of sleep do humans need, and why?”
Sleep medicine has only recently been recognized and we’ve seen major advancements in the last 50 years, since the discovery of REM in 1953. But, for a long time, scientists believed that sleep equaled inactivity of the brain because deep sleep waves are not dissimilar to those of a coma (we’ll see that this is not the case, and that even in the deepest sleep, the brain is active) but for that reason, there was not much interest in the study of sleep and therefore, not many studies done, so for a long time we didn’t understand the drive to sleep and frankly, we didn’t want to. In present time, sleep medicine is widely recognized but from a clinical perspective we’re still in the dark when it comes to sleep. There is so much we don’t know and are still learning.
One thing we do know for sure is that sleep is essential for normal motor and cognitive function. While we all know that after sleeping we feel rested and more alert, it is not just a mechanism that counteracts sleepiness, but rather a mechanism that is necessary for survival. So necessary in fact, that according to Matthew Walker, Professor of Neuroscience and Psychology at the University of California, Berkeley, and Founder and Director of the Center for Human Sleep, “there is no physiological system or operation of the brain that isn’t enhanced by sleep or impaired when you don’t get enough”. (1)
Unfortunately, our knowledge of sleep being at an all time high has coincided with our lack of sleep following suit. Americans are now sleeping less than ever, with the average down to 6 hours and 31 minutes. Yes, you read the correctly, the average. Society is becoming more and more sleep deprived and with that, disease and even fatalities have been escalating. We’ll get into that later on.
First, let’s start with the basics of the sleep-wake cycle that we all go through on a daily basis. Our sleep-wake cycle is regulated by two processes - the circadian rhythm (Process C) and sleep-wake homeostasis (Process S). This two-process model of sleep-wake regulation was originally posited in the early 1980’s by the Swiss sleep researcher Alexander Borbély. The two mechanisms are known to be able to work independently and there is limited evidence that they may influence one another. Together, they have an effect on most aspects of sleep and related variables such as sleepiness and alertness. (2)
Starting with process C, our circadian rhythms are regulated by our circadian clocks which are located in the brain’s hypothalamus, more specifically in the suprachiasmatic nucleus and is synchronized to external factors such as light, darkness, activity and social interactions. We can see the effect light has when we look at our production of melatonin. In humans, melatonin production has both a sleep-promoting effect and it plays a role in the synchronization of the cycle. With light being the biggest regulator, its levels are elevated in darkness and they decrease in light. (3; 4)
A great examples of the way our circadian biology is set up and how it can be affected, lies in taking a look night shift workers. While we continue to learn more about the long term effects of night work, we have seen that people with work hours that keep you awake during the night and sleeping during the day, wake up after only 4-6 hours and have trouble getting back to sleep, they are more subject to sleepiness, decreased performance, chronic disease risk and accident risk. (5)
So, get outside in the morning when you can! Since our circadian physiology is day-oriented, it’s important to get light in the morning in order to promote healthy sleep and a healthy life, including but not limited to more less addiction-prone behavior, less cravings, better blood glucose control, lower cortisol, and many performance related outcomes.
On to process S, sleep-wake homeostasis, essentially works as a timer, creating sleep pressure which rises during waking, decreases during sleep and increases when someone is sleep deprived. (6) Simply put, the less sleep you get, the more sleep pressure that builds up but it’s also important to keep in mind that when it comes to sleep, quality is just as important as quantity. Many of the health concerns attributed to sleep loss including but not limited to depression, anxiety, drowsy driving and even cancer and Alzheimer’s disease result from an epidemic of not just sleep deprivation but more specifically, rapid eye movement (REM) and non-rapid eye movement (NREM) sleep deprivation. (7)
Electrical activity of the brain during NREM sleep was first documented in 1937 by Dr. Alfred Lee Loomis. He documented the pattern we now know as non-REM sleep with an electroencephalogram (EEG) which records electrical activity of the brain. This pattern is made up of four stages which are vertex waves, sleep spindles, K complexes, and delta slowing/slow wave. (8)
The deepest of which is stage IV or slow-wave sleep (SWS). It is considered to be the deepest stage of sleep as it is most difficult to awaken people from and is seen on EEG recordings as high voltage, low frequency activity, with spindles called K-complexes that often occur after sensory stimulation or sometimes spontaneously. SWS consists of a general slowing of bodily functions and a decrease in response to sensory stimuli. During SWS, some activities, such as thermoregulation, still remain operative.
Following a period of slow-wave sleep, EEG recordings show that the stages of sleep reverse into a different state called REM sleep, in which the EEG recordings are surprisingly similar to those of the waking state - with low voltage, mixed frequencies. During this time, sensory response increases and it seems that those weird dreams you may have at this time, are a result of your cortex responding to said stimuli, while being isolated from the waking world. After about 10 minutes in the REM stage, the brain typically cycles back through the non-REM sleep stages and this happens, on average, about 5 times each night with each period having longer durations. (9)
This being the case, the last two hours of sleep are majorly composed of REM sleep which is why, if you hear someone bragging about being able to survive and function well on 6 hours of sleep, you might want to let them know that while they may think they’re only losing 2 out of the 8 hours and therefore “only” missing out on 25% of sleep, that this isn’t exactly the case.
Yes, you’re missing out on 25% of 8 hours but looking at it proportionately, you’re actually missing out on about 70% of REM sleep! You may be thinking, “sure 70% sounds like a lot, but why should I care?” Well, we still haven’t looked at what REM does exactly so let’s go ahead and do that.
From an evolutionary standpoint, all species experience NREM sleep but interestingly, REM sleep developed later and only seems to be present in avian and mammalian species. Little is known about this development but there are a few theories that may explain why this was the case. Some believe that REM came first, others that SWS came first and as there is high correlation found between body size and cycle duration in mammals, it’s possible that fragmentation between SWS and REM would be produced to maintain a constant temperature during sleep. We’re just not sure what the case is regarding the evolutionary development of sleep cycles. (10)
One thing that is seen very clearly in a child who hasn’t gotten much sleep, is that one of the main purposes of REM sleep is to re-calibrate the emotional centers of the brain. It is so important for emotional and behavioral health; hence lack of nap-time in kids leading to tantrums and emotional imbalances. Lack of sleep doesn’t just cause a bad mood though, that’s just the very tip of the iceberg.
Selective deprivation of REM sleep in experimental studies has impaired emotional balance and even induced post traumatic stress disorder (PTSD) like symptoms. On the other hand, increasing the duration and improving the quality of REM sleep with the use of medications such as Prazosin, which is oftentimes prescribed for PTDS has been shown to decrease PTSD symptoms.
We have seen high correlation between sleep disruption and addiction disorders as well, by looking at sleep’s interaction with our dopamine signals. This is observed in our mesolimbic reward system, which has been shown to be greatly affected by sleep deprivation leading to modified behaviors such as risk taking, sensation seeking and impulsivity.
In alcohol addiction, for example, sleep loss seems to predispose individuals to relapse. Even in childhood, sleep deprivation or disruption seems to predict substance use later in life. Another link is seen between sleep disruption and obesity, where following sleep restriction, decreased activity of decision regulation in the frontal cortex and enhanced reward sensitivity, the desire of high-calorie foods is increased. (11)
When it comes to deep sleep - studies have shown that NREM slow-wave activity plays a role in memory processing and that lack of it, impairs learning and encoding-related activity in the hippocampus.
We have also seen that lack of deep sleep leads to the immediate build up of beta amyloid and Tau protein in cerebrospinal fluid, both of which are big players in Alzheimer’s. At first, amyloid plaques and tau tangles form in specific, mostly non-overlapping brain regions, however as the disease progresses, they are eventually found throughout multiple brain regions, including those that are critical in the sleep–wake cycle, such as the cerebral cortex, the basal forebrain, the locus coeruleus and the hypothalamus and there appears to be a strong association between disrupted sleep and the development of Alzheimer’s. (12)
With that being said, it’s no surprise that the Guinness Book of Records actually banned any attempts at sleep deprivation. Whether you’re missing out on total sleep, deep sleep or REM sleep, it’s considered too much of a risk. This especially, when it became known that one of the biggest detriments to health caused by lack of sleep include depression, anxiety, and attempts and completions of suicide. And suicide isn’t the only fatal result of sleep deprivation.
The US National Highway Traffic Safety Administration estimates 100,000 accidents annually due to drowsiness or fatigue and statistics show that drowsy driving causes more accidents than drugs and alcohol combined. Aside from the obvious, which is falling asleep at the wheel, drowsy driving also includes microsleep which is essentially small lapses where the eyelid closes partially and you no longer react. There have been studies done examining driver performance during microsleep episode and these studies show impairment in reaction time and deterioration of performance. (13)
All in all, while we still haven’t scratched the surface on certain aspects of this vast topic, the take home message from the vast array of research should be that sleep has been demonstrated time and time again to be an extremely crucial if not the most crucial aspect to our overall health and well being. However, like most very important things it is often overlooked and replaced with quick fixes and tricks. Here’s a trick, you can’t trick your physiology. Understanding that is true in pretty much all cases gives you a one up on most people. If that ain’t a nifty trick I don’t know what is.
1. Attia, Peter. “#47 – Matthew Walker, Ph.D., on Sleep – Part I of III: Dangers of Poor Sleep, Alzheimer’s Risk, Mental Health, Memory Consolidation, and More.” #47 – Matthew Walker, Ph.D., on Sleep – Part I of III: Dangers of Poor Sleep, Alzheimer’s Risk, Mental Health, Memory Consolidation, and More, The Drive Podcast, 1 Apr. 2019.
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