What is sleep?

Sleeping is an intriguing enigma in the tapestry of human experience. We acknowledge its crucial role, yet the mystery surrounding its true essence persists. What is its ultimate purpose? How much sleep is ideal for our overall health and happiness? And why does falling into slumber come so naturally to some while others are in a perpetual struggle? As I pen down these thoughts at the age of 50, I can’t help but realise that roughly 16 years of my life have been spent in the embrace of sleep. With such a significant portion of our existence devoted to it, isn’t it time we unravel the secrets of sleep?

Why sleep is so important

Sleep isn’t just a luxury; it’s a fundamental necessity every inch of our body craves. Yet, its absence can be as devastating as its presence is rejuvenating. The consequences of prolonged sleep deprivation are stark and chilling: death. However, the manner in which this deprivation leads to demise remains shrouded in mystery.

In a morally contentious experiment conducted in 1989 by researchers at the University of Chicago, ten unfortunate rats were kept awake until they succumbed to their fate. Given its ethical implications, this test won’t likely be replicated. It only took between eleven and thirty-two days for these sleepless rodents to meet their end, as exhaustion ultimately conquered them.¹

Post-mortem examinations left scientists scratching their heads. There were no visible abnormalities to account for their demise. It was as if their bodies decided to call it quits. Given that a rat can survive without food for two to three weeks, these results demonstrate that sleep is just as crucial for our bodies as the food that sustains us.

What happens to the body when we sleep?

Sleep is intricately intertwined with numerous vital biological processes, serving as a linchpin for memory consolidation, hormonal equilibrium restoration, brain detoxification by eliminating accumulated neurotoxins, and immune system reset. Astonishingly, even minor adjustments in sleep patterns can yield significant improvements in health indicators like blood pressure, as evidenced by individuals with early hypertension who experienced notable enhancements by adding just one extra hour of sleep per night. Sleep can be likened to a nightly tune-up for the body, ensuring optimal functioning and well-being.

While commonly associated with memory transfer and processing, sleep’s role transcends mere cognitive functions. It beckons the question: Why do we willingly surrender consciousness, subjecting ourselves to vulnerability and disconnection from the external world during slumber, often rendered effectively paralysed? This paradox underscores the profound complexity and importance of sleep, a phenomenon that intrigues and mystifies researchers and sleepers alike.

Sleep and hibernation

If sleep were merely about rest, why do animals in hibernation still have periods of sleep? It might come as a surprise, but hibernation and sleep aren’t synonymous, at least not regarding neurological and metabolic functions. Hibernation resembles more a state of being concussed or anesthetised, where the subject is unconscious but not actively experiencing sleep. Therefore, even during hibernation, animals still need to obtain a few hours of conventional sleep each day within their more significant state of unconsciousness.

Do bears hibernate?

Here’s another twist: bears, often the poster animals for winter hibernation, don’t actually hibernate as traditionally defined. True hibernation entails deep unconsciousness and a significant drop in body temperature, often nearing 0 degrees Celsius.

According to traditional physiology teachings, bears like black bears and brown bears aren’t considered hibernators because they don’t experience a significant drop in body temperature during their winter dormancy, typically only going from 37°C to around 32–34°C (although some recent research has raised questions about this).² However, recent scientific discoveries over the past decade have shed new light on bear behaviour during hibernation. Despite their body temperature not dropping drastically, bears undergo a profound and prolonged period of dormancy, much like other animals that hibernate. Instead, their winter lethargy more accurately fits the description of a state of torpor.³

During this period, black bears, for instance, undergo a complete metabolic shutdown while in torpor, meaning their metabolic rate drops to about 25% of their Basal Metabolic Rate (BMR).⁴ Interestingly, bears don’t need to urinate or defecate throughout the 4 to 6 months of hibernation; instead, they recycle nitrogen from urea.⁵ Before hibernation, bears undergo a lengthy fasting period, which could last several months. This denning phase, which includes fasting and hibernation, coincides with the growth of bear cubs, posing a significant energy challenge for the mother bear. To sustain their cubs, female bears essentially consume themselves, beginning with their fat reserves and then tapping into lean muscle mass.⁶

Research suggests that a staggering 73% of the body mass lost during denning in brown bears is attributed to protein transfer from the mother to her cubs, surpassing the energy costs of maintaining maternal functions. This highlights the critical importance of protein turnover for the survival of these large mammals, even more so than stored energy in fats. Therefore, by reducing the energy spent on maintaining a constant body temperature (relatively low for large mammals), bears can alleviate the metabolic strain on their lean tissue, which is crucial for females nursing their offspring. In light of this, the optimal strategy for a hibernating bear is to conserve energy by dialling down euthermia (maintaining a constant body temperature) and channelling resources, both protein and energy, towards the survival and growth of their offspring.⁷

Sleep is more than rest

Whatever sleep bestows upon us goes beyond mere recuperation. A profound instinct drives us to crave sleep despite its inherent vulnerability to potential threats like brigands or predators. Yet, curiously, sleep doesn’t seem to offer anything essential that couldn’t be achieved through wakeful rest. The puzzle deepens when we consider why we spend a significant portion of our nights immersed in the surreal and often unsettling world of dreams. Whether navigating through a maze of endless corridors while being chased by hordes of zombies or facing the daunting abyss of falling endlessly, these nocturnal experiences hardly seem conducive to rest and rejuvenation. The enigmas of sleep and dreams persist, leaving us to ponder the more profound mysteries behind our nightly odysseys into the unknown.

Sleep and animals

Yet, sleep is universally believed to serve some fundamental elemental need—every creature, from the most basic nematodes to complex mammals, experiences periods of slumber. However, the amount of sleep required varies dramatically across species. For instance, elephants and horses manage with just two or three hours of sleep per night, a phenomenon whose rationale remains a mystery. On the other end of the spectrum, most mammals require significantly more sleep. Once hailed as the mammalian sleep champion with its reputed twenty-hour snooze fest, the three-toed sloth was primarily studied in captivity, where it faces no predators and little activity. In the wild, sloths actually sleep for around ten hours per day, not significantly more than humans. Remarkably, some birds and marine mammals have developed the ability to shut off one hemisphere of their brain at a time, allowing one side to remain vigilant while the other indulges in some well-deserved shuteye. This fascinating array of sleep patterns across the animal kingdom invites us to explore the intriguing mysteries of sleep’s role in our lives and in the broader world of biology.

What is sleep for?

Sleep serves several crucial functions that contribute to overall health and well-being. One essential function is the restoration of brain energy metabolism, which involves replenishing glycogen stores in the brain.⁸ These stores are depleted during waking hours and gradually replenished during non-rapid eye movement (NREM) sleep, helping to restore brain energy levels.⁹ Hormones such as cortisol, growth hormone, and prolactin also undergo pulsatile release during sleep, contributing to bodily restoration and homeostasis.¹⁰

Furthermore, sleep is vital in supporting the immune system’s efficiency, enhancing the body’s ability to fight off infections and promoting healing processes. Insufficient sleep can increase susceptibility to viral infections and compromise immune function.¹¹ Additionally, sleep is closely linked to thermoregulation, with the preoptic area of the brain regulating both sleep and body temperature to conserve energy and maintain optimal conditions for rest.¹²

Memory encoding and consolidation are also essential functions of sleep, with slow-wave sleep (SWS) and rapid eye movement (REM) sleep playing crucial roles in these processes. Sleep deprivation can impair memory consolidation and learning, highlighting the importance of adequate sleep for cognitive function.¹³  Synaptic potentiation and strengthening occur during sleep, allowing for the consolidation of relevant memories while weakening synapses associated with insignificant experiences, ultimately enhancing the brain’s capacity for new learning and experiences during wakefulness.¹⁴

Summary

Sleep is a vital physiological process supporting various bodily functions, including energy restoration, hormone regulation, immune system function, thermoregulation, and memory consolidation. Understanding the intricate relationship between sleep and these functions is essential for promoting overall health and well-being.

References:

1. Everson CA, Bergmann BM, Rechtschaffen A. Sleep deprivation in the rat: III. Total sleep deprivation. Sleep. 1989 Feb;12(1):13-21. doi: 10.1093/sleep/12.1.13. PMID: 2928622.
2. Schmidt-Nielsen K. 1995 Animal physiology: adaptation and environment. Cambridge, UK: Cambridge University Press.
3. Robbins CT, Lopez-Alfaro C, Rode KD, Toien O, Nelson OL. 2012 Hibernation and seasonal fasting in bears: the energetic costs and consequences for polar bears. J. Mammal. 93, 1493-1503.
4. Toien O, Blake J, Barnes BM. 2015 Thermoregulation and energetics in hibernating black bears: metabolic rate and the mystery of multi-day body temperature cycles. J. Comp. Physiol. B 185, 447-461.
5. Barboza PS, Farley SD, Robbins CT. 1997 Whole-body urea cycling and protein turnover during hyperphagia and dormancy in growing bears (Ursus americanus and U. arctos). Can. J. Zool. 75, 2129-2136.
6. Robbins CT, Lopez-Alfaro C, Rode KD, Toien O, Nelson OL. 2012 Hibernation and seasonal fasting in bears: the energetic costs and consequences for polar bears. J. Mammal. 93, 1493-1503.
7. Nespolo, R.F., Mejias, C. and Bozinovic, F., 2022. Why bears hibernate? Redefining the scaling energetics of hibernation. Proceedings of the Royal Society B, 289(1973), p.20220456.
8. Bennington JH Heller HC Restoration of brain energy metabolism as a function of sleep (1995)Prog Neurobiol1995453473607624482
9. Spiegel K Leproult R Van Cauter EImpact of sleep debt on metabolic and endocrine functionsLancet19993541435143910543671
10. Mullington J MEndocrine function during sleep and sleep deprivation Stickgold RWalker M The Neuroscience of SleepPhiladelphia,
11. Renegar KB Floyd RA KruegerJMEffects of short-term sleep deprivation on murine immunity to influenza virus in young adult and senescent
12. Szymusiak R Thermoregulation during sleep and sleep deprivation Stickgold R Walker M The Neuroscience of Sleep Philadelphia,
13. Stickgold R Walker MP Memory consolidation and reconsolidation: what is the role of sleep? Trends
14. Tononi G Cirelli C Sleep function and synaptic homeostasis Sleep Med