Infradian rhythms - have cycles which occur less than once a day e.g. the menstrual cycle
Circadian rhythms - have cycles which occur every 24 hours e.g. the sleep-wake cycle
Ultradian rhythms - have cycles which occur more than every 24 hours e.g. different stages of sleep
Biological rhythms are controlled by both endogenous pacemakers and exogenous zeitgebers.
Endogenous pacemakers: factors inside the body which regulate bio rhythms. A key structure is the suprachiasmatic nucleus (SCN) in the hypothalamus which seretes melatonin (a chemical which induces sleep). Morgan (1995) bred mutant hamsters so they had shorter circadian rhythms. He then transplanted their SCNs into normal hamsters and found the normal hamsters displayed the mutant rhythm.
Exogenous zeitgebers: factors outside the body which regulate bio rhythms. The most common example is light. Siffre (1975) spent six months in a cave without any external cues (e.g. light or clocks). His sleep-wake cycle went from 24 hours to between 25 and 30 hours which suggests that whilst endogenous pacemakers do still regulate biological rhythms, exogenous zeitgebers are needed to keep them in sync.
Lifespan Changes AO1
Infancy: babies sleep around 16 hours a day for about an hour at a time due to their sleep cycle being shorter than adults. They have immature versions of SWS (quiet sleep) and REM (active sleep). About half of infant sleep is active sleep. By 6 months a circain rhythm has become established,
Chilren: by age 5 EEG patterns look similar to adults but children still sleep more (around 12 hours a day) and have more REM sleep (around 30% of total sleep time). Boys sleep more than girls. Children are more likely to suffer from a variety of parasomnias.
Teenagers: the need for sleep increases slightly in adolescence, to about 9 to 10 hours. Teenagers also experience a phase delay of circadian rhythms (that they feel more awake at night and find it more difficult to get up in the mornings). In males, REM sleep is sometimes accompanied by ****** and *********** which is significantly less likely at other ages.
Adults: normal adult sleep is usually for around 8 hours per night, with 25% in REM. Childhoog parasomnias are more rare in adulthood however ther is an increase of other sleep disorders, such as insomnia and apnoea. As age increases lenght of sleep stays about the same but older people tend to wake up more in the night and have more difficulty falling asleep; they may compensate by napping during the day. REM is reduced to 20% and SWS is reduced to 5% or less. Older people also experience a phase advance of circadian rhythms.
Lifespan Changes AO2
- Babies sleep may be adaptive so that their parents can get on with their daily chores.
- Infants greater amount of REM may be due to the immaturity of their brain. REM has been linked with the production of neurotransmitters and the consolidation of memory. Premature babies spend about 90% of their sleep in active sleep.
- Teenage changes may be linked to hormonal changes. Hormones are primarily released at night which distrupts sleep patterns.
- Kripke et al (2002) found that too much sleep can be harmful to adults. Those sleeping 6 or 7 hours had a reduced morality risk whereas those who slept 10 hours had a 30% increase in risk of dealth. Highly correlational. Other factors, such as an underlying health issue, may lead to increased sleep.
- Problems staying asleep in older people may be due to the reduction of SWS making the sleeper more easy to wake.
- Reduction in SWS leads to the reduction of growth hormone production which could explain some of the symptoms associated with old age e.g. lower bone density (van Cauter et al, 2000)
- The resulting sleep defecit in old age may explain why older people experience impaired functions.
Restoration Theory of Sleep AO1
The main belief of the restoration theory is that we sleep to restore the body's ability to function. Oswald (1980) proposed that SWS enables the body to repair and REM sleep enables the brain to repair. 4 key predictions: inadequacy in functioning when sleep deprived, following sleep deprivation rebound effect occur, increase in REM during brain repair, increase in SWS during time of illness/injury.
SWS: growth hormone (GH) is secreted during SWS. GH enables protein synthesis and cell growth - vital because proteins are fragile and must constantly be renewed. SWS can also be linked to the immune system because antibodies are generated during protein synthesis and cell growth. Sassin et al (1969) found when sleep-wake cycle was reversed, the release of GH with sleep is also reversed.
REM: amount of REM sleep is proportional to the immaturity of the offspring at birth - babies have more REM for brain growth. Siegel and Rogawski (1988) suggest REM sleep allows for a break in neurotransmitter release which permits neurons to regain their sensitivity and allow the body to function properly. Support comes from MAOIs which increase the levels of neurotransmitters in the monoamine group; a side effect is the abolishment of REM activity. Suggests an increase in monoamine means monoamine receptors don't have to be revitalised and therefore there's no need for REM.
Restoration Theory of Sleep AO2
- Shapiro et al (1981) found ultra marathon runners slept for around an hour more on the two nights after the marathon. SWS in particular increased.
- Randy Gardner case study. Stayed awake for 11 days without stimulants. He was moody, delusional and confused and had problems concentrating and with his memory. He slept for 15 hours after his 'marathon'.
- Rechtschaffen et al (1983) deprived rats of sleep by making them fall into water whenever they fell asleep. All the sleep deprived rats died within 33 days.
- Horne and Millard (1985) gave participants several exhausting tasks. Found they fell asleep quicker but not for longer.
- Web and Bonnet (1978) participants gradually reduced their total amount of sleep over 2 months until they only slept for 4 hours a night and reported no adverse effects.
Restoration Theory of Sleep AO3
- Methodological issue: reliance on case studies and observational studies of small groups. Participants are likely to be unique and so study cannot be generalised to the majority of people. A lot of research relies on the use of animals due to the ethical implications of depriving humans of sleep. These cannot be applied to humans due to the different sleep requirements and cycles of animals.
- Reductionist: more applicable to humans than evolutionary theory however it completely ignores it. Inconsistencies in research suggest it isn't a full explanation of sleep. Some aspects of evolutionary theory are applicable to humans, such as the need to occupy unproductive hours. Horne suggests a combination of both evolutionary theory and restoration theory.
Evolutionary Theory of Sleep A01
4 key assumptions:
- sleep conserves energy; Webb (1982) describes this as hibernation theory. Animals (particularly the ones with high metabolic rates) use a lot of energy to maintain body heat; sleep is a period of inactivity where less energy is being used.
- sleep avoids predators. As sleep is vital, animals sleep at night to keep them out of harms way at a time when they are least vulnerable.
- time spent sleeping is limited by food requirements. Cows eat low nutrient food and so have to spend more of their time eating which reduces the amount of time they can spend sleeping. Carnivores have nutrient dense food and so can allow more time to sleep.
- Meddis (1975) proposed the waste of time assumption. Sleep helps animals to keep out of the way during times of the day when they may be vulnerable. This means sleeping at night or in a place where they are hidden. Meddis suggests sleep may ensure animals will stay still when they have nothing else to do. Siegel (2008) concurs with this view, pointing out that time spent asleep is better as the animal is less likely to be injured. Sleep enables energy conservation as well as keeping the animal out of danger.
Evolutionary Theory of Sleep A02
- Animals which have few natural predators often sleep around 12-15 hours a day, whereas animals which have many natural predators usually sleep no more than 5 hours a day.
- Zeppelin and Rechtschaffen found a relationship between metabolic rate and animal size with the amount of time spent sleeping. Smaller animals with higher metabolic rates spent more time asleep.
- Allison and Cicchetti (1976) found that species who had a higher risk of predation did sleep less.
- Brain is still active in REM sleep so it is only NREM sleep which conserves energy. Allison and Cichetti (1976) support this. They found that larger animals had less NREM sleep but not less REM sleep.
- Exceptions to Zeppelin and Rechtschaffen's research. Sloths are large with low metabolic rates yet they sleep for about 10 hours a day. Also the research is correlational not causational.
Evolutionary Theory of Sleep A02/3
- Exceptions to Allison and Cichetti's research. Rabbits have a higher predation risk yet sleep as much as moles which have a low danger rating.
- Capellini et al (2008) found a negative correlation between metabolic rate and sleep which is the opposite of what the theory would predict.
- Methodological: the majority of research is animal research which cannot be generalised to humans. It also can't be applied to domestic animals as they don't need to worry about predators or foraging.
- Reductionist: states that the main purpose of sleep is to avoid predators. Sleep does help prey avoid predators but it isn't the only reason. Evolutionary theory fails to explain why we crave sleep when sleep deprived and why prolonged deprivation proves fatal. Rechtschaffen et al (1983) found that rats deprived of sleep died within 33 days.