The nature of sleep
Sleep is a different state of consciousness where responsiveness to the external environment is diminished. Sleep is an example of both a circadian rhythm (it occurs daily) and an ultradian rhythm (with its cycle of separate stages). We sleep to repair the body and brain
Types of sleep
In the 1950s, the use of the electroencephalograph (EEG) led to considerable progress in the study of sleep, leading to the identification of different stages of sleep as well as different types of sleep.
An EEG uses electrodes on the scalp to record the electrical activity of the brain. Berger (1929 identified 2 major EEG patterns:
Desynchronised EEG – this is usually recorded when somebody is awake.
Synchronised EEG – a synchronised EEG indicates that millions of neurons are firing together in unison. This is usually recorded when someone is asleep.
In the 1950s Aserinsky and Kleitman (1953) and Dement and Kleitman (1957) were the first to use EEG recordings to show that sleep was a far more complicated brain state than was peviously thought. They demonstrated that each individual goes through the same stages of sleep, and that various states of human sleep and arousal could be identified through their EEG patterns. They also identified 2 types of sleep: REM and NREM
REM= dream. NREM = stage 1-4
Stage 1,2,3,4 + REM = 1 90 min cycle. (all this in 1st cycle, but not all will be in the other 4)
The nature of sleep
- Most people have 5 cycles of sleep per night that last approx 90 mins.
- SWS occurs in only the first 2 cycles; REM sleep occurs in all of the cycles, and increases during the course of the nights sleep
Each stage of sleep has its own corresponding EEG patterns and this is what has given us the knowledge that there are 4 stages in NREM sleep (which is also known as quiet sleep), as well as the distinctive stage of REM sleep.
- EEG characterised by theta waves (4-7 Hz).
- Over a period of approximately 15 minutes, patterns of activity become synchronised, and a regular pattern emerges.
- The parasympathetic nervous system is active and so the heart rate slows and muscles relax.
- The relaxed first stage of sleep is also known as the hypnogogic state, and hallucinatory images occurring here are linked to creativity.
- EEG is dominated by slow theta waves, but also with bursts of sleep spindles; these are fast (12-16 Hz)EEG activity that last for a second or so.
- These sleep spindles are linked to external stimuli which do not wake us up, eg the wind whistling.
- Heart rate, blood pressure, and body temperature continue to fall.
- This stage lasts for about 20 minutes.
- Delta waves (1-4 Hz) first appear and the number of spindles falls.
- Delta waves have larger amplitudes than theta waves.
- Alongside this slowing of brain waves heart rate and rate of breathing also slow.
- This stage lasts for about 15 minutes.
- The EEG is dominated by large, slow delta waves.
- This is the deepest stage of NREM, when people are most soundly asleep.
- This is when growth hormone is secreted.
- Metabolic rate is at its lowest and it is very difficult to wake from this state.
- This stage typically lasts 30-40 minutes and is the bottom of the ‘sleep staircase’.
- Although it is the deepest stage of quiet sleep, this is the stage in which sleepwalking is more likely to occur.
cont..REM (rapid eye movement)
- After about 30 minutes or so in Stage 4 sleep, the EEG trace speeds up the ‘sleep staircase’, through Stages 2 and 3, showing the brain is suddenly more active.
- The brain waves desynchronise and become complex (similar to the brain waves we experience when awake), as well as faster, and the brain’s oxygen and glucose demands increase.
- The eyes start to move under closed eyelids, and we enter REM (or dreaming) sleep. The is the most difficult stage to be awoken from.
- However, although cerebrally active, we are, in effect, physically paralysed.
- The RAS (reticular activating system) in the midbrain seems to set up a block, isolating the brain from the rest of the body.
- Heart and blood pressure increase and there may be increased blood flow in sexual organs, but the individual is still difficult to awaken.
- This contrast between an aroused brain and a sleeping state and largely unresponsive body led to REM also being termed paradoxical sleep.
- When you first sleep: stage 1,2,3,4,REM then you go back up, 3,2,REM
- Paralysis important when in REM otherwise you would act out your dream which is dangerous
- REM sleep lasts for about 10-15 minutes and completes each cycle of sleep. We then go back down through Stages 2-4 again, and this cycle repeats about every 90 minutes through the night, although the time spent in Stages 3 and 4 gets progressively less until only Stages 1 and 2 of NREM plus REM sleep are returned to by the end of the night.
- This 90-minute cycle is the ultradian rhythm of sleep.
Lifespan changes of sleep. (nature of sleep part 2
Lifespan changes are a crucial determinant of the amount of sleep people need. Sleep needs vary by age, both qualitatively, in terms of the different stages of sleep, and quantitatively, in terms of how much sleep is needed.
- Babies sleep for about 16 hours a day, but this sleep is broken up into short periods since their sleep cycles are shorter than the adult 90-minute cycle.
- Infants have sleep stages which are similar to adult stages, called quiet sleep and active sleep: these are immature versions of SWS (slow wave sleep) and REM.
- After birth there is more active sleep than the equivalent REM sleep in adults, with about half an infant’s sleep being active.
- Infants also often enter active sleep immediately, and it is not until they are about 3 months old that the sequence of REM/NREM sleep is established.
- Over the first few months of life, the proportion of REM sleep decreases rapidly.
- By about 6 months a circadian rhythm has become established (one main sleep-wake cycle), and by the age of one year infants usually sleepmainlyat night, with one or two naps during the day.
- By the age of 5, children have EEG patterns that look like those of an adult, but they are still sleeping more (about 10-12 hours a day) and having more REM activity (about 30% of sleep time).
- Boys sleep slightly more than girls.
- During childhood it is not uncommon for children to experience a variety of parasomnias – sleep disorders such as sleep walking or night terrrors.
- During childhood the amount of sleep needed gradually decreases, but in adolescence it increases slightly to about 9-10 hours a night.
- Adolescents have less REM sleep than children (but longer). Circadian rythms also change so that teenagers feel naturally more awake later at night and have more difficulty getting up in the morning.
- These changes may be linked to a decrease in melatonin signals, which determine the biological clock in all cells in the body; these decrease during adolescence as a signal to the body to start puberty.
- Furthermore, external pressures on teenagers (school work, part-time work, socialising, etc) may lead to a less regular sleep cycle.
- The ‘normal’ adult sleep pattern involves approximately 8 hours sleep per night, with 25% in REM sleep.
- Childhood-related parasomnias are relatively rare in adulthood, but there is an increasing frequency of other sleep disorders, eg sleep apnoea (stopping breathing for a few seconds at a time) and insomnia.
- During middle age a shallowing and shortening of sleep may occur. There is a decrease in the amount of deep stage 4 sleep, and it may be harder to stay awake or feel refreshed on waking.
- Such changes may be linked to lifestyle factors, such as being overweight, lack of exercise, alcohol or caffeine consumption. Hormone changes (for both men and more significantly women) may also contribute to a reduction to about 7 hours sleep a night, and an increase in lighter stages of sleep.
- The percentage of REM sleep remains fairly constant.
- Sleep at this age is characterised by frequent interruptions and periods of wakefulness.
- Many suffer a form of sleep apnoea, and Stage 3 and 4 sleep (slow wave, deeper sleep) is much reduced, perhaps because of a reduced need for growth hormones.
- REM sleep decreases to about 20% of total sleep time.
- Older people also experience a phase advance of circadian rhythms, feeling sleepier early in the evening and waking up earlier
- Research into sleep has often been conducted in sleep laboratories, using objective measurements. Changes noted in the different stages of sleep, involving quantity and quality of sleep have been replicated and are well established. This is particularly true for infant sleep patterns since much research has been carried out into sleep in order to investigate the cause of SIDS (sudden infant death syndrome).
- However, on the other hand, the location and conditions under which the research is carried out may mean that findings lack ecological validity. Being connected to machines, with electrodes on your scalp, whilst aware that you’re being watched may not ensure you have the best night’s sleep possible!
- There has been little research into normal sleep among the middle-aged. Dement (1999) suggested this is because they’re busy getting on with their lives and are reluctant to volunteer for research unless they have a problem. (IDA issue = generalisability)
- Sleep is often affected by external factors, eg work, stress, medication; even sleeping beside someone may affect the quality and quantity of sleep a person gets.
- Older individuals often have little slow wave sleep, which affects hormone production and may explain why physical injuries take longer to heal.
Infants’ sleep patterns may have become established to ensure that parents have time to carry out other tasks. In other words, the differences seen in babies have an adaptive function: they help ensure survival. Night waking also ensures that babies, with their smaller stomachs, get enough food to grow and develop. This point is further supported by the fact that premature babies spend 90% of their time in active sleep.
The increase in REM sleep shown by babies may reflect their immature brains, and the considerable amount of learning that is taking place. This point is further supported by the fact that premature babies spend 90% of their time in active sleep.
Hormonal changes occurring during adolescence may well explain the differences in sleep patterns experienced by adolescents.
Too much sleep in adulthood has been linked to an increased risk of mortality. Kripke et al (2002) found that people sleeping only 6 or 7 hours a night had a reduced mortality risk, whereas those sleeping for an average of 8 hours had a 15% increase in risk of death, and the risk was over 30% for people sleeping 10 hours. But take note, this data is correlational, which means there's no causal effect
- Medical/physical issues may underlie the reduced sleep experienced by older people. On the other hand, sleep deficit may explain why older people experience impaired functions, eg alertness.
- Floyd et al (2007) reviewed nearly 400 sleep studies, finding that REM sleep decreased by about 0.6% a decade.
- Dement (1999) reported that over 40% of a group of healthy men and women aged 65-88 had some form of sleep apnoea, often involving frequent ‘micro-arousals’, lasting 3 seconds or less.
- Baird et al (2009) found that early sleep patterns (and also the mental health of women prior to conception) can have effects on later behaviour and learning.
- Borbely et al (1981) questioned adults aged 65-80 on their sleeping habits, finding that 60% of them reported taking frequent daily naps. Sleep in the elderly is more interrupted, but they continue to need the same amount of sleep as they did in early adulthood, hence the need for naps.
(Look at essay plan and handout for IDA, Practical applications and cultural bias and page 14)
Functions of sleep, incl. evolutionary & restorati
During sleep, important biological functions take place, restoring the biological system to good working order; this is the basis of the restoration theories. Alternatively, sleep may not have specific benefits other than conserving energy, or keeping an animal safe from predators; this is dealt with by explanations adopting an evolutionary approach.
OSWALD'S RESTORATION THEORY (ALL AO1)
Two of the stages of sleep – slow wave sleep (SWS) and REM sleep – are associated with particular benefits. Oswald (1980) proposed that these have different functions: SWS enables body repair and REM enables brain recovery.
SLOW WAVE SLEEP (ALL AO1)
Growth hormone is secreted during SWS. This stimulates growth and is, therefore, particularly important in childhood. It is also important in adulthood because it enables protein synthesis and cell growth to take place, both of which are vital in restoring body tissue since proteins are fragile and must be constantly renewed.
Growth hormone is secreted in pulses through the day, but a significant amount is released at night, mainly during SWS sleep. Sassin et al (1969) found that if the sleep-waking cycle is reversed by 12 hours, the release of GH is also reversed, showing a link between neural mechanism related to SWS and release of GH. Van Cauter and Plat (1996) also found that the amount of GH released correlates with the amount of SWS, and the decline of GH in older age has also been associated with reduced SWS (van Cauter and Plat, 2000).
The immune system - lack of SWS has also been associated with reduced functioning of the immune system. The immune system consists of various protein molecules, antibodies, which are regenerated during cell growth and protein synthesis in SWS.
REM SLEEP (AO1)
Brain growth – the percentage of REM sleep is much greater in babies than in adults, and even higher in premature babies. This has been explained in terms of their rapid brain growth.
Neurotransmitter activity – this may be affected by REM sleep. Siegal and Rogawski (1988) suggest that REM sleep allows for a break in neurotransmitter release, which allows neurons to regain their sensitivity. Support for this view comes from the action of some anti-depressant drugs, which increase levels of neurotransmitters and seem to abolish REM sleep suggesting a link between the two.
REM sleep and memory – Crick and Mitchison (1983) proposed that during REM sleep unwanted memories are discarded, thus allowing more important memories to be easily accessible. Recent research, eg Stickgold (2005) suggest the relationship between REM sleep and memory is more complex. It seems that REM sleep may be important in allowing the consolidation of procedural memories, ie memory for skills such as riding a bike, whereas SWS sleep is important in the consolidation of semantic memory (related to knowledge and the meaning of things) and episodic memory (memory for events).
- Oswald’s restoration theory is only one such theory. Horne’s core sleep model suggests that many restorative processes occur when we’re awake, suggesting that sleep doesn’t provide any repair function in humans, except for the brain. Horne referred to core sleep, which he believed is necessary for restoration, whereas other types of sleep he called optional, the main purpose being energy conservation (see evolutionary explanations). Stage 4 and REM sleep experienced in the early hours of sleep are seen as necessary for a healthy brain function: during these stages the brain is seen as refreshing and restoring itself, ready for the challenges of the next day.
- Findings from studies of sleep deprivation have been mixed.Examples of total sleep deprivation are rare and usually single cases, therefore findings may lack generalisability.However, findings suggest that lack of sleep doesn’t always lead to long-term damage, and there is no need to recover anything like the amount of sleep that was lost.However, when individuals have been deprived of sleep for more than 72 hours, it seems they have short periods of microsleep while apparently remaining awake.Williams et al (1959) found that EEG readings for such individuals show that microsleep produce the same brain wave patterns as real sleep.
- Animal studies have found that sleep deprivation may have fatal consequences. RECHTSCHAFFEN - Sleep deprivation leads to death. Done on rats, kittens and puppies. Rats lasted 11 days then died & kittens and puppies lasted 8 days then died. Animal studies raise issues of ethics, validity and generalisability because can't do on people as they'd die (so unethical), animals have different sleep requirements and different pattern (e.g. dolphins don't have REM sleep)
- Partial sleep deprivation may lead to ‘REM rebound’ – the need for more REM sleep after a night deprived of REM sleep. This has been investigated by researchers waking sleeping volunteers as soon as they enter REM sleep. The result is that people show an increased tendency to go into REM sleep when they go back to sleep, and on subsequent nights a higher percentage of time is spent in REM sleep than normal. ‘SWS rebound’ has also been found when SWS has been prevented by acoustic stimulation (noise) whenever the volunteers EEG appear to be going into deep sleep/SWS.
- Patients with brain trauma, either through injury or ECT, spend an increased amount of time in REM sleep, suggesting that it is the increased blood flow during REM sleep that aids brain repair and restoration.
- Oswald’s findings of increased protein synthesis during SWS have not been supported by others who find a decrease rather than an increase in protein synthesis of the whole body during sleep in humans. This decrease may be linked to overnight fasting as protein synthesis remains constant when individuals are fed continuously via intragastric types throughout the 24-hour period.
- Fatal familial insomnia is a rare human condition, usually starting in middle age, where sufferers cannot sleep and usually die within 2 years, suggesting support for the restoration theory.
look on handout an extra sheet for more points and mindmap
Evolutionary explanations (adopting the ecological approach, ie based on observations of animals in their natural environment)
Evolutionary explanations see sleep as serving some adaptive function related to survival, and therefore occurring through natural selection. Different species have evolved different sleep patterns, in response to different environmental needs, such as predator avoidance, conservation of energy and dietary requirements, so it may be that sleep has evolved not simply to fulfil some vital physiological function, as suggested by restoration explanations, but also to provide other benefits.
Energy conservation – mammals expend a lot of energy maintaining a constant body temperature. All other activities also use energy, and animals with a high metabolic rate use even more energy. Sleep serves the purpose of providing a period of enforced inactivity, much as hibernation, and is therefore a means of conserving energy. Webb (1982) described this as the hibernation theory of sleep.
Foraging requirements – some animals spend a lot of time gathering food, eg herbivores such as cows and horses spend most of the day eating plants relatively poor in nutrients. As a result of the amount of time spent eating, they cannot ‘afford’ to spend much time sleeping. Carnivores, on the other hand, eat food that is high in nutrients and so do not need to eat continuously. Therefore, they can ‘afford’ to rest much of the time, and by resting they can conserve energy
Predator avoidance – Meddis (1979) believes that sleep evolved to keep animals hidden from predators when usual activities, like foraging, are not required. An alternative view is that those animals most at risk from predators will sleep less as they must remain constantly vigilant.
Energy, foraging or predation – one way to investigate the comparative costs and benefits of sleep is to compare sleep habits across different species. Zepelin and Rechtschaffen (1974) found that smaller animals, with high metabolic rates, sleep more than larger animals. This supports the view that energy conservation might be the main reason for sleep. However, there are many exceptions, eg sloths who sleep for 20 hours a day and yet are very large. Allison and Cicchetti (1976) found that species who had a higher risk of predation did sleep less, though again there were exceptions, such as rabbits who had a very high danger rating yet slept as much as moles who had a low danger rating.
Research by Capellini et al (2008) suggests that energy conservation hypothesis may be wrong, whereas the foraging and predator avoidance explanations are right. Capellini et al (2008) argued that methods used to collect data on sleep in different animals should be standardised to make comparisons more meaningful. They carefully selected data from studies which involved animals who were habituated to lab conditions. The study focused on only land mammals.
- They found a negative correlation between metabolic rate and sleep, which doesn’t support the energy hypothesis
- Data supports the view that there is a trade-off between sleep and foraging – greater foraging requirements are linked to less time spent sleeping
- The relationship between predation risk and sleep is a complex one. Animals that sleep in exposed positions sleep less, but time spent sleeping is also reduced in species that sleep socially – yet they ought to be able to sleep longer because there is safety in numbers.
Greatest support for evolutionary approach is species which are genetically closely related show greater behavioural similarities in sleep patterns
REM and NREM sleep – energy output only drops in NREM sleep; during REM sleep the brain is still relatively active. This supports the view that NREM sleep has evolved for energy conservation as suggested by Allison and Cicchetti (1976) who found that larger animals had less NREM sleep but not less REM sleep. In contrast Capellini et al (2008) found no correlation between body size and NREM sleep.
More primitive animals, eg reptiles, only have NREM sleep. This suggests that NREM sleep evolved first for energy conservation, whereas REM sleep may have evolved later to maintain brain activity. This is supported by the greater need for REM sleep in infants whos brains are developing. Again, this hypothesis is not supported by Capellini et al (2008).
A combined approach – Horne’s (1988) theory (mentioned earlier) combines elements from adaptive theories with those from restorative theories. He suggested a distinction between core and optional sleep. Core sleep is equivalent to the deep sleep we have in the first hours of sleep and is important for essential body and brain processes. The later hours of sleep are seen as dispensable. Horne believes that optional sleep has the function of occupying unproductive hours, and, in the case of small mammals, conserving energy.
(Look at a4 sheet on evolutionary approach for AO2 and IDA)