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September 3, 2022Deprivation & Allostasis – The Cost of Sleep Loss and of Adaptation
In the first installation of this series, we discussed the conservation of sleep across species, the ecological intelligence of sleep behavior, and the Two-Process Model of sleep regulation. In part 2, I’ll dive deep into the cost of sleep loss for the individual and society and introduce the ideas of allostasis and allostatic load.
Diversity in Sleep Requirements
Each of us has a biologically- and behaviorally determined need for sleep – the timing of which is influenced both by our routines and by our genetics, and the optimal amount of non-rapid eye movement (NREM) and rapid eye movement (REM) sleep for each person will depend on activity levels, health status, age, and even biological sex. Social, family, and career pressures are often incongruent with our biological expectation for sleep of a certain timing, duration, or quality. Because of our diversity of needs, these pressures can have unequal negative impacts on health and quality of life. A person’s age, sex, or genes can predispose them to thrive in a sleep-wake routine while others suffer.
What if waking up “on time” was actually bad for you? Evidence from a Washington state school district suggests standard school start times (SSTs) may be at odds with the biological needs of high schoolers. Pubertal teenagers have an average biological preference of waking up 1-3 hours later than adults [1] but are typically expected to be present and contributing during morning hours, despite standard SSTs being associated with chronic sleep deprivation [2]. In 2016 local researchers in Seattle, WA successfully petitioned school boards to adopt a later start time for students, citing chronic sleep deprivation as a source of truancy and poor scholastic performance, and a probable driver of future health problems. Dr. Horacio De La Iglesia and colleagues (University of Washington) found that this change was associated with a > 30-minute increase in total sleep time, significantly improved scholastic performance, and even an improvement in school attendance [3]. These results and numerous others demonstrate that the timing of our sleep changes dramatically as we age [4-7].
Diversity of sleep needs can also be found among individuals of the same age, as genetic polymorphisms in circadian genes can have a significant impact on biological predisposition toward sleep timing [8] and sleep amount [9], and sex steroids are a major predictor of both insomnia and the successful recovery from sleep loss [10]. This heterogeneity in sleep requirements, coupled with individual obligations to career, family, and community is often the source of persistent sleep deprivation.
Acute Sleep Deprivation
Poor sleep is an unfortunate hallmark of modern society. Self-reported sleep data suggests that 30% of Americans between 30-64 years old get fewer than 6 hours of sleep nightly [11], and in 2006, the National Health Institute Survey (NHIS) estimated that as many as 70 million Americans suffered from chronic sleep loss or sleep disorders [12]. In 2017, the National Highway Traffic Safety Administration estimated that drowsy driving was responsible for greater than 90,000 vehicle accidents in the United States alone (for the calendar year 2017), resulting in nearly 50,000 injuries and more than 700 deaths.
While sleep restriction may be normalized – or even encouraged (in the case of startups, academia, etc.) – the consequences for the individual are anything but normal. A single night of lost sleep is associated with decreased insulin sensitivity [13], with one study reporting that even 4 hours of lost sleep is sufficient to increase insulin resistance in healthy participants [14]. As sleep loss accumulates, so do its consequences. By the time a healthy individual has been awake for 40 hours (i.e. the evening after losing a single night of sleep), they have increased systolic blood pressure, decreased vascular function, and increased pro-inflammatory cytokines [15].
These changes in metabolic and immune function are accompanied by altered cognitive function and stress hormones. Healthy subjects subjected to 24 hours of sleep deprivation have increased cortisol and norepinephrine, and produce significantly more errors on a cognitive task. In US military service members, 26 hours of sleep deprivation (which is not at all uncommon in the field) is correlated with degraded performance on the Attention Network Test (ANT), a standard tool for assessing attention and reaction time [16]. Evidence from rats suggests that 96 hours of sleep restriction decreases the proliferation of new cells in the hippocampus by as much as 50% and prolonged total sleep deprivation in mice results in inevitable death without clear anatomical cause [17] (these experiments cannot be conducted in humans for obvious ethical reasons – and they likely would meet strict resistance from animal use and care committees today – but they demonstrate the great biological drive within organisms to satisfy sleep need). Taken together, even restricted sleep produces major biological and behavioral consequences. While herculean efforts to stay awake for work or school may be well-intentioned, they are almost certainly driving broad, undesirable changes to health and performance that negatively impact individuals of every age and demographic across society.
The Cost of Adapting to Bad Sleep
In some cases (early parenthood [18], military deployment [19], shift work [20,21], etc.), regular sleep loss may be inevitable. Humans are remarkably capable of adapting to these extreme sleep challenges, and can sometimes even do so without significant depreciation to cognitive and physical performance (for a time). Under “normal” conditions, your sleep-wake homeostasis is actively maintained by Process S (for a refresher on this, check out part one of this series). Changes to this set-point typically occur when a new routine or environmental challenge demands additional (or different) waking time. With time, the brain and body will adapt to a new sleep-wake balance, but this adjustment will come with a physiological cost. The adjustment of a previously determined set-point is called allostasis, and the physiological cost of adjusting that homeostatic balance is the allostatic load. While an individual may become accustomed to limited sleep (adapting to a 3 am wake-up for work, for example), in almost all cases long-term sleep restriction of fewer than 6-7 hours/night comes with diverse physical consequences, including increased risk for hypertension, cardiovascular disease, obesity and diabetes, increased oxidative stress, and increased evening cortisol and insulin [22,23].
Beyond the physiological cost of chronic poor sleep, researchers now know that sufficient sleep is essential for mental health. Chronic sleep loss increases the likelihood of depression [24, 25], can precipitate or exacerbate manic events in individuals with bipolar disorder [26, 27, 28], and may even increase the risk of substance abuse [29].
Conclusion
While individual variation in sleep need and in lifestyle drivers make it difficult to say exactly “how much sleep you need” (7-8 hours is probably right for most people), it likely comes as no surprise that restricted sleep is disastrous for the individual. Further complicating the importance of sleep is the fact that sleep is not unitary: sleep in mammals is made up of several sleep stages, each with its own unique electrophysiological and neurochemical markers, and specific contributions to behavior, cognition, emotion, and learning. In Part 3 of this series, we will explore the biological basis of NREM sleep and learn more about the contributions of specific neural oscillations to the perfection of complex motor behavior and the recovery from sleep loss.
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- Carvalho-Mendes, R. P., Dunster, G. P., de la Iglesia, H. O., & Menna-Barreto, L. (2020). Afternoon school start times are associated with a lack of both social jetlag and sleep deprivation in adolescents. Journal of Biological Rhythms, 35(4), 377-390.
- Dunster, G. P., de la Iglesia, L., Ben-Hamo, M., Nave, C., Fleischer, J. G., Panda, S., & de la Iglesia, H. O. (2018). Sleepmore in Seattle: Later school start times are associated with more sleep and better performance in high school students. Science Advances, 4(12), eaau6200.
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