Sleeping: Function
Function
The multiple theories proposed to explain the function of sleep reflect the as yet incomplete understanding of thfffe subject.
It is likely that sleep evolved to fulfill some primeval function, but has taken over multiple functions over time as organisms have evolved as with the larynx which today performs multiple functions such as controlling the passage of food and air, phonation for communicating, and social purposes.
Some of the many proposed functions of sleep are as follows.
Restoration
Wound healing has been shown to be affected by sleep. A study conducted by Gumustekin et al. in 2004 shows sleep deprivation hindering the healing of burns on rats.
It has also been shown that sleep deprivation affects the immune system and metabolism. In a study by Zager et al. in 2007, rats were deprived of sleep for 24 hours. When compared with a control group, the sleep-deprived rats' blood tests indicated a 20% decrease in white blood cell count, a significant change in the immune system.
A study by Bonnet and Arand in 2003 indicates that sleep affects metabolism, is indeed a metabolic phase—anabolism. Comparing normal human sleepers and sleepers with sleep state misperception insomnia, where patients complain of poor sleep but have normal sleep by electroencephalographic (EEG) criteria, the researchers found significantly greater metabolism values for the normal sleepers.
It has yet to be clearly proven that sleep duration affects somatic growth. One study by Jenni et al.in 2007 recorded growth, height and weight, as correlated to parent-reported time-in-bed in 305 children over a period of nine years (age 1–10). It was found that "the variation of sleep duration among children does not seem to have an effect on growth". It has been shown that sleep, more specifically slow-wave sleep (SWS), does affect growth hormone levels in adult men. During eight hours sleep, Van Cauter, Leproult, and Plat found that the men with a high percentage of SWS (average 24%) also had high growth hormone secretion, while subjects with a low percentage of SWS (average 9%) had low growth hormone secretion.
There are multiple arguments supporting the restorative function of sleep. We are rested after sleeping and it is natural to assume that this is a basic purpose of sleep. The metabolic phase during sleep is anabolic; anabolic hormones such as growth hormones as mentioned above are secreted preferentially during sleep. The duration of sleep among species is, in general, inversely related to the animal size and directly related to basal metabolic rate. Rats with a very high basal metabolic rate sleep for up to 14 hours a day where as elephants and giraffes with lower BMRs sleep only 3–4 hours per day.
Energy conservation could as well have been accomplished by resting quiescent without shutting off the organism from the environment, potentially a dangerous situation. A sedentary non-sleeping animal is more likely to survive predators, while still preserving energy. Sleep therefore does something else other than conserving energy. Most interestingly, hibernating animals that wake up from hibernation go into rebound sleep because of lack of sleep during the hibernation period. They are definitely well rested and are conserving energy during hibernation, but need sleep for something else.Rats kept awake indefinitely develop skin lesions, hyperphagia, loss of body mass, hypothermia, and eventually septicemia and death.
Anabolic/catabolic
Non-REM sleep may be an anabolic state marked by physiological processes of growth and rejuvenation of the organism's immune, nervous, muscular, and skeletal systems (with some exceptions). Wakefulness may perhaps be viewed as a cyclical, temporary, hyperactive catabolic state during which the organism acquires nourishment and reproduces.
Ontogenesis
According to the ontogenetic hypothesis of REM sleep, the activity occurring during neonatal REM sleep (or active sleep) seems to be particularly important to the developing organism (Marks et al., 1995). Studies investigating the effects of deprivation of active sleep have shown that deprivation early in life can result in behavioral problems, permanent sleep disruption, decreased brain mass (Mirmiran et al. 1983), and an abnormal amount of neuronal cell death (Morrissey, Duntley & Anch, 2004).
REM sleep appears to be important for development of the brain. REM sleep occupies the majority of time of sleep of infants, who spend most of their time sleeping. Among different species, the more immature the baby is born, the more time it spends in REM sleep. Proponents also suggest that REM-induced muscle inhibition in the presence of brain activation exists to allow for brain development by activating the synapses yet without any motor consequences which may get the infant in trouble. Additionally, REM deprivation results in developmental abnormalities later in life.
However, this does not explain why older adults still need REM sleep. Aquatic mammal infants do not have REM sleep in infancy REM sleep in those animals increases as they age.
Scientists have shown numerous ways in which sleep is related to memory. In a study conducted by Turner, Drummond, Salamat, and Brown[28] working memory was shown to be affected by sleep deprivation. Working memory is important because it keeps information active for further processing and supports higher-level cognitive functions such as decision making, reasoning, and episodic memory. Turner et al. allowed 18 women and 22 men to sleep only 26 minutes per night over a 4-day period. Subjects were given initial cognitive tests while well rested and then tested again twice a day during the 4 days of sleep deprivation. On the final test the average working memory span of the sleep deprived group had dropped by 38% in comparison to the control group.
Memory also seems to be affected differently by certain stages of sleep such as REM and slow-wave sleep (SWS). In one study cited in Born, Rasch, and Gais multiple groups of human subjects were used: wake control groups and sleep test groups. Sleep and wake groups were taught a task and then tested on it both on early and late nights, with the order of nights balanced across participants. When the subjects' brains were scanned during sleep, hypnograms revealed that SWS was the dominant sleep stage during the early night representing around 23% on average for sleep stage activity. The early night test group performed 16% better on the declarative memory test than the control group. During late night sleep, REM became the most active sleep stage at about 24%, and the late night test group performed 25% better on the procedural memory test than the control group. This indicates that procedural memory benefits from late REM-rich sleep whereas declarative memory benefits from early SWS-rich sleep.
Another study conducted by Datta indirectly supports these results. The subjects chosen were 22 male rats. A box was constructed where a single rat could move freely from one end to the other. The bottom of the box was made of a steel grate. A light would shine in the box accompanied by a sound. After a 5 second delay an electrical shock would be applied. Once the shock commenced the rat could move to the other end of the box, ending the shock immediately. The rat could also use the 5-second delay to move to the other end of the box and avoid the shock entirely. The length of the shock never exceeded 5 seconds. This was repeated 30 times for half the rats. The other half, the control group, was placed in the same trial but the rats were shocked regardless of their reaction. After each of the training sessions the rat would be placed in a recording cage for 6 hours of polygraphic recordings. This process was repeated for 3 consecutive days. This study found that during the post-trial sleep recording session rats spent 25.47% more time in REM sleep after learning trials than after control trials. These trials support the results of the Born et al. study, indicating an obvious correlation between REM sleep and procedural knowledge.
Another interesting observation of the Datta study is that the learning group spent 180% more time in SWS than did the control group during the post-trial sleep-recording session. This phenomenon is supported by a study performed by Kudrimoti, Barnes, and McNaughton. This study shows that after spatial exploration activity, patterns of hippocampal place cells are reactivated during SWS following the experiment. In a study by Kudrimoti et al. seven rats were run through a linear track using rewards on either end. The rats would then be placed in the track for 30 minutes to allow them to adjust (PRE), then they ran the track with reward based training for 30 minutes (RUN), and then they were allowed to rest for 30 minutes. During each of these three periods EEG data were collected for information on the rats' sleep stages. Kudrimoti et al. computed the mean firing rates of hippocampal place cells during pre-behavior SWS (PRE) and three 10-minute intervals in post-behavior SWS (POST) by averaging across 22 track-running sessions from seven rats. The results showed that 10 minutes after the trial RUN session there was a 12% increase in the mean firing rate of hippocampal place cells from the PRE level, however after 20 minutes the mean firing rate returned rapidly toward the PRE level. The elevated firing of hippocampal place cells during SWS after spatial exploration could explain why there were elevated levels of SWS sleep in Datta's study as it also dealt with a form of spatial exploration.
The different studies all suggest that there is a correlation between sleep and the many complex functions of memory. Harvard sleep researchers Saper and Stickgold[32] point out that an essential part of memory and learning consists of nerve cell dendrites sending information to the cell body to be organized into new neuronal connections. This process demands that no external information is presented to these dendrites, and they suggest that this may be why it is during sleep that we solidify memories and organize knowledge.
Further information: Sleep and learning, Sleep and creativity
[edit] Preservation
The "Preservation and Protection" theory holds that sleep serves an adaptive function. It protects the person during that portion of the 24-hour day in which being awake, and hence roaming around, would place the individual at greatest risk. Organisms do not require 24 hours to feed themselves and meet other necessities. From this perspective of adaptation, organisms are safer by staying out of harm's way where potentially they could be prey to other, stronger organisms. They sleep at times that maximize their safety, given their physical capacities and their habitats. (Allison & Cicchetti, 1976; Webb, 1982).
However, this theory fails to explain why the brain disengages from the external environment during normal sleep. Another argument against the theory is that sleep is not simply a passive consequence of removing the animal from the environment, but is a "drive": animals alter their behaviors in order to obtain sleep. Therefore, circadian regulation is more than sufficient to explain periods of activity and quiescence that are adaptive to an organism, but the more peculiar specializations of sleep probably serve different and unknown functions.
Moreover, the preservation theory does not explain why carnivores like lions, which are on top of the food chain, sleep the most. By the preservation logic, these top carnivores should not need any sleep at all. Preservation does not explain why aquatic mammals sleep while moving. Lethargy during these vulnerable hours would do the same, and will be more advantageous because the animal will be quiescent but still be able to respond to environmental challenges like predators etc. Sleep rebound that occurs after a sleepless night will be maladaptive, but still occurs for a reason. For example, a zebra falling asleep the day after it spent the sleeping time running from a lion is more and not less vulnerable to predation.
It is likely that sleep evolved to fulfill some primeval function, but has taken over multiple functions over time as organisms have evolved as with the larynx which today performs multiple functions such as controlling the passage of food and air, phonation for communicating, and social purposes.
Some of the many proposed functions of sleep are as follows.
Restoration
Wound healing has been shown to be affected by sleep. A study conducted by Gumustekin et al. in 2004 shows sleep deprivation hindering the healing of burns on rats.
It has also been shown that sleep deprivation affects the immune system and metabolism. In a study by Zager et al. in 2007, rats were deprived of sleep for 24 hours. When compared with a control group, the sleep-deprived rats' blood tests indicated a 20% decrease in white blood cell count, a significant change in the immune system.
A study by Bonnet and Arand in 2003 indicates that sleep affects metabolism, is indeed a metabolic phase—anabolism. Comparing normal human sleepers and sleepers with sleep state misperception insomnia, where patients complain of poor sleep but have normal sleep by electroencephalographic (EEG) criteria, the researchers found significantly greater metabolism values for the normal sleepers.
It has yet to be clearly proven that sleep duration affects somatic growth. One study by Jenni et al.in 2007 recorded growth, height and weight, as correlated to parent-reported time-in-bed in 305 children over a period of nine years (age 1–10). It was found that "the variation of sleep duration among children does not seem to have an effect on growth". It has been shown that sleep, more specifically slow-wave sleep (SWS), does affect growth hormone levels in adult men. During eight hours sleep, Van Cauter, Leproult, and Plat found that the men with a high percentage of SWS (average 24%) also had high growth hormone secretion, while subjects with a low percentage of SWS (average 9%) had low growth hormone secretion.
There are multiple arguments supporting the restorative function of sleep. We are rested after sleeping and it is natural to assume that this is a basic purpose of sleep. The metabolic phase during sleep is anabolic; anabolic hormones such as growth hormones as mentioned above are secreted preferentially during sleep. The duration of sleep among species is, in general, inversely related to the animal size and directly related to basal metabolic rate. Rats with a very high basal metabolic rate sleep for up to 14 hours a day where as elephants and giraffes with lower BMRs sleep only 3–4 hours per day.
Energy conservation could as well have been accomplished by resting quiescent without shutting off the organism from the environment, potentially a dangerous situation. A sedentary non-sleeping animal is more likely to survive predators, while still preserving energy. Sleep therefore does something else other than conserving energy. Most interestingly, hibernating animals that wake up from hibernation go into rebound sleep because of lack of sleep during the hibernation period. They are definitely well rested and are conserving energy during hibernation, but need sleep for something else.Rats kept awake indefinitely develop skin lesions, hyperphagia, loss of body mass, hypothermia, and eventually septicemia and death.
Anabolic/catabolic
Non-REM sleep may be an anabolic state marked by physiological processes of growth and rejuvenation of the organism's immune, nervous, muscular, and skeletal systems (with some exceptions). Wakefulness may perhaps be viewed as a cyclical, temporary, hyperactive catabolic state during which the organism acquires nourishment and reproduces.
Ontogenesis
According to the ontogenetic hypothesis of REM sleep, the activity occurring during neonatal REM sleep (or active sleep) seems to be particularly important to the developing organism (Marks et al., 1995). Studies investigating the effects of deprivation of active sleep have shown that deprivation early in life can result in behavioral problems, permanent sleep disruption, decreased brain mass (Mirmiran et al. 1983), and an abnormal amount of neuronal cell death (Morrissey, Duntley & Anch, 2004).
REM sleep appears to be important for development of the brain. REM sleep occupies the majority of time of sleep of infants, who spend most of their time sleeping. Among different species, the more immature the baby is born, the more time it spends in REM sleep. Proponents also suggest that REM-induced muscle inhibition in the presence of brain activation exists to allow for brain development by activating the synapses yet without any motor consequences which may get the infant in trouble. Additionally, REM deprivation results in developmental abnormalities later in life.
However, this does not explain why older adults still need REM sleep. Aquatic mammal infants do not have REM sleep in infancy REM sleep in those animals increases as they age.
Scientists have shown numerous ways in which sleep is related to memory. In a study conducted by Turner, Drummond, Salamat, and Brown[28] working memory was shown to be affected by sleep deprivation. Working memory is important because it keeps information active for further processing and supports higher-level cognitive functions such as decision making, reasoning, and episodic memory. Turner et al. allowed 18 women and 22 men to sleep only 26 minutes per night over a 4-day period. Subjects were given initial cognitive tests while well rested and then tested again twice a day during the 4 days of sleep deprivation. On the final test the average working memory span of the sleep deprived group had dropped by 38% in comparison to the control group.
Memory also seems to be affected differently by certain stages of sleep such as REM and slow-wave sleep (SWS). In one study cited in Born, Rasch, and Gais multiple groups of human subjects were used: wake control groups and sleep test groups. Sleep and wake groups were taught a task and then tested on it both on early and late nights, with the order of nights balanced across participants. When the subjects' brains were scanned during sleep, hypnograms revealed that SWS was the dominant sleep stage during the early night representing around 23% on average for sleep stage activity. The early night test group performed 16% better on the declarative memory test than the control group. During late night sleep, REM became the most active sleep stage at about 24%, and the late night test group performed 25% better on the procedural memory test than the control group. This indicates that procedural memory benefits from late REM-rich sleep whereas declarative memory benefits from early SWS-rich sleep.
Another study conducted by Datta indirectly supports these results. The subjects chosen were 22 male rats. A box was constructed where a single rat could move freely from one end to the other. The bottom of the box was made of a steel grate. A light would shine in the box accompanied by a sound. After a 5 second delay an electrical shock would be applied. Once the shock commenced the rat could move to the other end of the box, ending the shock immediately. The rat could also use the 5-second delay to move to the other end of the box and avoid the shock entirely. The length of the shock never exceeded 5 seconds. This was repeated 30 times for half the rats. The other half, the control group, was placed in the same trial but the rats were shocked regardless of their reaction. After each of the training sessions the rat would be placed in a recording cage for 6 hours of polygraphic recordings. This process was repeated for 3 consecutive days. This study found that during the post-trial sleep recording session rats spent 25.47% more time in REM sleep after learning trials than after control trials. These trials support the results of the Born et al. study, indicating an obvious correlation between REM sleep and procedural knowledge.
Another interesting observation of the Datta study is that the learning group spent 180% more time in SWS than did the control group during the post-trial sleep-recording session. This phenomenon is supported by a study performed by Kudrimoti, Barnes, and McNaughton. This study shows that after spatial exploration activity, patterns of hippocampal place cells are reactivated during SWS following the experiment. In a study by Kudrimoti et al. seven rats were run through a linear track using rewards on either end. The rats would then be placed in the track for 30 minutes to allow them to adjust (PRE), then they ran the track with reward based training for 30 minutes (RUN), and then they were allowed to rest for 30 minutes. During each of these three periods EEG data were collected for information on the rats' sleep stages. Kudrimoti et al. computed the mean firing rates of hippocampal place cells during pre-behavior SWS (PRE) and three 10-minute intervals in post-behavior SWS (POST) by averaging across 22 track-running sessions from seven rats. The results showed that 10 minutes after the trial RUN session there was a 12% increase in the mean firing rate of hippocampal place cells from the PRE level, however after 20 minutes the mean firing rate returned rapidly toward the PRE level. The elevated firing of hippocampal place cells during SWS after spatial exploration could explain why there were elevated levels of SWS sleep in Datta's study as it also dealt with a form of spatial exploration.
The different studies all suggest that there is a correlation between sleep and the many complex functions of memory. Harvard sleep researchers Saper and Stickgold[32] point out that an essential part of memory and learning consists of nerve cell dendrites sending information to the cell body to be organized into new neuronal connections. This process demands that no external information is presented to these dendrites, and they suggest that this may be why it is during sleep that we solidify memories and organize knowledge.
Further information: Sleep and learning, Sleep and creativity
[edit] Preservation
The "Preservation and Protection" theory holds that sleep serves an adaptive function. It protects the person during that portion of the 24-hour day in which being awake, and hence roaming around, would place the individual at greatest risk. Organisms do not require 24 hours to feed themselves and meet other necessities. From this perspective of adaptation, organisms are safer by staying out of harm's way where potentially they could be prey to other, stronger organisms. They sleep at times that maximize their safety, given their physical capacities and their habitats. (Allison & Cicchetti, 1976; Webb, 1982).
However, this theory fails to explain why the brain disengages from the external environment during normal sleep. Another argument against the theory is that sleep is not simply a passive consequence of removing the animal from the environment, but is a "drive": animals alter their behaviors in order to obtain sleep. Therefore, circadian regulation is more than sufficient to explain periods of activity and quiescence that are adaptive to an organism, but the more peculiar specializations of sleep probably serve different and unknown functions.
Moreover, the preservation theory does not explain why carnivores like lions, which are on top of the food chain, sleep the most. By the preservation logic, these top carnivores should not need any sleep at all. Preservation does not explain why aquatic mammals sleep while moving. Lethargy during these vulnerable hours would do the same, and will be more advantageous because the animal will be quiescent but still be able to respond to environmental challenges like predators etc. Sleep rebound that occurs after a sleepless night will be maladaptive, but still occurs for a reason. For example, a zebra falling asleep the day after it spent the sleeping time running from a lion is more and not less vulnerable to predation.