What happens when we don’t get enough sleep?

An increasing number of studies are finding that teenagers simply aren’t getting enough sleep, with some suggesting that up to a half of all teens are now suffering from sleep deprivation. But what exactly is it to be sleep deprived and what impact does it have on the body? Three A-level Psychology students attempt to unpick some of the evidence and discover that the situation is more complicated than many might at first believe.

Looking at the evidence – Florence Roberts-Graham

UT 85: D6109 Gardner sitting biting on unidentified object

Randy Gardner during his attempt

Sleep deprivation is the condition of not having enough sleep. A few studies into sleep deprivation have been conducted to find the effects of sleep deprivation on the body and how long you can be deprived of sleep.

Randy Gardner holds the scientifically documented record for the longest period a human being has intentionally gone without sleep not using stimulants of any kind. Gardner (18years old) was a high school student at the time and stayed awake for a total of 264 hours (eleven days) as part of a school science project. At a press conference, on the final day he said, “I wanted to prove that bad things didn’t happen if you went without sleep,” said Gardner. “I thought, I can break that record and I don’t think it would be a negative experience.” Stanford University’s sleep researcher William Dement observed Gardner’s ‘science project’.

After the experiment Gardner slept for 14 hours 40 minutes, which isn’t a huge amount of sleep after the 11 days he’d been awake for! Overall Gardner only made up 11 hours of the 90 hours he lost. Surely his body would have needed to catch up on all the sleep he had missed to follow the restoration theory. The restoration theory (Ian Oswald, 1966) is the theory that the purpose of sleep is to restore the body during inactivity. However the sleep that he did make up, made up a disproportionate amount of SWS4 and REM.
* 68% Stage 4
* 53% REM
This therefore supports Oswald’s restoration theory of sleep.

The picture of results from the sleep deprivation is confusing. It is often claimed that Gardner’s experiment demonstrated that extreme sleep deprivation has little effect, other than the mood changes associated with tiredness. Dement claims that there was very little in terms of cognitive dysfunction to Gardner. This is primarily due to a report by researcher William Dement, who stated that on the tenth day of the experiment, Gardner had been, among other things, able to beat Dement at pinball. However, John J. Ross, who monitored his health, reported serious cognitive and behavioral changes. These included moodiness, problems with concentration and short term memory, paranoia, and hallucinations.
No long-term psychological or physical effects have been observed since Garners ‘school project’ took place.

Another study into sleep deprivation is a study of Peter Tripp who was a radio DJ. He did the experiment as part of a radio program to raise money for charity. Tripp stayed awake for 201 hours and 10 minutes (8 days). Although unlike Gardner, Tripp was under the influence of drugs for the last 66 hours. This means that the study isn’t as reliable as Gardner’s study. He used drugs to get through the experiment as he was doing it in the face of the public.

After the experiment Tripp slept for 13 hours and 13 minutes, this also isn’t a lot of sleep compared to the amount he had missed.

The results from Tripp’s experiment aren’t as reliable as Gardner’s as Tripp also experienced hallucinations but as Tripp was using drugs, it isn’t clear whether it was the sleep deprivation or the drugs causing the hallucinations. Tripp also started seeing things that weren’t there after just a few days. After watching his brain waves, doctors found that even though Tripp was awake, his brain waves and hallucinations shadowed the 90minute REM cycle, and he was having hallucinations instead of dreams. Also over the time, Tripp’s mean body temperature declined and he because abusive to everyone around him including people who he had known for years.

Weeks later after the experiment was all over, Tripp himself believed he had completely recovered back to his normal self and was suffering no effects from the experiment, although close people around him including his wife didn’t agree with him and they soon after got a divorce. As well as his divorce, Tripp’s career nose-dived after becoming embroiled in a scandal. Tripp had hoped that the experiment would boost his career but found the opposite happening. Tripp blamed the sleep deprivation for his fall from grace.

With both these experiments you can’t generalize the results over the whole population for the results of sleep deprivation, as they are both single person experiments and these case studies aren’t very reliable to generalize. But they have given us some valuable points and results about some cases of sleep deprivation.

Randy Gardner and Peter Tripp- The truth behind sleep deprivation – Tom Acaster

For a long time people have argued about the effects of the deprivation of sleep, both cognitively and physically. Some argue that, without sleep, our bodies cannot cope with hassles of everyday life. Others argue that, whilst the deprivation of sleep has an effect, it is not significant enough to affect the functions of everyday life.
The truth behind this debate lies, as always, with evidence. More specifically, case studies have the answer as to how humans react to sleep deprivation, while experiments from the minds of Rechtschaffen and Jouvet tell us how animals behave when confronted with sleep deprivation.

An important study into the effects of sleep deprivation comes from Huber-Weidman, who performed a meta-analysis to find out the effects of sleep loss on humans. He came up with a table, which showed the varying effects of nights without sleep. He came to the conclusion that it would only take 6 days for someone to be contracted with “sleep deprivation psychosis” which essentially means that the person becomes depersonalised, and loses they’re sense of self identity. There are also case studies showing the effects of a lack of sleep.


Radio DJ Peter Tripp used drugs to keep him awake

Peter Tripp was a radio DJ who, as a publicity stunt, decided to see how long he could go for without sleep. He lasted 201 hours and 10 minutes, before succumbing to sleep. After a few days, he began to hallucinate, seeing cobwebs, mice, and kittens; looking through drawers for money that wasn’t there and insisting that a technician had dropped a hot electrode into his shoe. Randy Gardner also did something similar as part of a science project. He stayed up for 11 days, during which time it was said that he hallucinated, had blurred vision and extreme paranoia.

There are also several animal studies that show the effects of sleep deprivation. These are Jouvet and Rechtschaffen. Rechtschaffen et al placed rats on a metal disc in water. Every time its EEG suggested it was falling asleep, the disc began to spin. A second rat was used as control, but was allowed to sleep. After 33 days the rats died. Jouvet also did a similar experiment that later became known as the “upturned flowerpot” experiment. Kittens were kept on an upturned flowerpot, and when they began to fall asleep, they fell off. They soon realised they had to stay awake to not fall off. The cats died after 35 days.

However, there are obviously some issues when considering these results as definite proof. The problem with Randy Gardner comes from the witness accounts. The man who observed him claimed that there was no real difference between him and his normal functions, whereas the paramedic claimed that he was suffering with extreme paranoia and slurred speech as well as hallucinations. Therefore the accuracy of the accounts can question the reliability of the study.  Peter Tripp was also on drugs for the last 60 hours of his experiment, so we don’t know if his hallucinations and such were because of the effects of drugs. It also hard to generalise the studies on a larger scale, when only one participant was involved. There also clear problems with generalisability in the animal studies, and the animal studies have ethical issues associated with them as well.
In conclusion, it’s incredibly hard to successfully pinpoint the exact effects of sleep deprivation. The animal studies show that sleep deprivation is undeniably bad… for cats and rats. As for humans, it isn’t so easy. The evidence we do have is tainted by drugs and inaccurate witnesses, which makes it increasingly hard to successfully decide the true effects of sleep deprivation. We may have some understanding, but we don’t have anywhere near enough.

The Sad Case of Peter Tripp – Rachel Gilby

Sleep deprivation is an ever growing problem with over half of all teenagers sleep deprived according to experts.

Sleep is important for cognitive functioning

Sleep is important for cognitive functioning

Not only is lack of sleep evident in young people and having a negative impact on their achievements, but many adults also aren’t getting enough sleep. Many car accidents in fact (around 3,000 deaths and serious injuries on UK roads each according to the AA) are caused by overly tired drivers at the wheel. But why is sleep so important and what are the true risks of sleep deprivation?

Sleep is an altered state of consciousness. It is a naturally occurring process of many stages including different stages of slow wave sleep and REM sleep. There are a number of different theories as to why sleep is needed. It is said that a good theory of sleep needs to: provide a plausible explanation as to why sleep is found in such a variety of different animals, explain the findings of research studies into sleep deprivation and be able to explain the differences that exist in sleep patterns between different species. The 2 main theories of sleep are the restoration theory and the evolutionary theory. The restoration theory by Oswald suggests that sleep is needed to restore our brain and body to help them function properly. His research suggests that slow wave sleep may restore the biological processes in our body and REM restores the brain. The opposing theory is the evolutionary theory, including the hibernation theory by Webb. In this it is suggested that humans and animals need to sleep to conserve energy. The theory is very much based on what is needed for survival, with diet predators and food availability all being factors influencing the amount of sleep an animal can have.

Studies into sleep deprivation provide much of the evidence as to why we really need sleep; if we know whathappens if we don’t sleep, we are able to piece together why we need it in the first place. The 2 main human studies used are those of Randy Gardener, a school boy who stayed awake for 264 hours as part of a school project, and Peter Tripp, a radio DJ who stayed awake for 201 hours as part of a record breaking wakeathon publicity stunt. Randy Gardener, who was observed by Stanford university sleep research William Dement, was claimed to have not suffered cognitively during the exercise, though medical staff did disagree with this claim. Afterwards he slept for 14 hours and 40 minutes with the total sleep he made up only accounting for 11 hours of the 90 he lost. This showed that it is not important to make up sleep lost. However it was found that he did make up a disproportionate amount of Slow wave sleep 4 and REM suggesting that these forms of sleep may be the most important to the body, also supporting the restoration theory of sleep. Peter Tripp on the other hand was seen to have suffered cognitively as a result of his sleep deprivation. During his stunt, which involved him staying in a glass box in Times Square for much of the time, Tripp experienced hallucinations within the first 6 days, worsened further by the fact his last 66 hours were spent under the influence of drugs to keep him awake. After the stunt he slept for 13 hours and 13 minutes suggesting again that you do not need to fully catch up on the hours of sleep that you have lost. After the event Tripp’s career nose dived with him being involved in many scandals, his involvement of which he put a strong emphasis of blame on the wakeathon stunt and its psychological impact.

Many other studies into the impact of sleep deprivation have also been conducted. A meta analysis (study of studies) by Huber-Weidman provides us with a better picture of overall what symptoms a lack of sleep appears to bring. Below is a table of their results.

As you can see the impacts of sleep deprivation show that sleep really is something that is needed, even if it isn’t something that needs to be caught up on. Whichever way you look at it, sleep is clearly important and the ability to concentrate it brings, needed especially for the tasks we do in day to day life such as driving, as well as for the learning of the many sleep deprived teens in the UK.

How can you stop those workplace mistakes?

As an owner of a factory you may have noticed silly but costly mistakes made by your employees, you may believe this is just due to lack of concentration but this could be due to the shifts they are having to work. Shift work can dramatically disrupt workers sleep making them unable to concentrate and focus on important work. Amy Allsopp looks at why mistakes are more common amongst shift-workers and how employers can help to reduce these costly errors.

People who work shifts are prone to lack of sleep due to the hours they have to work, the time they work is often the time they want to be asleep while the time in the day when they do not want to sleep is often when they are not at work. This can cause massive disruption of biological rhythms such as the sleep wake cycle and could lead to long term sleep deprivation. Although night workers are vital to the smooth running of the factory they are more often than not the cause of big mistakes and accidents that can often have a costly effect on the business. This is because of the decreased alertness because of their lack of sleep.

As well as the shift work causing potential accidents within the workplace studies have shown that shift work can also cause major health problems. In 2008 MM Martino discovered a strong correlation between shift work and a range of different organ diseases. He found that workers who have been working shift work for the around 15 years are at greater risk of developing heart or kidney disease.  He believed that this is because of the constant desynchronisation of biological rhythms which would often lead to major sleep deprivation.


The impact of shift-work on the body

Many researchers have looked into how you could reduce the risk of mistakes and sleep deprivation for shift workers. One of the most common suggestions is to enforce the rotation of shifts where the shifts would alternate around every three days. Also forward rotating shifts have been considered where for example an employee may have a morning shift one week followed by afternoon shifts the second week and then finally a night shift on the third week, this would then be repeated. This is believed to be much more beneficial causing much less damage to the workers health, also lessening the chance of mistakes. A final suggestion is the use of artificial lighting in the workplace. Our biological rhythms (particularly the circadian rhythm)  are massively influenced by the light, for example when there is bright lighting we feel much more awake compared to if we are somewhere dark. The use of artificial lighting has been proven to make the circadian rhythm last longer meaning people were less tired and will be more able to focus on the work they have to do.

Who knows the costly mistakes that could be caused by unfocused and tired employees but there are successful ways that have been proven to be effective ways to make sure shift workers are always on high alert, preventing anything from going wrong.

Coping with a disrupted circadian rhythm

With the holiday season upon us, many Brits will be jetting off in search of adventure. Florence Roberts-Graham has been looking into how the body reacts to traveling long distances and what holidaymakers can do to lessen the impact.

What is jet lag?

Jet lag is the effect of a sudden switch of time zones in air travel. This often results in an unpleasant feeling of extreme tiredness, depression and slowed mental and physical reactions andjetlag_caribb sometimes sickness.

Why might people suffer from Jet Lag?

Jet lag occurs when you fly east to west (easier to adjust) or west to east (harder to adjust). You don’t get jet lag when you fly north to south or vice-versa. This is because you only switch time zones when flying horizontally across the globe. The actual jet lag is caused by the dislocation between the body clock and local zeitgebers (external or environmental cue of time and date e.g. watches). Our biological rhythms are not equipped to cope with sudden and large changes.

Other factors affecting the severity of Jet Lag

Firstly the number of times zones crossed can have an effect as the more you pass the more dislocation your body will feel between the body clock and local zeitgebers. Secondly age. Sack et al (2007) found that jet lag decreases with age.

Individual differences mean some people might suffer awfully with jet lag even just crossing one time zone, but others might not be affected them at all, this is called “phase tolerance”.

Reducing jet lag

Even though we can’t do a lot to avoid jet lag, there are a few things that might help to reduce it.

  • Sleeping well before your flight is always a good idea as you might struggle to sleep at a different time once the time zone has changed.
  • Adjust flight behaviour to time zone of destination this is to slowly prepare yourself as much as you can so it’s not such a sudden change.
  • Avoid caffeine or alcohol
  • On arrival, use and adjust to local zeitgebers
  • Go out in the morning daylight as soon as possible, sunlight is very effective at resynchronising body clock.

Evidence found that also reduces jet lag

Beaumont et al (2004) found that melatonin given at bedtime three days before travel and for five days after arrival significantly reduced the symptoms of jet lag.

Is a later start to the school day really the answer?

Tom Acaster isn’t convinced that a later start to the school day is the best way to improve educational outcomes. The science, he argues, might be sound – but is society ready?

Evidence supporting an earlier start to the school day is based on research conducted into the circadian rhythm,625427-tired-teen a biological rhythm that lasts about 24 hours, and is associated with things such as the sleep-wake cycle. This cycle works when external factors (known as exogenous zeitgeibers) affect the SCN (or supercharismatic nucleus) in the brain, which then sends information to the pineal gland, which releases the hormone melatonin, the hormone that, when released into the bloodstream, makes us feel tired. Because this feeling of tiredness is associated with these external factors, we then know when to feel tired based on these triggers. Whilst an adult’s circadian rhythm is more accustomed to earlier starts, teenagers often work better with a later start.

Paul Kelley (formally headteacher at Monksheaton High School) has  argued that by starting the day later, the body’s cognitive functions are improved, and the students at the school can function better. The fact is, there is both evidence for and against the effectiveness of starting the school day later on. Kelley himself found that, after applying the research of Russell Foster of Oxford University to his own school, found that students improved they’re performance after this research was put in place. This obviously shows great benefit for the future of education, as this research can lead to a brighter, enthusiastic future for the next generation. This argument can be justified through a variety of reasons. Insufficient sleep causes lowered ability to think, learn, and remember. We know this because in a multitude of studies, adults have voluntarily deprived themselves of sleep and had their cognitive functioning tested. Moreover, sleep deprivation results in irregular emotional control. Also, although experimental studies of extreme sleep deprivation are not ethically possible, in a few studies, children whose sleep was restricted by an hour or so showed some degree of dysfunction similar to those seen in adults. It’s also worth noting that a much larger number of studies have established a correlational association between reduced sleep quantity and quality and poorer performance on a wide variety of tasks including attention, learning, and academic performance.

However, if this theory works so well, why is not compulsory for all schools across the country. One large problem resulted in some negative problems with the teachers; they declined in performance as they were accustomed to the early starts, and their circadian rhythm was accustomed to the typical daily ritual. Whilst this proves that it would be more beneficial to teach the students later in the day, it would also cause problems for the staff as well. Also, if the school hours start later and end later, this will cause problems with the rest of society, as they would also have to change the way they work around the kids. As most of modern society is dominated by adults, this would cause problems for they’re circadian rhythms as well. After school activities would also take a fall, as there would be no time to engage in extra curricular activities, which could train the brain in ways the school cannot, for example sports outside of school.

And what about the children? This may sound cliché, but high school students aren’t exactly clamouring for change. Teens, much like the adults in their lives, will be resistant to change and will worry about the impact on their after-school activities. Therefore, it is equally important to educate the students themselves about the benefits and disadvantages of a later start time. A change will also affect the younger kids. If a change is made, chances are it will affect both primary and secondary education. Whilst this may be advantageous to the high school students, the younger kids will not have the same mindset as teenagers. Consequently, the younger children may not perform as well whilst the older kids thrive.

The main issue I have is that society will change to adapt around the needs of education if the school times are changed. People assume that because of the better grades, unemployment would decrease. However, this is assuming the companies do not change their hiring policies, or university’s not changing their entry requirements. Everything in society will have to adapt or perish should the school times change, but the adaptations may not be for the better.

It may seem like the change of school start times may be something wonderful, but there are a lot of factors that go unanswered. Truthfully, there is no definitive evidence backing either side of the argument, making it hard to justify one way or the other. Also, any evidence that is recorded cannot be taken at face value. There are always issues with validity and reliability of studies, so nothing can really be taken as true. It may appear as though this is the breakthrough the country has been looking for in terms of education, but when you scratch the surface, not all is as it appears.

(Postscript: A later start to the school day was only one of the ‘experiments’ conducted by headteacher Dr Paul Kelley, leading some to claim that he was using his school as some kind of laboratory. In early 2012 Dr Kelley resigned from his position following a highly critical report by the local council. It appears that despite the many revolutionary interventions, he was unable to ensure high exam results. MS)

Why a later start to the school day could improve educational outcomes.

Schools in the UK and abroad are currently investigating the possibility that a later start to the day will actually benefit their students. But are we simply pandering to the lazy teenager? Adam Hindmarsh looks at evidence suggesting teenagers’ biological rhythms might provide clues as to why many schools are now opting for a later start to the school day.

Sixty-six percent of teenagers complain of being tired during the day as reported by their parents, this could be sleepy_teenagerbecause of having to get up early and biologically being unable to go to sleep earlier, also 15% of teenagers fall asleep during the day from lack of sleep.

Dr. Judith Owens, director of sleep medicine at Children’s National Medical Center explains that teenager’s bodies aren’t designed to sleep before 11 p.m., but they are still being made to get up around 6 or 7 a.m. meaning they are not getting the right 9 hours sleep that they should be getting, instead they are getting around 6 hours or less. Surveys into 8th, 10th, and 12th grade found that they get 7 hours or less sleep per night on week days due to having to get up early, not only does this effect the education of the children but can also put them in danger as having to get up early makes them more drowsy when driving leading to more car accidents.

Neuroscientist Russell Foster conducted memory tests on students at Brasenose College, Oxford, he found that pupils performed substantially better in the afternoon than they did in the morning. This suggests that actually their bodies work differently to an adults and actually they are biologically programmed to get up later in the day, therefore schools should open later on in the day because they need to sleep later enabling them to have the correct amount of sleep. Dr Paul Kelley, headteacher at Monksheaton High School in the UK (who has already implemented a later start)  saidevidence had shown rousing teenagers from their beds early resulted in abrupt mood swings, increased irritability, depression, weight gain and reduced immunity to disease.”, furthermore he claimed that we are the ones who are making teenagers how they are by disrupting their sleeping pattern, so to change how they are in society, we need to change how we make them get up too early, it would be like making an adult wake up at 4 a.m. every day. The peak for teenagers mental function is in the middle of the afternoon, Russell Foster said that making the start times later for the students would benefit them because they have higher cognitive function later on in the day.

Another option to starting later on in the day could be still starting early in the morning, but then just doing more simple things in the morning and more academically difficult things in the afternoon as teenagers have higher cognitive function in the afternoon. Kyla Wahlstrom of the University of Minnesota found that a teenager’s body has a late to bed, late to get up cycle, this is because melatonin, the chemical released in the brain to make you sleepy is released later on in the day, after 11 p.m. then is stopped being released later on, so at 7 a.m. they are still sleepy and the body is wanting to sleep.

All of this research and points towards the same conclusion, that the teenage body clock has slightly altered since the beginning of puberty, so this also points to the idea that schools need to open later and begin lessons later on because it means that the teenagers have higher cognitive function later in the day compared to adults because of their cycle being slightly out.

The role of pheromones in the synchronisation of the human menstrual cycle


Martha McClintock

Is there any evidence to support the oft quoted belief that women who live together have synchronised menstrual cycles? Rachel Gilby investigates the possible role of pheromones and the research that might (but only might) offer some explanation for this curious phenomenon.

The human menstrual cycle is an infradian rhythm because it lasts for more then 24 hours. Menstruation itself occurs at the end of the cycle when the lining of the womb is shed as no eggs have been fertilised since the last menstruation. As part of the human menstrual cycle, oestrogen and progesterone, as well as other hormones, control the release of eggs from the ovary, the thickening of the lining of the uterus and then the menstruation. This cycle happens repetitively throughout a woman’s life from when she hits puberty until she reaches the menopause at around the age 50 and prepares the body for the possibility of carrying a child.

Much research has been done into how, in some cases of females all living together, there can be synchronisation of their menstrual cycle. The first notable piece of work is from Martha McClintock in 1971. Whilst at university she noticed how her and her roommates seemed to have a synchronised menstrual cycle and that this could not simply be a coincidence. She then looked at the idea that pheromones, biochemical substances that are released into the air, acting like external hormones, may play a role as they are chemical messengers that pass on messages from one member of a species to another. She took a number of female friends and dorm sharers and took pheromones from them and wiped them onto the upper lip of each other. The upper lip as it was assumed this would be the best place for them to be taken in through the nostrils and go into the body in a way that mimicked how it would do naturally in real life. She then repeated this process and found that eventually, a large proportion of women in the experiment now had synchronised menstrual cycles.

In a follow up ten year longitudinal study, in 1988, McClintock and Stern then looked at how pheromones from other women effected the menstrual cycles of women with infrequent, random, ovulation. They took 29 females between the ages of 20 and 35 with a history of irregular ovulation and gathered pheromones from 9 of them at different points in their cycles. They did this to see if they could lengthen or shorten the women’s cycles by placing pheromones from different stages of other women’s cycles on their upper lip. Their study showed that 68% responded to the pheromones with their cycles being lengthened or shortened depending on where in the cycles the pheromones they were given were from.

McClintock’s studies were both in natural settings making them high in ecological validity. The fact that the women’s lives were not altered in anyway other than by the addition of the independent variable (pheromones) and the dependent variable (menstrual cycle times) is a huge benefit as if they were put in a synthetic environment/situation this could have lead to synthetic results which are not true in real life situations. Another strength of the studies is that they could be easily replicated and that her second study continued to support and give evidence for what was found in her first study: that pheromones played a role.

As with many studies, there were also some weaknesses in McClintock’s studies. Firstly no control group was used such as looking at how much of her results were affected by the pheromones and how much was effected by the placebo effect and knowledge of the other women’s cycles. Being a quite naturalistic study that was performed over a long period of time, it was also hard for other factors to be controlled such as diet and stress levels, known to have an effect on the cycles too. The samples are also something McClintock’s studies can be criticised for, only taking into account a small amount of females with a small age range. Her original sample was hugely unrepresentative of the population, with only people in that area, of that class, with that type of lifestyle and that age range were used as they were all university students living together. The small sample size also meant that each synchronised person had a huge effect on results, how can we be sure that this wasn’t due to other causes or when their cycles were originally? McClintock’s studies also did not show significant enough results to suggest that the pheromones are the complete cause of the synchronisation of the human menstrual cycle.

Strassmann suggested that the results of the work done by McClintock may have been influenced by bias. McClintock and Stern admitted that they had made adjustments slightly to the data in order to take into account the effect of nasal congestion on the results. With the small sample size, this could have resulted in a big change in results and Strassmann responded to this by saying that ‘it would be useful to know what a priori criteria were employed in making such adjustments, and whether the data analysis part of the project was done blind’, suggesting that there may have been a lack in logical facts used in making the adjustments and that it was not done objectively, instead, the adjustments were done to influence the results and help prove the hypothesis.

The study has also been criticised for not showing cause and effect, only a correlation between the pheromones and menstrual cycles. There has been little investigation into other factors and if they have more or less of an effect than the pheromones. As with things such as heart disease, could there be a number of influencing factors?  

To conclude, the research into the synchronisation of the human menstrual cycle may not be extensive enough to give a definite indication as to what causes the synchronisation. Although there are many strengths to McClintock’s work, there are also many weaknesses and the reliability of her data is also questionable.