By Emmanuel H. During, MD
A recent report from the Centers for Disease Control and Prevention (CDC) (1) estimated that over one-third of the adult population in the U.S. sleeps less than the recommended minimum of 7 hours each night (2, 3). This alarming statistic has not received much press, though it raises important concerns regarding our lifestyle and the attention we pay to an activity that consumes no less than a third of our lives. The "7-hour rule" is not just another medical factoid: it is a concrete conclusion by a panel of experts based on a review of over 5,000 published scientific studies on the topic. Sleep deprivation is associated with a shorter lifespan and an increased risk of several health conditions, including heart disease, type 2 diabetes, and cancer. All too often in medicine, chronic diseases, however devastating, seem intangible to us until it is too late. As tangible and concrete experiences form the basis of change in our lives, let's review a few tangible facts.
We accept that sleep deprivation slows reaction time to environmental stimuli. But how does it do so, and to what extent? In a famous experiment conducted by Belenky and colleagues (4), healthy young adults were assigned to four groups and allowed to sleep no more than 9, 7, 5, or 3 hours each night. For one week, participants were administered a daily test called a "psychomotor vigilance task." It is a simple test that resembles a slot machine, where three digits appear for a short period of time on a computer screen and randomly freeze. Participants are instructed to press on a button as soon as the digits freeze. The finding was striking: compared to fully rested individuals, sleep-deprived individuals reacted markedly slower (reduced by 25 to 50%), and the degree of sleep deprivation was directly associated with their reaction time. In fact, although the extent of slowing stabilized for most participants after the fourth night, the group that was most sleep deprived continued to decline in reaction time throughout the week. This finding suggests that there may be a critical threshold below which lack of sleep exceeds our capacity to cognitively plateau. Van Dongen's research team (5) found that threshold to be 4 hours, below which sleep deficit exceeds our capacity to cognitively compensate and maintain our performance, however poor it may be.
But sleep deprivation impairs emotional intelligence and stability in more insidious ways as well. For example, it impairs one’s ability to discern emotions based on facial expressions. Similarly, sleep-deprived individuals tend to overemphasize negative emotions over positive ones. Gujar and his team (6) gave two groups of participants the same set of pictures with subtle facial expressions of four basic emotions: fear, happiness, sadness, and anger. Each participant was tested and re-tested within a few hours, either after a long nap or after a relaxing activity. They found that after sleep, participants in general overrated happiness and underrated expressions of fear and anger. This was noted specifically in the group of nappers who had entered the rapid eye movement (REM) stage of sleep – the part of sleep involving more vivid dreaming while at the same time being in a state of complete muscle relaxation. Another experiment analyzed unconscious responses to emotionally disturbing pictures and their relation to sleep (7). Involuntary emotional reactions such as subtle sweating or slight frowning were recorded while participants were shown pictures of either neutral scenes or scenes of violence and pain. Again, participants were tested twice, with or without an intervening nap. Those who had slept manifested much less distress the second time. The underlying mechanism for this "habituation" (emotional dampening) to negatively charged emotions was revealed in a recent study using functional MRI. The investigators showed that REM sleep resulted in a relative decrease in the activity of the brain’s fear center, the amygdala (8).
REM sleep is not just important for emotional health; it also enhances creativity and problem solving. Anagrams consist of the rearrangement of letters within words or phrases that give rise to semantically related yet structurally distinct words or phrases (e.g., listen becomes silent, or the country side becomes no city dust here). Anagrams rely on several cognitive functions including concentration, language, and mental agility or creativity. An experiment conducted in healthy volunteers assigned to solve complex anagrams under various conditions showed that individuals awakened during REM sleep perform best (9). Conversely, another study showed that creative thinking during the daytime increases the total duration of REM sleep the following night, as though the network of molecules and cells underlying creativity require a recharge during this stage of sleep (10). In addition, REM sleep is critical for learning new skills, particularly "procedural skills" such as new motor skills (typing on a keyboard, learning new guitar chords, learning how to drive, etc.), whereas deep "slow-wave sleep" is necessary for the recall of facts and experiences.
As we better understand the critical role that sleep plays in productivity and well-being, the question arises: how can we be so unaware of such a fundamental need? How can there be such a disconnect between how we are actually performing and how we think we are performing?
One reason is that this problem presents a classic Catch-22: sleep deprivation not only diminishes cognitive performance, but also impairs judgment and self-monitoring. Lack of sleep hampers our insight and critical thinking, particularly when it comes to our own errors (11, 12). Furthermore, sleepiness is a poor marker of one's actual sleep deficit. When Van Dongen and his team allowed subjects to sleep no more than 6 hours for two consecutive weeks, they discovered that although cognitive performance reached a nadir, participants reported feeling only "slightly" sleepy when asked to estimate their degree of sleepiness on the Stanford Sleepiness Scale (5). This experiment suggests that individuals subjected to chronic sleep deprivation cannot evaluate their sleep deficit and cognitive and functional status based on sleepiness alone. Stabilization of cognitive performance at a new baseline, together with relative resilience to sleepiness, appear to provide a false sense of one’s ability to overcome or cope with lack of sleep.
Resistance to some degree of sleep deprivation does nonetheless present certain advantages. From an evolutionary standpoint, individuals most sensitive to insufficient sleep are unlikely to survive very long in a hostile and threatening environment. Sleep and stress are like the yin and yang of our internal bodily functions: sleep dampens some of the effects of stress, while stress masks the consequences of sleep deprivation. Whatever the origin of this mechanism, sleep and stress connect our brains to our fight-or-flight hormones, causing increased secretion of corticosteroid, the stress hormone, as well as molecules that act like brain boosters. As a result, sleep deprivation in the short term can be a potent antidepressant. Unfortunately, this effect, which can be so powerful that it has been used in medical settings (13), does not last. By fueling the stress causal chain over days and weeks, sleep deprivation causes insidious changes in several bodily and physiological functions. In fact, after just a few days, lack of sleep increases the sense of hunger and hampers our capacity to metabolize sugar by altering its normal metabolism, as seen in individuals with type 2 diabetes (14).
In today’s modern, productivity-driven society, sleep is generally perceived as a passive, non-productive state. Perpetuating the stigma of "sleep equals laziness," influential figures often boast about their reduced need for sleep. A large part of the population appears to be trapped in a double bind: despite a chronic sense of mild fatigue and sleepiness — signals easily masked by regular caffeine intake — they continue to favor wake time over sleep. Unfortunately, this situation slowly transforms into a vicious cycle resulting in decreased efficacy and an even lower level of functioning, which in turn translates into longer wake time with lower return on time and effort.
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Emmanuel H. During, MD, is dually trained in neurology and psychiatry and specialized in sleep medicine. After completion of medical school at Louis Pasteur University and psychiatry residency in Paris, France, he relocated to the U.S. to conduct neuroimaging research at NYU in New York. He then completed a second residency training in neurology at Cedars-Sinai Medical Center in Los Angeles, followed by a sleep medicine fellowship at Stanford University where he joined the faculty as assistant professor with a dual appointment in neurology and sleep medicine. Stanford Center for Sleep Sciences and Medicine is the birthplace of sleep medicine and includes research, clinical and educational programs that have advanced the field and improved patient care for decades. To learn more, visit us at: http://sleep.stanford.edu/