Considering the functions of sleep and dreams, it has been suggested that humans spend approximately one third of their lives asleep (cited in Goldsmith, 2005). However, there still remains no clear agreement on what the function of sleep is. This essay focuses on exploring how humans sleep, and possible explanations for why we sleep and why we dream. It is believed that sleep follows a circadian rhythm, effecting sleep onset and stages (cited in Sanei & Chambers, 2007). The human biological circadian rhythm is roughly a 24 hour cycle, which is controlled by a circadian pacemaker.
This “pacemaker” is the section of the brain known as the suprachiasmatic nucleus (SCN), which is situated in the hypothalamus (cited in Stickgold & Walker, 2009). Signals produced by the SCN travel to different regions of the brain which controls the sleep-wake cycle within humans (cited in Kramer et al, 2001). The SCN regulates other functions associated with the sleep cycle such as body temperature, hormone secretion, urine production, and changes in blood pressure, which are all known to decrease during the sleep process.
The SCN controls and is entrained to the sleep-wake cycle which is dependent on the cycle of light and dark, and on body temperature. A change in these could shift or disrupt the cycle. One internal factor affecting the circadian rhythm is melatonin (cited in Bermudez, Forbes & Indiji, 1983). The SCN regulates the pineal gland’s secretion of melatonin, which has a day/night function. Melatonin is a hormone that helps to regulate sleep. The release of melatonin is dependent on the availability of light on the ganglion cells which contain the photopigment melanopsin.
The ganglion cells receive light information through receptors known as rods and cones. This information travels along the retinohypothalamic pathway, where the SCN receives and interprets the environmental light, which determines the release of melatonin (cited in Hannibal ; Fahrenkrug, 2002). Peak levels of melatonin arise in the darkness at night when the release of melatonin is not suppressed. The lowest levels of melatonin arise in daylight, when the suppression of melatonin occurs due to the availability of light.
This process allows the body to sleep effectively at night, and wake in the day (cited in Olive, 2006). The importance of melatonin in the process of sleep is emphasised in a study carried out by Czeisler et al in 1995. This study looked into the effects of light and melatonin in blind people. The findings indicated that, in general, most blind people are not entrained to a 24-hour day, and tend to suffer from sleeping disorders such as insomnia in spite of sleep, work and social contact.
One of the suggestions for this is that due to blind people having a limited exposure to light, melatonin is not suppressed during the day. This leads to a dysfunction in the 24-hour cycle which can possibly lead to sleep disorders. External factors that affect the circadian rhythm are called zeitgebers. A zeitgeber is an external cue which influences the operations of the internal clock in humans (cited in Smith, Comella ; Hogl, 2008).
One of the most powerful zeitgebers is light, which is known as an ‘exogenous zeitgeber’; a natural cue to influence the sleep-wake cycle in humans. However, a study by Aschoff and Weber (cited in Kleitman, 1963), used an underground bunker, where student participants were allocated to stay for a period of time. Although the participants did not have any cues to light, they were still found to settle into a regular sleep-wake cycle of approximately 25-27 hours. The results of this suggest that humans may have internal mechanisms to influence the pattern of sleep.
It is suggested therefore that humans possess some form of a natural alarm clock, which can trigger a human’s internal clock, such as an internal cue to eat, wake or make an important meeting (cited in Grondin, 2008). The way in which humans sleep has been observable since the introduction of the electroencephalograph (EEG). The EEG, introduced by Berger in 1929 (cited in Smith, Van Gils & Prior, 2006), records the electrical activity of billions of cortical neurons using a number of small metal electrodes on the surface of the skull.
The electro- oculography (EOG), measures eye movement, whereas the electromyography (EMG), measure muscle tension. This non-invasive procedure can be ‘synchronised’ with a repeated and recognisable waveform, or ‘desynchronised’, with an apparently random pattern of spikes and waves, where there is no consistent waveform. The EEG has a number or frequency of spikes or waves per second, which is measured as cycles per second, known as herz (Hz). Aserinksy and Kleitman in 1953 (cited in Siegel, 2002), concluded that there are two distinct stages of sleep; ‘slow-wave’ sleep (SWS) and ‘rapid eye movement’ sleep (REM).
There are believed to be four stages of SWS (cited in Sircar, 2008). In stage one, the EEG pattern demonstrates ‘theta waves’, with a frequency of 4 to 7 Hz. Stage two occurs approximately ten minutes after this, where the EEG pattern becomes synchronised with slower and larger waves which are interrupted by bursts of fast spiking activity, known as the ‘sleep spindles’, with a frequency of 12 to 16 Hz. Stage three of SWS is dominated by large ‘delta waves’ with a frequency if 1 to 3 Hz, with sleep spindles becoming less common.
Heart rate, respiration, metabolic rate and body temperature continue to fall at this stage. In stage four, the EEG recording consists only of delta waves, with a remaining frequency of 1 to 3 Hz. At this stage, metabolic rate is at its lowest. The arousal threshold (a measure how difficult it is to wake an individual from sleep) is very high. Sleep is believed to be a dynamic process; after approximately 30 minutes in stage four of SWS, it is suggested that humans ascend through the sleep stages to the light SWS of stage two (cited in Hobson, 1994).
At this point, approximately 90 minutes after going to sleep, the EEG activity suddenly shifts into the fast, desynchronised pattern of the aroused person. This stage of sleep is known as ‘rapid eye movement’ sleep (REM) (cited in Bear, Connors & Paradiso, 2007). At the point where an individual reaches REM sleep, arousal thresholds are very high and the skeletal muscles relax completely, leaving the individual effectively paralysed. At the same time, respiration and heart rate increases and rapid movement of the eyes occur (cited in Butkov & Lee-Chiong 2007).
After approximately 15 minutes into REM sleep, the individual returns back into light SWS sleep before descending into the deeper stages three and four of SWS sleep. This pattern repeats itself approximately every 90 minutes and the individual experiences this cycle five or six times in one night (cited in Fadem, 2004 . The importance of the function of REM sleep is emphasised in a sleep deprivation study carried out by Dement in 1960 (cited in Foulkes, Pivik & Ahrens, 1968). Dement’s investigation involved depriving participants of REM sleep.
His findings indicated that the lack of REM sleep lead to increased aggression and poor concentration. His study also found that when participants were deprived of sleep, this lead to an attempt to increase REM sleep in subsequent nights by approximately 20 to 30 per cent. The findings of Dement’s study suggest that REM sleep possibly satisfies a mental or biological need within humans (cited in Loker, 2003). The function of non-REM sleep has also been indicated through studies carried out to observe its function.
Shapiro et al (1981, cited in Shapiro, Bortz, Mitchell, Bartel & Jooste, 1981) carried out an investigation into the effects of exercise on sleep, where he studied athletes over four consecutive nights after they had completed a 92 kilometre race. Shapiro found significant increases in sleep time and SWS over the four nights, suggesting that SWS is vital for restoration within humans. However, Horne and Moore (1985, cited in Billard & Kent, 2003) suggest that exercise does not lead to abnormal sleep patterns. They suggest instead that it is the increase in body temperature which is responsible.
In their study, they found that spraying athletes with water while they ran led to their body temperature reducing. As a result, they found that the athletes demonstrated regular sleep patterns. The findings of Horne and Moore, suggest that high body temperature leads to increased SWS and irregular sleep patterns. The findings of a number of studies indicate that both SWS and REM sleep are vital for healthy human functioning. When considering the reasons as to why humans sleep, there are two main theories that offer an explanation for the function of human sleep; the evolutionary theory and the restoration theory.
The central focus of the evolutionary theory proposes that the function of sleep in humans has evolved from their ancestral past. Webb (1974, cited in Kalat, 2009) proposes an adaptation hypothesis, suggesting that the amount of sleep an organism requires is reliant on food availability. Webb suggests that the amount of sleep is dependent on how readily available food resources are to an individual. The reasoning behind Webb’s suggestion is that the more scarce food is, the more sleep an organism requires in order to conserve what energy they have.
His theory proposes that during sleep, behavioural activity stops, meaning less energy is spent on movement. He suggests further that body temperature and metabolic rate slow down during sleep, saving energy. Humans in modern times, have a more easily accessible food resource, resulting in a biological need of only approximately 8 hours per day. Although it is suggested that their ancestors slept slightly longer than they do in the modern day due less readily available food resources (cited in Zastrow ; Kirst-Ashman, 2007), humans have evolved to still require approximately 8 hours of sleep in a day (cited in Stevens, 2004).
Meddis (1975, cited in Singhal, Johnson ; Lander, 2007) offers a further evolutionary explanation and proposes a theory of protection to explain why humans sleep. Meddis suggests that in the past, night time would have been a period of great danger. He implies further that as a species with generally poor night vision, it would have been unintelligible to forage during the night. Meddis offers this explanation on the grounds that humans would have been more likely to fall or hurt themselves, or be at increased risk of danger.
The protection theory suggests that that it is an evolutionary advantage to sleep during the night as it keeps humans protected from harm and danger. Meddis proposes a genetic explanation as part of an evolutionary approach, suggesting that those who slept during the night were more likely to have survived to maturity and passed on their genes, ensuring that as an activity, sleep would have been retained in our behavioural response. The protection theory is however open to criticism.
It is suggested that if the protection theory is to be accepted then it would be expected that those who are exposed to more risk of danger and predation would sleep more (cited in Moorcroft ; Belcher, 2005). However this is does not appear to be the situation in some cases. Research indicates that species such as herbivores, who are at most risk sleep less, whereas big cats such as lions who are at little risk of danger, tend to sleep for longer periods of time (cited in Moorcroft, 1993).
As a result of these findings, Meddis concluded that food intake has an influence on the amount of time an individual spends asleep, suggesting that those who spend more time eating have less time to sleep and vice versa . Although the protection theory suggests that sleep serves an important adaptive function, it is not clear why such a complex physiological mechanism as sleep would evolve simply to keep vulnerable individuals safe. It is suggested therefore that a state of behavioural inactivity, would serve much the same purpose (cited in Schedlowski ; Tewes, 1999).
In addition, the adaption hypothesis, put forward by Webb 1974 can also be criticised for its suggestion that sleep is a function of energy conservation. Meddis (1975, cited in Archer, 1992) suggested that the hypothesis is far too simplistic, and proposes instead that the function of sleep is a combination of protection and dietary requirements. In general, although the evolutionary theory offers a sound explanation of human sleep in terms of a developmental process from their ancestral past, criticisms cast doubt over the validity of the theory as a whole.
The second theory of sleep is the restoration theory. The general suggestion of the theory is that sleep is necessary to repair the body and restore the body to its full ‘waking’ capacity, repairing the general wear and tear caused by daily activities. A specific way in which the body ‘restores’ the materials of the day comes from the finding that growth hormone is released during SWS (cited in Latta et al, 2005). As well as being highly involved in the growth process, growth hormone is also has an important role in the metabolism of proteins.
Protein synthesis is vital for the restoration of body tissue, which must be constantly renewed and replaced during sleep (cited in Carlson, 1994). One model that is a component of the restoration theory is Horne’s core sleep/ optional sleep model (1988, cited in Toates, 2007). As a result of a number of controlled studies, Horne’s model concluded that although sleep deprivation in normal participants was not dramatic, there were some problems with cognitive abilities, such as memory, attention and perception.
Horne also discovered that in such situations, sleep recovery of stage 4 SWS and REM sleep occurs, suggesting that these are critical phases. As a result, Horne suggests therefore that ‘core sleep’, consisting of stage 4 SWS and REM sleep is essential for effective brain functioning in humans and is essential for our cognitive abilities. Horne suggests further that the lighter stages of SWS are not essential, and he refers to these as ‘optional sleep’. One unique study of sleep deprivation is that carried out by Dement in 1978 (cited in Ewert, 1997).
This study investigated a case of a 17 year old boy who remained awake for 262 hours. He developed blurred vision, some perceptual disturbances, incoherent speech and a mild degree of paranoia. Although this situation is not typical of regular sleep deprivation, this study is important for development the possible negative effects sleep deprivation may have on cognitive functioning. Horne concluded that although sleep is responsible for brain recovery, he suggests that as sleep deprivation does not have dramatic effects on bodily functions.
He suggests therefore that relaxed wakefulness is vital for body restoration, leaving core sleep to provide for the restoration of brain systems (cited in Siegel, 2002). Another model that ties in with the restoration theory is Oswald’s restoration model (1980). Oswald’s model is similar to that put forward by Horne. Both models agree that REM sleep is vital for brain development, repair and recovery. However, the models differ slightly with regards to the function of SWS sleep.
Whereas Horne suggests that SWS is not responsible for body recovery, Oswald suggests that the release of growth hormone during SWS implies that this stage of sleep is vital for body recovery. Both models however, do agree on the importance of REM sleep. Oswald in particular noted that as the months before and after birth are a time of rapid brain growth and development, REM sleep is vital for these processes, therefore it is logical that a baby should show increased amounts of REM sleep. This corresponds with findings that new-born babies experience approximately 50 to 60 per cent of REM during their sleep cycle (cited in Oswald, 1980).
Although the models put forward by Horne and Oswald are slightly different, they encompass the restoration theory and emphasise the function sleep, REM sleep in particular, as being vital to brain restoration and recovery. A criticism of the restoration theory is the suggestion that sleep is vital for repairing the effects of wear and tear during the day. Rosenzweig et al (1999) proposes that it would be expected therefore that intense exercise during the day would lead the individual to sleep for longer, but suggests however that this is not the case. This therefore questions the suggestion that sleep has a restorative function.
A further criticism of the restoration model is the suggestion that growth hormone is important for protein synthesis. Horne (1988) indicates that as amino acids are only freely available for five hours after a meal, most individuals eats several hours before sleeping, implying that not a vast amount of protein synthesis occurs during the night. Although the restoration theory highlights the importance of sleep for recovery and restoration, it appears than more research into this field is vital to gain a deeper understanding. A distinctive feature of REM sleep is dreaming.
It is suggested that although people who are woken during non-REM sleep may report dreaming, there are much more likely to report dreams if they are woken during a phase of REM sleep (cited in Toates, 2007). One approach taken as to why humans dream is encompassed in the neurobiological theories of dreaming. One neurobiological model of dreaming is that put forward by Crick and Mitchison in 1983. Their model proposes that the brain is ‘off line’ during dreaming, and that during this phase it sifts through the information gathered during the day’s waking activities and throws out all unwanted material.
According to this model, we dream in order to forget and this involves a process of ‘reverse learning’. Their model suggests that the cortex cannot cope with the vast amount of information received during the day without developing ‘parasitic’ thoughts, which would disrupt the efficient organisation of memory. During REM sleep, these unwanted connections in cortical networks are wiped out by impulses bombarding the cortex from sub-cortical areas. The actual content of dreams represents there parasitic thoughts as they are erased form memory.
One criticism of the reverse-learning model however is that dreams are often organised into clear stories or narratives. The problem with this model is that if dreams consist only of parasitic thoughts then it would be feasible that they should be organised in this systematic way (cited in Runco & Pritzer, 1999). Another neurobiological model of dreaming is the activation-synthesis model, proposed by Hobson and McCarley in 1988 (cited in Hobson, 1988). The activation component of this model concerns the regular switching on of REM sleep as part of the cycle of sleep stages.
This model proposes that the REM mechanism is based in the brain stem and when activated, it inhibits skeletal muscles, producing ‘paralysis’, exciting activity in the forebrain via pathways ascending from the brainstem. Hobson and McCarely suggest that this leads to the arousal of sensory and motor information, forming the basis of dream experiences. The activation synthesis hypothesis states that dreaming is an automatic part of the brain’s sleep mechanisms and be seen as an ‘in-built’ process, which may have no significance beyond the brain’s natural drive to organise material into a coherent sequence.
Hobson and McCarley suggest that dreams can either be meaningless or creative. This point in particular has led to much criticism, as due to the breadth of model, it is therefore difficult to explain any type of dream experience and it has little predictive power. It is suggested therefore that it is difficult to validate the model. Psychological theories are also used to explain the functions of dreams. One explanation from this perspective is that there is a link between dreams and learning. Stickgold (1998) carried out a study into self-reported dream imagery.
Stickgold found that participants often report dream imagery that includes elements of a current task that is required to be learned. He concluded that both dreams and REM sleep are important for the learning process. Such studies however have been criticised for being carried out in laboratories, questioning the validity of such findings (cited in Globus, 1987). One key explanation of dreams from the psychological approach comes from Sigmund Freud, who looked at dreaming from a psychodynamic perspective (cited in Freud, 1955).
Freud believed that a dream is a disguised fulfilment of desires repressed into the unconscious mind. He suggested that these desires have to be disguised as they may be sexual or aggressive urges which will be unacceptable to the dreamer when awake. He implies that that the two functions of dreams are to protect the sleeper, but simultaneously allow some expression of these repressed urges. Freud suggests that due to this, the dream has a ‘manifest content’ which the dreamer reports and the analyst interprets in order to reveal the ‘latent content’, which directly reflects the repressed urges.
The transformation of hidden desires and anxieties into the manifest symbolism of the dream is called ‘dream work’. Several mechanisms may be involved, including condensation (refers to the way in which a particular dream symbol acts as a focus for several hidden thoughts), displacement (occurs particularly with emotional reaction which may be disproportionate to the event itself) and representability (involved in the visual representation of underlying thoughts within dreams.
In order to interpret the latent content of dreams, Freud developed a vocabulary of dream symbols, drawing on cultural symbols found in mythologies, stories and jokes, as examples. He suggested, for instance, that dreams of flying represented sexual intercourse. Freud emphasised also that dream imagery tends to contain representations of the day’s events, which can be used to disguise the latent content. He did accept however that dream imagery could be accepted at face value, where certain imagery did not have underlying meaning.
Freud’s theory has been criticised however, as it relies on the subjective reports of the dreamer which may be inaccurate (cited in Mattoon, 1984). However, as dreams are personal and inaccessible, it is impossible to carry out such interpretation in any different manner. One general criticism of dream research is that many studies are carried out using participants who are deprived of REM sleep. As a result, it is suggested that the content of dreams within these studies may not be valid, and the imagery and content may be influenced by the deprivation of such stages of sleep (cited in Schwartz, 1978).
Although there are many different views with regards to the function of sleep and dreams, many theories have offered sound explanations of the processes. However, there is still no clear-cut, widely accepted explanation as to why humans sleep and dream. As a result, more intense and extensive research into these areas may assist in offering more conclusive theories that may offer more concise explanations as to the functions of sleep and dreams.