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Body rhythms
 Circadian rhythms
• One biological rhythm is the 24-hour circadian rhythm (often known as the ‘body clock’),
which is reset by levels of light.

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• The word circadian is from the Latin ‘circa’ which means ‘about’, and ‘dian’, which means

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‘day’ (around a day).

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• A circadian rhythm, or circadian cycle, is a natural, internal process that is part of the

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body’s internal clock that regulates the sleep–wake cycle and repeats roughly every 24

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• The sleep-wake cycle is an example of a circadian rhythm, which dictates when humans
and animals should be asleep and awake.

• Light provides the primary input to this system, acting as the external cue for sleeping or
waking. For example, waking up when the sun rises and going to sleep when its dark.
• Circadian rhythms are daily rhythms. These include body temperature, the sleep–wake cycle and some
hormonal changes.

• The sleep–wake cycle consists of a daily cycle from being awake to sleeping; this is around a 24-hour
cycle.

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• Jet lag affects the sleep–wake cycle and happens when we cross time zones. Our sleep–wake cycle is

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triggered by light and time cues. When we are in a different time zone, these cues are different for us,

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such as it being light outside at the time when we would be ‘set’ to sleep. This can cause daytime

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irritability and poor concentration, underlining the importance of the sleep–wake cycle for normal

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functioning.

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• Shift work also affects the sleep–wake cycle. People doing shift work change what they do at different
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times in the day, such as having to work when it is dark. Cues to time and light are different and this
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affects their biological rhythms, including their sleep–wake cycle.

• The SCN then uses this information to coordinate the activity of the entire circadian system.

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• Sleeping and wakefulness are not determined by the circadian rhythm alone, but also by

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homoeostasis. When an individual has been awake for a long time, homeostasis tells the

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body that there is a need for sleep because of energy consumption. This homeostatic drive for

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sleep increases throughout the day, reaching its maximum in the late evening, when most

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people fall asleep.

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• Body temperature is another circadian rhythm. Human body temperature is at its lowest in
the early hours of the morning and at its highest in the early evening. Sleep typically occurs
when the core temperature starts to drop. The body temperature starts to rise towards the end
of a sleep cycle, promoting feelings of alertness first thing in the morning.
• Biological rhythms are regulated by endogenous (internal) pacemakers and exogenous
(external) zeitgebers.

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• Endogenous (internal) pacemakers are the body’s internal biological clocks.

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• Exogenous zeitgebers are external cues, including light, that help to regulate the internal

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biological clocks.

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 Internal pacemakers
• Endogenous (internal) pacemakers are internal mechanisms that govern biological rhythm,
particularly the circadian sleep-wake cycle.

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• Although endogenous pacemakers are internal biological clocks, they can be altered and

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affected by the environment. For example, although the circadian sleep-wave cycle will

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continue to function without natural cues from light, research suggests that light is required

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to reset the cycle every 24 hours.

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• The most important endogenous pacemaker is the suprachiasmatic nucleus, which is
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closely linked to the pineal gland, both of which are influential in maintaining the circadian
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sleep/wake cycle.

• The SCN and pineal glands work together as internal pacemakers; however, their activity
is responsive to the external cue of light.
• The suprachiasmatic nucleus (SCN), which lies in the hypothalamus, is the main endogenous
pacemaker (or master clock).

• It controls other biological rhythms, as it links to other areas of the brain responsible for

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sleep and arousal. The SCN also receives information about light levels (an external

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zeitgeber) from the optic nerve, which sets the circadian rhythm so that it is in

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synchronization with the outside world, e.g. day and night.

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• The SCN sends signals to the pineal gland, which leads to an increase in the production of
melatonin at night, helping to induce sleep.

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• Melatonin is a hormone that signals the need for sleep and is produced in the pineal gland.

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• Melatonin is triggered by darkness. In animals, it is involved in the synchronizing of

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circadian rhythms, such as the sleep– wake cycle and blood pressure.

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• Melatonin can be used as a medication to help with insomnia and with jet lag, though its

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benefits are questionable when used for shift workers.
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• The pineal gland regulates sleep–wake patterns, which in turn affect stress levels and
physical performance.
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 External zeitgebers
• External zeitgebers influence biological rhythms: these can be described as environmental
events that are responsible for resetting the biological clock of an organism.

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• Zeitgebers, meaning ‘time givers’, are environmental or external cues that regulate the

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body’s circadian rhythms. Zeitgebers include the light–dark cycle and the 12-month cycle of

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the seasons.

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• They can include environmental cues such as meal-times, environmental stress, any

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medications we are on, and social activities, but the most important zeitgeber is light, which

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is responsible for resetting the body clock each day, keeping it on a 24-hour cycle.
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• The SCN contains receptors that are sensitive to light and this external cue is used to
synchronize the body’s internal organs and glands.

• Light comes into our eyes and light sensitive cells in the retina of the eye let the internal
body clock know whether it is day or night

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• Melanopsin, which is a protein in the eye, is sensitive to light and carries the signals to the

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SCN in the brain to set the 24-hour daily body cycle.

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• For example, the signal may be to wake up. When the sun rises, there is increased light. This

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is the time when humans wake up and start the day. Whereas, during and after sunset, there is

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decreased light. The night period is when humans usually sleep.
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• In addition, social cues, such as mealtimes, can also act as zeitgebers and humans can
compensate for the lack of natural light, by using social cues instead.
• Another way light affects the circadian rhythm is via the release of melatonin. The pineal
gland secretes melatonin. Melatonin is released when the receptors indicate low light levels
within your eye (SCN).

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• The two work in tandem, so light, an external zeitgeber, will affect the SCN, an internal

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pacemaker, to regulate your sleep-wake cycle (your biological rhythm).

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• Other external zeitgebers include temperature and food.

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 Research into the circadian sleep-wake cycle-strengths
• Research has been conducted to investigate circadian rhythms and the effect of external cues like light
on this system.

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• Siffre (1975) found that the absence of external cues significantly altered his circadian rhythm: he

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stayed in a cave for 6 months alone and when he returned from an underground stay with no clocks or

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light, he believed the date to be a month earlier than it was. This suggests that his 24-hour sleep-wake

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cycle was increased by the lack of external cues, making him believe one day was longer than it was,

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and leading to his thinking that fewer days had passed.

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• There is evidence from animals that the suprachiasmatic nuclei (SCN) have the role of setting the

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biological clock. For example, if the SCN is removed from hamsters their bodily rhythms stop, meaning
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their sleep–wake patterns become random.
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• There is also evidence from humans. A study by Miles et al (1977) looked at a blind man whose bodily
rhythm was nearer to 25 hours (all his circadian rhythms were desynchronized from the 24-hour societal
schedule), suggesting we have circadian rhythms governed by external factors.
• There are practical applications for understanding body clocks in humans. Understanding
zeitgebers can help shift workers, for example by darkening their bedroom in the daytime.

• Li-You Chen et al (2015) confirm that the pineal hormone melatonin is important for

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controlling sleep. The study, using rats, found that early sleep deprivation (ESD) led to

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reduction in melatonin levels. The researchers suggested that a way of preventing this

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reduction is to give children melatonin supplements. Such studies support the suggestion that

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melatonin influences sleep.

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 Weaknesses
• Much of the evidence is from animal studies and there can be problems generalizing these
findings to humans.

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• The evidence relating to human blindness is weak. There are individual differences in 'being

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blind' and it can be difficult to draw conclusions leading to universal explanations. For

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example, people with NLP blindness might perceive light and colors (e.g. Rose, 2015).

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• Studies like Siffre's are sleep-deprivation studies, so the situation is not 'natural' and there

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may be a lack of validity.

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• A weakness of the explanation relating to light as an external influence on sleep is that there
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are differences in findings. Siffre (1975) wanted to check the findings of studies that claim a
'natural' sleep–wake cycle could be 48 hours, when other studies suggest it is nearer to the
expected 24 hours. It is hard to draw firm conclusions about the sleep–wake cycle and the
role of zeitgebers when results of studies differ.
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Infradian rhythms
• Another important biological rhythm is the Infradian rhythm.

• Infradian rhythms last longer than 24 hours and can be weekly, monthly or annually.

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• A monthly Infradian rhythm is the female menstrual cycle, which is regulated by hormones

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that either promote ovulation or stimulate the uterus for fertilization.

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• Ovulation occurs roughly halfway through the cycle when estrogen levels are at their

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highest. After the ovulatory phase, progesterone levels increase in preparation for the

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possible implantation of an embryo in the uterus.

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• It is also important to note that although the usual menstrual cycle is around 28 days, there is
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considerable variation, with some women experiencing a short cycle of 23 days and others
experiencing longer cycles of up to 36 days.
• A second example of an Infradian rhythm is related to the seasons. It is an annual rhythm.

• Research has found seasonal variation in mood, where some people become depressed in the
winter, which is known as seasonal affective disorder (SAD).

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• SAD is an Infradian rhythm that is governed by a yearly cycle. Psychologists claim that

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melatonin, which is secreted by the pineal gland during the night, is partly responsible.

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• The lack of light during the winter months results in a longer period of melatonin secretion,

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which has been linked to the depressive symptoms.

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• Weekly Infradian rhythms include seven-day rhythms for blood pressure and heart rate.
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 Research into Infradian rhythms
• Research suggests that the menstrual cycle is, to some extent, governed by external zeitgebers. Reinberg
(1967) examined a woman who spent three months in a cave with only a small lamp to provide light.
Reinberg noted that her menstrual cycle shortened from the usual 28 days to 25.7 days. This result

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suggests that the lack of light in the cave affected her menstrual cycle.

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• There is research to suggest that Infradian rhythms such as the menstrual cycle are also important

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regulators of behavior. Penton-Volk et al. (1999) found that woman expressed a preference for feminized

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faces at the least fertile stage of their menstrual cycle, and for a more masculine face at their most fertile

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point. These findings indicate that women’s sexual behavior is motivated by their Infradian rhythms,

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highlighting the importance of studying Infradian rhythms in relation to human behavior.

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• Finally, evidence supports the role of melatonin in SAD. Terman (1988) found that the rate of SAD is
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more common in Northern countries where the winter nights are longer. For example, Terman found that
SAD affects roughly 10% of people living in New Hampshire (a northern part of the US) and only 2% of
residents in southern Florida. These results suggest that SAD is in part affected by light (exogenous
zeitgeber) that results in increased levels of melatonin.
 Research into Infradian rhythms
• Research into infradian rhythms using humans is often done as case studies, or using small samples. The
use of case studies means that the data is from an individual so there may be something unique about that
person, meaning the results are not generalisable.

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• Research into the use of light therapy has led to the use of light boxes for people who have seasonal

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affective disorder, to help reduce their symptoms.

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• It is hard to prove that a change in infradian rhythms leads to an evolutionary advantage as it is hard to

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scientifically test the ideas due to evolution happening over several centuries.

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• Research into infradian rhythms often collect objective, numerical data. As the data is usually objective
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then the studies can be seen as scientific, and reliable as the data is fact and not open to interpretation.
 Pheromones
Pheromones are substances which are secreted to the outside by an individual and received by a second
individual of the same species.

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Many examples exist in animals but their role in humans remains uncertain since adults have no

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functioning organs, which processes pheromone signals in animals

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Pheromones are airborne chemical messengers released by the body (for instance, through sweat and

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urine). Pheromones have physical or emotional effects on other members of the same species. In

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animals, pheromones can carry all sorts of messages - they can warn others of nearby predators, for

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example.

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Do human pheromones even exist? If they do exist, can they really influence behaviour? The honest
answer to these questions is - we simply don't know for sure. Our knowledge of human pheromones is
incomplete.
You might find it surprising that our knowledge of pheromones is so lacking. After all, with all of our
modern lab methods and technology, shouldn't we be able to find out?
There are a number of reasons why reliable knowledge of human pheromones is so difficult to acquire. Here
are a few reasons why:

• Complexity of human attraction: Why we feel attracted to one person - and not someone else - is

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remarkably complex. It encompasses all five of ours senses, not to mention our culture, memories, and

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personality. This makes it very difficult to isolate and measure the effect of just one factor - such as

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human pheromones - on attraction.

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• Complexity of human scent. Your "odorprint" - the unique smell you give off - is made up of hundreds

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of unidentified chemicals, including bacteria that mixes with your bodily chemicals. Furthermore,

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humans have over 400 odour receptors - and each odour receptor has genetic variants, meaning that each

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person processes odours differently. Given this complexity, it is very difficult to test how just one
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chemical - such as a pheromone - could make a significant difference in attraction.
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• Difficulty in replicating findings. When a study produces new, startling findings, other scientists will
try to replicate the original study to confirm the results. In many cases, research studies on pheromones
have not been successfully replicated. But then how do we know if the original study was flawed, or the
attempt at replication was flawed?
 Pheromones
• McClintock (1971) concluded that male pheromones reset a females infradian rhythms and increase
ovulation, which he stated has an evolutionary advantage.

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Russell (1980) used pheromones and found that four out of five women synchronised their menstrual

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cycle to the menstrual cycle of the donor woman Russell (1980) used a single blind study, so the women

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did not know which group they were in, adding control to the study and so increasing reliability..

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Dalton (1964) found that PMS can affect female behaviour as it is associated with an increase in

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accidents and crime. Dalton’s (1964) study only found an association, not a cause and effect so it may

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not be PMS that causes the changes in female behaviour

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 Treatment for SAD

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THERAPY MEDICATIONS

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 Medications
• Some people with SAD benefit from antidepressant treatment, especially if symptoms are
severe.

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• Antidepressants are thought to be most effective if taken at the start of winter before

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symptoms appear and continued until spring.

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• Selective Serotonin Reuptake Inhibitors (SSRIs) are the preferred type of antidepressant

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for treating SAD. They increase the level of the neurotransmitter serotonin in your brain,

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which can help lift your mood.

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• It can take up to 4 to 6 weeks for the medicine to take full effect.

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• Light therapy mimics natural outdoor light and appears to cause

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a change in brain chemicals linked to mood. It increases the production

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of the hormone serotonin, which improves the mood.

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• Light therapy is one of the first line treatments for fall-onset SAD. It generally starts working in a

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few days to a few weeks and causes very few side effects.

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• Research on light therapy is limited, but it appears to be effective for most people in relieving
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SAD symptoms.

• It's thought that light therapy is best for producing short-term results. This means it may help
relieve your symptoms when they occur, but you might still be affected by SAD next winter.
 Psychotherapy
• Psychotherapy, also called talk therapy, is another option to treat SAD.A widely used and very
effective psychotherapy that helps people with SAD is cognitive behavioral therapy (CBT).

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• CBT is based on the principle that “what you do and what you think influences how you feel”.

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• CBT focuses on helping individuals learn healthy ways to cope with SAD, especially with

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reducing avoidance behavior and scheduling meaningful and pleasurable activities to relieve the

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symptoms.

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• It also helps individuals identify and change negative, irrational thoughts and behaviors that may
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be making them feel worse.

• It also helps individuals to learn how to manage stress and build in healthy behaviors, such as
increasing physical activity and improving your sleep patterns.
 CBT VIDEOS
https://www.youtube.com/watch?v=q6aAQgXauQw

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https://www.youtube.com/watch?v=ZdyOwZ4_RnI

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Thank you!