Sleep can be understood as fulfilling many different functions but intuition suggests there is one essential function. The discovery of this function will open an important door to the understanding of biological processes.
The sleep cycle involves several stages, but these theories generally focus on rapid-eye movement REM sleep —which is when you dream —and the other stages lumped together as non-REM sleep. The restorative theory of sleep, first proposed in , is among the more accepted explanations for why people need sleep. It suggests that the purpose of sleep is to store memory and restore our brains and bodies for the next day. Some of the brain chemicals that accumulate during the day are associated with the plaques that characterize Alzheimer's disease , so it's theorized that the brain-clearing activities of sleep may protect you against Alzheimer's.
Also called the evolution theory or preservation theory, the original version of this early theory suggested that while humans evolved, hiding out overnight increased the ability to survive. For our early ancestors, nighttime was dangerous—especially because the predators who hunt at night function better in the dark than humans do—so it made sense to seek a safe refuge.
Also, because they couldn't be out finding food themselves, their bodies slowed down to conserve energy for when they could be active. People who avoided dangers in this way, the theory argued, lived longer and were more likely to reproduce. Thus, sleep became an adaptive or evolutionary advantage and became part of the neurochemistry of the species. However, most experts reject this idea because sleeping leaves animals including humans vulnerable and defenseless, which doesn't support the idea that sleeping made our ancestors safer.
A flaw in this theory, some experts say, is that sleeping limits productivity, such as finding food and reproducing, so staying awake longer would be an evolutionary benefit. According to some sleep researchers, the daily need for sleep combined with the incentive not to be out in the dark caused us to adapt to function best during daylight, which prevented us from adapting to the dark. In a similar vein to aspects of other theories, some experts theorize the primary purpose of sleep is conserving energy.
By sleeping, they say, you're able to spend part of your time functioning at a lower metabolism. That lowers the number of calories you need to eat. For early humans, that extra food requirement could have been the difference between life and death, or survival of the species versus extinction. It was harder to gather food at night, so it made sense to stay hidden then. They also point to the brain's need to replenish its reserve of glycogen, which is an important fuel.
However, while it's true that metabolism slows during non-REM, the brain is extremely active during REM sleep, which some say is a strike against the energy conservation theory. Among the more recent theories deals with brain plasticity also called neuroplasticity , which is the brain's ability to change and adapt in response to experience.
It can change both functional aspects such as re-learning skills in a new area after damage and structural aspects such as forming new pathways due to learning. The brain plasticity theory says that sleep is necessary for the brain to make structural changes.
Support for this theory comes from many places. As in restorative theory, this concept deals with information processing and memory formation. Research suggests that sleep loss leads to less structural plasticity, which can have a negative effect on alertness, cognition, and mood. Sleep deprivation also compromises memory formation, which is related to learning and plasticity.
It's believed that the plasticity theory explains why babies and young children require a lot of sleep—they're learning so much about the world that their brains need more time to process it.
Researchers are even trying to promote less sleep interruption for preterm babies in neonatal intensive care, citing studies about sleep's long-term impact on brain development and plasticity. Finally, sleep as a behavior for recovery and return to homeostasis is more flexible than often perceived.
Depending on the sleep debt or circadian time, it can be associated with a faster recovery with a more dangerous loss of consciousness when sleep deprived , or a slower recovery using lighter sleep and thus a less decreased arousal threshold [ 55 , ]. Homeostatic and circadian regulation: independent or intimately linked? Circadian and homeostatic regulation of sleep are usually considered distinct [ 20 ].
Although this holds true under various experimental conditions, it would be strange if these processes, functionally linked by environmental conditions such as light and dark, had not evolved molecular links.
Most recently, it has been reported that sleep deprivation can impair expression of circadian genes [ ] and modulate electrical activity within the SCN, a known regulator of circadian rhythms [ ]. Further, many circadian mutants [ , ] have abnormal sleep homeostasis, and half of sleep-regulated transcripts are also modulated by circadian time [ 95 , 96 ], suggesting that a simple dichotomy between circadian and sleep homeostasis may not be valid.
The problem of REM sleep : Considering the fact that NREM sleep may have localized restorative effects in particular slow-wave sleep in the cortex , it is tempting to speculate that REM sleep could have a similar role in some noncortical regions.
Problematically, however, whereas NREM sleep is associated with decreased metabolic activity and unit firing, REM sleep is an active state with increased energy expenditure [ 19 ] and enhanced activity in the pons, amygdala, and most of the cortex [ 92 ]. A cessation of neuronal activity during REM sleep is however observed in some key regulatory areas e.
REM-off neurons are also present in many brainstem regions [ ], and are often dismissed as passive monitors of motor activity, as these units often fire during phasic REM sleep.
Homeostasis could thus be specifically restored in these networks during REM sleep. This hypothesis does not explain how REM sleep, a hybrid state with decreased activity in a few networks and increased activity elsewhere, could have evolved. To solve this puzzle, it has been argued that the function of REM sleep has changed across evolution. A primitive state more akin to REM sleep may have emerged first to restore homeostasis in locomotor many neurons are activity-on or -off in the brainstem , sensory, autonomic, and subsequently thermoregulatory networks [ 1 ].
To take thermoregulatory networks offline may have been less costly energetically in animals like reptiles, who have achieved partial endothermy [ ]. Such a primitive state may still exist in echidnae, which are partially endothermic mammals [ ].
Constant endothermy subsequently evolved, favoring continuing activity in the cold and dark of temperate climates and also rendering sleep more and more costly energetically. Sleep would have then diverged into two states: NREM sleep to restore metabolic homeostasis in most of the brain, and REM sleep to restore selected primitive networks mentioned above.
Activation of forebrain and limbic areas during REM sleep [ 92 ] would have finally been selected to optimize learning and creativity, increasing survival and mitigating the negative effects of increased energy expenditure.
Further, REM sleep deprivation has strong effects on memory consolidation [ 80 , 81 ]. This hypothesis may also explain why long-term REM sleep deprivation is lethal, as it would also perturb primitive networks involved in energy homeostasis and basic functions.
The fact that REM sleep is not easily observed in some rare mammals may only reflect difficulties in measuring this process in the right networks, variations in forebrain activation, and strong effects of natural selection in selected ecological instances.
It may also explain the complex phenotype of REM sleep erections, atonic and phasic motor activity, and rapid eye movement , as it is possible that REM sleep is ancestrally the summation of several distinct substates. These speculations strongly argue for the need to study molecular and electrophysiological changes within important structures e.
We predict for example that recovery in sleep-promoting networks will occur during wake, with a similar molecular signature but allowing for neurochemical diversity, as many of the wake-active systems currently reported are glutamatergic or monoaminergic while most known sleep-promoting systems are GABAergic; see Figure 1.
Molecular and anatomical studies of sleep and sleep regulatory networks across species : To conduct molecular studies in exotic species is increasingly easy, thanks to genomic sequencing efforts, yet there are almost no data on the functional organization of sleep regulatory networks across species. Recent studies have shown that hypocretin, a major regulator of monoaminergic tone and sleep in mammals, does not have similar anatomic connections, has only one rather than two receptors, and is not strongly wake-promoting in fish [ 32 ], where light and melatonin have more effects [ 29 , 31 , 32 ].
Similarly, birds, which are very sensitive to light and melatonin, also have a single hypocretin receptor, as does the marsupial opossum, an animal with large amounts of REM sleep. This suggests that the top neural networks orchestrating the occurrence of sleep are more variable across species than are cellular, molecularly based changes; this is analogous to the circadian system, where clock genes are more conserved than SCN organization [ ].
Further studies in selected species will be extremely instructive in understanding sleep across evolution, confirming or rejecting some of the hypotheses discussed above. Sleep is as necessary as water and food, yet it is unclear why it is required and maintained by evolution. Recent work suggests multiple roles, a correlation with synaptic plasticity changes in the brain, and widespread changes in gene expression, not unlike what has been recently discovered in circadian biology. Functional data are however still largely lacking, and studies such as functional genomic screens in model organisms, comparative sleep neuroanatomy through phylogeny, and the study of molecular changes within specific wake, REM sleep, and NREM sleep regulatory systems are needed.
The resilience of behavioral sleep in evolution and after experimental manipulations may be secondary to the fact that it is grounded at the molecular, cellular, and network levels. The author would like to thank the three anonymous reviewers who helped significantly in improving the manuscript. Part of the ideas developed benefited greatly from a panel discussion on the topic in the World Federation of Sleep Society meeting, , Cairns, Australia.
Box 1. Sleep in Other Organisms Sleep as a behavior is universal [ 1 , 19 , 23—25 ]. Download: PPT. Figure 1. The Limitations of Brain Organization Models for Sleep Regulation Brain localization models are generally insufficient to explain brain function. Current Theories on Why We Sleep Decreased energy demands : Current theories on why we sleep can be divided into three main groups Table 1.
Table 1. Robustness May Favor Temporal Organization of Sleep Internal stability homeostasis , whether at the cellular or organismal level, is a prerequisite for life, yet is constantly challenged by external factors. What Remains To Be Solved? Conclusion Sleep is as necessary as water and food, yet it is unclear why it is required and maintained by evolution.
Acknowledgments The author would like to thank the three anonymous reviewers who helped significantly in improving the manuscript. References 1. Principles and practice of sleep medicine. Philadelphia: Elsevier Saunders. J Neurosci — View Article Google Scholar 3. Pillay P, Manger PR Testing thermogenesis as the basis for the evolution of cetacean sleep phenomenology. J Sleep Res — View Article Google Scholar 4.
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Sleep 68— Physiol Behav — Nature E10—E Nature E9—E In general, beta wave activity is associated with alert, conscious thinking and behavior, alpha with deep relaxation and meditative states, theta with light sleep, and delta with deep sleep. EEG recordings made during sleep are painstaking performed by sleep researchers who monitor this activity of volunteer subjects throughout the night. In addition to EEG, the electromyograph EMG - muscular activity and tension , electrooculograph EOG - eye movements, and electrocardiograph EKG - heart contractions are also recorded to consider possible contamination of the EEG signal, and because these measures are of interest in their own right.
Breathing and pulse rates and body temperature are also routinely measured. The activity of brain neurons both while awake and asleep are controlled by nuclei found in the medulla and pons see Figure 8. Figure 20 illustrates the characteristics of the human EEG while awake and during the four stages of sleep.
The discussion focuses on the nature of consciousness and physiology associated with each of these stages. We spend approximately one third of our life in the state of sleep.
Volition the will and choice to respond to the environment and consciousness are partially or completely deferred, sensitivity to external stimuli is reduced, and physiological functions are partially suspended.
What is the purpose of this behavior that constitutes such a major portion of our existence yet provides us with such little conscious gratification? Two primary theories are proposed to answer this question: recuperation theories and circadian theories. The foundation of the recuperation theories argues that the homeostasis or internal stability of the body is disturbed by the waking state; sleep is needed to restore normal function.
The circadian theories argue that animals sleep during the time of day when it is unnecessary for them to be engaged in survival behaviors. Wakefulness is necessary only to allow the animal to engage in behaviors that will satisfy basic needs. In this context, sleep conserves energy for the behaviors that are necessary for survival.
There is significant variation across species in the time each spends sleeping. Humans typically need 8 hours of sleep; the giant sloth needs 20 hours of sleep, and the horse only 2 hours. These differences provide a backdrop for contrasting the two major theories. For example, recuperation theories would predict that animals that expend more energy would need more sleep.
There is, however, no correlation between the activity level of a species and the amount of time spent in sleep. This is further supported by findings that many of the major restorative functions in the body like muscle growth, tissue repair, protein synthesis, and growth hormone release occur mostly, or in some cases only, during sleep. Other rejuvenating aspects of sleep are specific to the brain and cognitive function. For example, while we are awake, neurons in the brain produce adenosine , a by-product of the cells' activities.
The build-up of adenosine in the brain is thought to be one factor that leads to our perception of being tired. Incidentally, this feeling is counteracted by the use of caffeine , which blocks the actions of adenosine in the brain and keeps us alert.
Scientists think that this build-up of adenosine during wakefulness may promote the "drive to sleep. During sleep, the body has a chance to clear adenosine from the system, and, as a result, we feel more alert when we wake.
One of the most recent and compelling explanations for why we sleep is based on findings that sleep is correlated to changes in the structure and organization of the brain. This phenomenon, known as brain plasticity, is not entirely understood, but its connection to sleep has several critical implications. It is becoming clear, for example, that sleep plays a critical role in brain development in infants and young children.
Infants spend about 13 to 14 hours per day sleeping, and about half of that time is spent in REM sleep, the stage in which most dreams occur. A link between sleep and brain plasticity is becoming clear in adults as well. This is seen in the effect that sleep and sleep deprivation have on people's ability to learn and perform a variety of tasks. This theory and the role of sleep in learning are covered in greater detail in Sleep, Learning, and Memory.
Although these theories remain unproven, science has made tremendous strides in discovering what happens during sleep and what mechanisms in the body control the cycles of sleep and wakefulness that help define our lives.
While this research does not directly answer the question, "Why do we sleep? Getting adequate sleep the first night after learning a new skill is important for improving memory and performance. Why Do We Sleep, Anyway?
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