Since the 1950s, sleep researchers have attempted to determine the causes of sleep. Given the fact that every phenomenon is caused by a large number of interacting factors (see multifactorial causation in Section 1-6), it often helps to distinguish between proximal causes — those that occur close in time to (but before) the phenomenon being explained — and distal causes — those that occur farther back in time. In the case of sleep, the proximal causes of greatest interest to researchers are brain structures, brain activity, and the biochemicals[∂] that affect and result from this activity. The distal causes of greatest interest to researchers are the adaptive consequences of sleep — those consequences that help individuals to survive longer and reproduce more — which would have been the factors most important for the evolution of sleep.
Proximal Causes of Sleep
The functioning of the brain affects the functioning of the rest of the body; and the functioning of the rest of the body affects the functioning of the brain. The spinal cord carries sensory information from the rest of the body into your brain; and it also carries information from your brain out to the rest of the body. When the spinal cord enters the skull, it becomes the brain stem. Electrical activity in the brain stem is important for causing animals to fall asleep and awaken. If we cut the brain stem of a cat where it connects to the spinal cord, for example, the cat still shows normal sleep/wake patterns because brainstem activity still affects activity in the rest of the brain. On the other hand, if we cut the brainstem of a cat several inches above this point, the cat will appear to be permanently in SW sleep because brainstem activity no longer affects activity in the rest of the brain. Thus, the area between these two cuts must be important for producing daily patterns of sleeping and waking. This area includes the reticular activating system (RAS) — also known as the reticular formation — which is a large pathway that begins in the spinal cord, travels through the brain stem, and ends in the cerebral cortex (see Figure 1).
Figure 1. The reticular activating system (the image can be found here)
Increases in RAS activity wake up an animal, whereas decreases in RAS activity may cause an animal to fall asleep. If you are daydreaming and a loud noise occurs behind your head, you will immediately become alert. This is because the noise caused activity in the RAS which “woke up” the rest of the brain, especially the cerebral cortex. Structures in the brain stem are inportant "nodes" in the RAS. Activity within a structure called the pons seems to be important for initiating REM sleep: it becomes active immediately before a person enters REM. You will learn more about the brain stem and the rest of the brain in Section 3.
There are a number of brain chemicals important for inducing sleep. For example, if a compound called serotonin is removed from a cat’s brain (by destroying neurons that produce this biochemical), the cat will sleep very little or not at all for at least several days. Another biochemical called melatonin is important for making us feel sleepy: when it gets dark outside, the body's production of melatonin increases. Still other biochemicals influence the onset of SW sleep and REM sleep. Abnormalities involving these biochemicals may be important for the development of various sleep disorders. For example, insomnia (sleeping too little) may occur when there is too little melatonin in the brain, whereas increased activity of serotonin may be associated with hypersomnia (sleeping too much).
Distal Causes of Sleep
Almost all mammals and many birds show both REM and NREM sleep. The fact that this pattern of physiological activity occurs in many different species suggests that sleep and its separate stages have some very important functions — functions that first evolved in ancestral species shared by these modern animals.
When we ask questions about the functions of sleep, we are concerned with what sleep does for us, with what its biological and psychological purposes are. When you get a good night’s sleep, your body and mind feel rested the next morning. On the other hand, when you don’t get a good night’s sleep, your body and mind do not feel rested. These findings suggest that one way to develop an adequate theory of the functions of sleep might be to deprive subjects of sleep and see what happens. Many researchers did this during the 1950s and 1960s, using both humans and nonhuman animals. In total sleep-deprivation studies, participants are not allowed to sleep, typically for 2-3 nights in a row. In partial sleep-deprivation studies, on the other hand, the amount of sleep that participants are allowed per night is reduced — typically to only about 3-5 hours per night — sometimes for up to several months, but more often for only a week or two.
In general, total-sleep deprivation studies found that, when people are prevented from getting any sleep at all, they became very sleepy (they develop a strong desire to sleep), an unsurprising finding to say the least. In addition, participants in these studies experienced difficulties concentrating on long and/or boring tasks (such as listening to a lecture), but often had little or no difficulty concentrating on short and/or exciting tasks (such as playing a game requiring a lot of physical activity). These sleep-deprived participants tended to become uninterested in things that normally would interest them when they were well rested; and also tended to make more mistakes on boring tasks, but not on exciting ones that could hold their attention. Nevertheless, in order to continue performing at normal levels, the bodies of sleep-deprived participants had to work much harder than normal. Therefore, they used more energy to perform tasks. Some total-deprivation studies showed that participants' physical fitness decreased the longer that they went without sleep. In addition, immune-system functioning decreased during prolonged sleep deprivation.
A commonly believed but apparently false claim is that people undergoing total-sleep deprivation for several days sometimes become psychotic (experience hallucinations and delusions). But, unless participants are taking stimulants (which sometimes cause the development of psychotic symptoms) or are already predisposed to developing psychotic episodes, it does not seem to be true that short stretches of total-sleep deprivation will cause people to become psychotic. On the other hand, it can cause people to experience unusual perceptions (illusions). For example, an object such as a book may be mistaken for a cat, and the subject may even try to pet it. The difference between an illusion and a hallucination is that a hallucination involves perceiving something that is not there, whereas an illusion involves misperceiving something that is there.
Other psychological changes that tend to occur with total-sleep deprivation are increased emotionality, irritability, and aggressiveness. In addition, many people begin to have problems with remembering events that have occurred recently, which suggests that sleep may be important for memory formation.
What makes it so difficult to perform total sleep-deprivation studies is that people cannot stay awake after a day or two without sleep. The longest total sleep-deprivation study on record lasted for 11 days (Dement, 1999). This feat was accomplished in 1965 by a 17-year-old boy named Randy Gardner. However, Randy was unable to stay awake the entire time: after prolonged sleep deprivation, he entered brief periods of Stage-1 (light) sleep called microsleeps. During a microsleep, people often appear to be awake and may even be engaged in some activity, but their EEGs show that they are in a transitional stage between sleep and waking (similar to Stage 1). This can be very dangerous when sleep-deprived people are engaged in activities such as driving: they often don't realize that they are falling into a light sleep, which increases greatly the probability of a serious accident. Thus, if you have not slept well (or at all) for some time, and you feel drowsy while driving but think that this isn't affecting your driving, pull over and take a short nap anyways: people in Stage 1 sleep usually don't realize that they were just sleeping when they wake up, so your perception that your driving is OK may be incorrect.
In animals, researchers can use more extreme methods to keep subjects awake. For example, they can deprive animal subjects of sleep for much longer periods, although they are not able to completely prevent short periods of light sleep. Studies performed by Allan Rechtschaffen and his colleagues involved placing rats on a disk that was floating in water (Rechtschaffen, Gilliland, Bergmann, & Winter, 1983). The disk was set up in such a way that, when the rat fell asleep, the disk would rotate, and the rat would have to begin walking in order to avoid falling in the water. In this way, the rat would get only about 10% of its normal sleep per day. When rats are sleep-deprived in this manner, they typically end up dying within two to four weeks. Some of the problems observed before death were weight loss (even with increased eating), sores on the paws and tail, increased energy use, and changes in hormone levels. Because a severe lack of sleep results in problems such as these — problems that lead eventually to the death of these animals — it is reasonable to conclude that sleep must have some very important biological functions linked to survival.
People who get less than their normal amount of sleep also exhibit some mild to moderate problems depending on how much sleep they are getting and how long this deprivation lasts. The most consistent finding is that, when sleep is decreased to less than about six hours per night, people often report that they feel very sleepy, again a very unsurprising finding. There are, however, few consistent effects on the performance of partially sleep-deprived subjects on various kinds of tasks. Tasks that require a person to pay attention for longer periods of time are the ones most likely to show performance deficits after a couple of days of reduced sleep. Nevertheless, when sleep is reduced gradually over several months so that subjects get acclimated to less sleep, few or no effects are seen on performance measures, even though subjects continue to complain about fatigue.
One interesting finding of long-term partial sleep-deprivation studies is that, when sleep was reduced to about five hours per night for about seven months, participants continued to sleep from one to almost three hours less per night after the study was completed compared to their previous sleep habits. The major problem with all of these older studies, however, is that the measures used to look at the effects of sleep deprivation are limited to rough measures of performance and mood. They are not able to detect the more subtle physiological and psychological effects that may occur with prolonged sleep deprivation. Newer studies that have used better measures of physiological and psychological processes have found negative effects (see, for example, Lee-Chiong, 2008). We will discuss some of these in Section 5, when we look at sleep and memory formation.
Given these relatively unsatisfactory findings, you may not be suprised to hear that we still do not have very good answers to the question of what sleep is doing for us:
Like hunger or thirst, the drive for sleep appears to satisfy an elementary need. However, unlike eating and drinking, the purpose for sleep remains obscure. Sleep is the one major biological process whose raison d’être [reason for existing] has not yet been specified. Unraveling its essence constitutes a major challenge for biological research in general and for neuroscience in particular. (Borbély & Tononi, 1998; p. 167)
Several theories about the functions of sleep have been developed over the years, but we still don’t have conclusive evidence supporting any of them. I will mention here only three.
(1) Restorative theory. This theory suggests that sleep helps to replenish bodily resources that have been depleted during the day. It states that REM sleep, in particular, helps to restore the brain, whereas NREM sleep (stages 1-4) helps to restore the rest of the body. Sleep-deprivation studies support this theory: people prevented from sleeping tend to feel very fatigued. Your experience of going to bed feeling fatigued and waking up the next morning feeling reinvigorated also is partial evidence supporting this theory. Athletes who have engaged in heavy exercise spend more time sleeping, on average, after the exercise (especially slow-wave sleep, which may be especially important for restoring depleted bodily resources).
(2) Energy-conservation theory. This theory suggests that sleep helps us to conserve energy by allowing for a period of minimal energy use sometime during each 24-hour period. Species who use higher amounts of energy during the day (such as “warm-blooded” mammals and birds) tend to spend more time sleeping. Slow-wave sleep is thought to be especially important here since the other stages (especially REM) tend to be associated with higher expenditures of energy. Certain animals with very low rates of energy use (such as many amphibians and reptiles) don’t even have stages of slow-wave sleep.
(3) Predation theory. This theory suggests that humans sleep during the night because this protects us from certain dangers, especially from the danger posed by predators. Humans are relatively helpless when it is dark because our eyes no longer function well and the rest of our senses are not as good as those of our potential predators. Other animals with different vulnerabilities sleep on different schedules. For example, goats and some other grazing animals sleep less than four hours per day in several very short periods of sleep, perhaps to lessen the possibility of experiencing a surprise attack by a predator in the open grasslands in which they are found.
In essence, these three theories together suggest that sleep has evolved in humans and other animals as a way to promote the survival of individuals as well as to increase their reproductive success. Each theory may explain, in part, why we sleep; but there probably are other explanations that will be added with time. Nevertheless, there is little conclusive evidence for any current theory of sleep.
To summarize, although many studies have attempted to discover the major functions of sleep, we still do not have good answers to this research question:
Like hunger or thirst, the drive for sleep appears to satisfy an elementary need. However, unlike eating and drinking, the purpose for sleep remains obscure. Sleep is the one major biological process whose raison d’être [reason for existing] has not yet been specified. Unraveling its essence constitutes a major challenge for biological research in general and for neuroscience in particular. (Borbély & Tononi, 1998, p. 167)
Several theories have been formulated over the years, but none have much evidence to support their claims.
Study Questions for Section 2-10
- Where is the reticular activating system (reticular formation) and what does it do?
- How is the brain stem involved in sleeping and waking?
- What do you think would happen with respect to REM sleep if you completely destroyed the pons in a cat?
- What would happen to your sleep if your brain produced too little serotonin?
- What would happen to your sleep if your brain produced too little melatonin?
- How are total sleep-deprivation and partial sleep-deprivation studies similar? How are they different?
- What happens to humans who are kept awake for several nights in a row?
- What happens to humans whose sleep per night is reduced for one week to several months?
- What happens to rats who are kept awake for weeks in a row?
- What are the major functions of sleep?