How Does Sleep Deprivation Affect the Brain?

How many hours of sleep does the average person require? The American Academy of Sleep Medicine and the Sleep Research Society recently convened an expert panel which reviewed over 5,000 scientific articles and determined that sleeping less than 7 hours in adults (ages 18-60) was associated with worsening health, such as increased obesity and diabetes, higher blood pressure as well as an increased risk of stroke and heart disease. In addition to increasing the risk for illnesses, inadequate sleep is also linked to impaired general functioning, as evidenced by suppressed immune function, deficits in attention and memory, and a higher rate of errors and accidents. Since at least one third of adults report that they sleep less than 7 hours a day (as assessed by the Centers for Disease Control and Prevention in a survey of 444,306 adults), one can legitimately refer to insufficient sleep as a major public health issue. Even though insufficient sleep and other sleep disorders have reached epidemic-like proportions affecting hundreds of millions of adults world-wide, they are not adequately diagnosed and treated when compared to medical risk factors and conditions. For example, in most industrialized countries, primary care physicians perform annual blood pressure and cholesterol level checks, but do not routinely monitor the sleep duration and quality of their patients.

One reason for this may be the complexity of assessing sleep. Checking the blood cholesterol level is quite straightforward and provides a reasonably objective value which is either below or above the recommended cholesterol thresholds. However, when it comes to sleep, matters become more complicated. The above-mentioned expert panel acknowledged that there can be significant differences in the sleep requirements of individuals. Those who suffer from illnesses or have incurred “sleep debt” may require up to 9 hours of sleep, and then there are also significant environmental and genetic factors which can help determine the sleep needs of an individual. The average healthy person may need at least seven hours of sleep but there probably groups of individuals who can function well with merely 6 hours while others may need 9 hours of sleep. Then there is also the issue of the sleep quality. Sleeping for seven hours between 10 pm and 5 am has a higher quality of sleep than sleeping between 6 am and 1 pm because the latter will be associated with many more spontaneous awakenings and interruptions as well as less slow-wave sleep (a form of “deep sleep” characterized by classical slow wave patterns on a brain EEG recording during sleep). Unlike the objective cholesterol blood test, a true assessment of sleep would require an extensive sleep questionnaire asking details about sleep history and perhaps even recording sleep with activity monitors or EEGs.

Another reason for why insufficient sleep is not treated like other risk factors such as cholesterol and blood pressure is that there aren’t any easy fixes for poor sleep and the science of how poor sleep leads to cognitive deficits, diabetes and heart disease is still very much a topic of investigation.

In the case of cholesterol, numerous studies have shown that cholesterol levels can be effectively lowered by taking a daily medication such as a statin and that this intervention clearly lowers the risk of heart attacks and stroke. Furthermore, the science of how cholesterol causes stroke and heart disease has been worked out quite well by identifying the molecular mechanisms of how cholesterol contributes to the build-up of plaque in the arteries which can then lead to heart attacks and stroke. When it comes to sleep, on the other hand, multi-faceted interventions are required to restore healthy sleep levels. Medications to help patients sleep can be used in certain circumstances for a limited time but they are not a long-term solution. Instead, improving sleep requires individualized solutions such as developing a sleep schedule of fixed bed-times, minimizing the use of digital screens in the bedroom, and avoiding caffeine, large meals, nicotine or alcohol just before bedtime. The complexity of assessing and treating insufficient sleep also makes it very difficult to prove the efficacy of interventions. Controlled clinical studies can demonstrate that a cholesterol-lowering medication reduces the risk of heart attacks by treating thousands of patients with the active medication when compared to thousands of patients who receive a placebo, but how do you test the efficacy of individualized sleep interventions in thousands of patients?

Understanding the precise mechanisms by which insufficient sleep impairs our functioning and health has therefore become a major topic of research with significant advances that have been made in the past decades. Correlative studies which link poor sleep to worse health cannot prove that it is the inadequate sleep which caused the problems, but studies in which human subjects undergo well-defined sleep deprivation for a defined number of hours coupled with EEGs, brain imaging studies and cognitive assessments are providing important insights into how poor sleep can affect brain function. The sleep researcher Matthew Walker at the University of California and his colleagues recently reviewed some of the key studies in sleep research and identified some of the major categories of brain function impairment as a consequence of sleep deprivation:

1.      Attention:

Several studies of human subjects have consistently shown that sleep deprivation leads to a significant decrease in the ability to pay attention to tasks. Some studies have kept subjects awake for 24 hours at a stretch whereas other studies merely restricted sleep to a few hours a night and monitored the performance. Importantly, one study that restricted sleep to less than 3 hours for one week was able to show that the attentiveness and performance of subjects recovered rapidly once the sleep-deprived subjects were allowed to sleep for 8 hours but it still did not return back to the levels of those without sleep deprivation. This means that the after-effects of sleep deprivation can linger for days even when we start sleeping normally.

2.      Memory:

The impairment of working memory (the temporary memory we use to make decisions and complete tasks) is another key feature of sleep deprivation. Brain imaging studies have been able to identify specific abnormalities in certain areas of the brain that are critical for the “working memory” function such as the dorsolateral prefrontal cortex and thus provide somewhat objective measures of cognitive impairment. Interestingly, placing magnetic coils around the head of sleep-deprived subjects to initiate TMS (transcranial magnetic stimulation) has been reported to help restore some of the loss of visual memory, however, Walker and colleagues note that the benefits of TMS in sleep deprivation are not always consistent and reproducible.

3.      Responding to negative stimuli

Sleep deprivation increases responses to negative stimuli such as fear. For example, when subjects who had one night of sleep deprivation were shown images of weapons, snakes or mutilations, their aversion responses were much stronger than those of control subjects. Hyper-responsiveness of the amygdala, the part of the brain which processes emotional reactions, is thought to be one major element in these exaggerated responses of sleep-deprived subjects.

Walker and colleagues note that not all changes seen in the brain imaging studies are necessarily detrimental. In fact, some of these changes may be adaptations that have evolved to help our brains cope with the stress of sleep deprivation. Even though significant progress has been made in sleep deprivation research, understanding differences between individuals in terms of how and why they respond differently to sleep deprivation, distinguishing the mechanisms of beneficial adaptations in brain function from detrimental responses and also developing new studies that study the effects of chronic sleep deprivation – one that occurs over a period of weeks and months and thus mimics real-life sleep deprivation – instead of the short-term acute sleep deprivation studies that are currently performed in the laboratory are major challenges for sleep researchers. Hopefully, advances in sleep research will lead to a better understanding of sleep health and ultimately also translate into sleep becoming an integral part of medical exams in order to address this burgeoning public health problem.


Watson NF, Badr MS, Belenky G, Bliwise DL, Buxton OM, Buysse D, Dinges DF, Gangwisch J, Grandner MA, Kushida C, Malhotra RK, Martin JL, Patel SR, Quan SF, Tasali E. Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society. J Clin Sleep Med 2015;11(6):591–592.

Liu Y, Wheaton AG, Chapman DP, Cunningham TJ, Lu H, Croft JB. Prevalence of Healthy Sleep Duration among Adults — United States, 2014. MMWR Morb Mortal Wkly Rep 2016;65:137–141

Krause AJ, Simon EB, Mander BA, Greer SM, Saletin JM, Goldstein-Piekarski AN, Walker MP. (2017). The sleep-deprived human brain. Nature Reviews Neuroscience


Note: An earlier version of this article was first published on the 3Quarksdaily blog.

Typical Dreams: A Comparison of Dreams Across Cultures

But I, being poor, have only my dreams;

I have spread my dreams under your feet;

Tread softly because you tread on my dreams.

                                    William Butler Yeats – from “Aedh Wishes for the Cloths of Heaven



Have you ever wondered how the content of your dreams differs from that of your friends? How about the dreams of people raised in different countries and cultures? It is not always easy to compare dreams of distinct individuals because the content of dreams depends on our personal experiences. This is why dream researchers have developed standardized dream questionnaires in which common thematic elements are grouped together. These questionnaires can be translated into various languages and used to survey and scientifically analyze the content of dreams. Open-ended questions about dreams might elicit free-form, subjective answers which are difficult to categorize and analyze. Therefore, standardized dream questionnaires ask study subjects “Have you ever dreamed of . . .” and provide research subjects with a list of defined dream themes such as being chased, flying or falling.

Dream researchers can also modify the questionnaires to include additional questions about the frequency or intensity of each dream theme and specify the time frame that the study subjects should take into account. For example, instead of asking “Have you ever dreamed of…”, one can prompt subjects to focus on the dreams of the last month or the first memory of ever dreaming about a certain theme. Any such subjective assessment of one’s dreams with a questionnaire has its pitfalls. We routinely forget most of our dreams and we tend to remember the dreams that are either the most vivid or frequent, as well as the dreams which we may have discussed with friends or written down in a journal. The answers to dream questionnaires may therefore be a reflection of our dream memory and not necessarily the actual frequency of prevalence of certain dream themes. Furthermore, standardized dream questionnaires are ideal for research purposes but may not capture the complex and subjective nature of dreams. Despite these pitfalls, research studies using dream questionnaires provide a fascinating insight into the dream world of large groups of people and identify commonalities or differences in the thematic content of dreams across cultures.

The researcher Calvin Kai-Ching Yu from the Hong Kong Shue Yan University used a Chinese translation of a standardized dream questionnaire and surveyed 384 students at the University of Hong Kong (mostly psychology students; 69% female, 31% male; mean age 21). Here are the results:

Ten most prevalent dream themes in a sample of Chinese students according to Yu (2008):

  1. Schools, teachers, studying (95%)
  2. Being chased or pursued (92 %)
  3. Falling (87 %)
  4. Arriving too late, e.g., missing a train (81 %)
  5. Failing an examination (79 %)
  6. A person now alive as dead (75%)
  7. Trying again and again to do something (74%)
  8. Flying or soaring through the air (74%)
  9. Being frozen with fright (71 %)
  10. Sexual experiences (70%)

The most prevalent theme was “Schools, teachers, studying“. This means that 95% of the study subjects recalled having had dreams related to studying, school or teachers at some point in their lives, whereas only 70% of the subjects recalled dreams about sexual experiences. The subjects were also asked to rank the frequency of the dreams on a 5-point scale (0 = never, 1=seldom, 2= sometimes, 3= frequently, 4= very frequently). For the most part, the most prevalent dreams were also the most frequent ones. Not only did nearly every subject recall dreams about schools, teachers or studying, this theme also received an average frequency score of 2.3, indicating that for most individuals this was a recurrent dream theme – not a big surprise in university students. On the other hand, even though the majority of subjects (57%) recalled dreams of “being smothered, unable to breathe“, its average frequency rating was low (0.9), indicating that this was a rare (but probably rather memorable) dream.

How do the dreams of the Chinese students compare to their counterparts in other countries?

Michael Schredl and his colleagues used a similar questionnaire to study the dreams of German university students (nearly all psychology students; 85% female, 15% male; mean age 24) with the following results:

Ten most prevalent dream themes in a sample of German students according to Schredl and colleagues (2004):

  1. Schools, teachers, studying (89 %)
  2. Being chased or pursued (89%)
  3. Sexual experiences (87 %)
  4. Falling (74 %)
  5. Arriving too late, e.g., missing a train (69 %)
  6. A person now alive as dead (68 %)
  7. Flying or soaring through the air (64%)
  8. Failing an examination (61 %)
  9. Being on the verge of falling (57 %)
  10. Being frozen with fright (56 %)

There is a remarkable overlap in the top ten list of dream themes among Chinese and German students. Dreams about school and about being chased are the two most prevalent themes for Chinese and German students. One key difference is that dreams about sexual experiences are recalled more commonly among German students.

Tore Nielsen and his colleagues administered a dream questionnaire to students at three Canadian universities, thus obtaining data on an even larger study population (over 1,000 students).

Ten most prevalent dream themes in a sample of Canadian students according to Nielsen and colleagues (2003):

  1. Being chased or pursued (82 %)
  2. Sexual experiences (77 %)
  3. Falling (74 %)
  4. Schools, teachers, studying (67 %)
  5. Arriving too late, e.g., missing a train (60 %)
  6. Being on the verge of falling (58 %)
  7. Trying again and again to do something (54 %)
  8. A person now alive as dead (54 %)
  9. Flying or soaring through the air (48%)
  10. Vividly sensing . . . a presence in the room (48 %)

It is interesting that dreams about school or studying were the most common theme among Chinese and German students but do not even make the top-three list among Canadian students. This finding is perhaps also mirrored in the result that dreams about failing exams are comparatively common in Chinese and German students, but are not found in the top-ten list among Canadian students.

At first glance, the dream content of German students seems to be somehow a hybrid between those of Chinese and Canadian students. Chinese and German students share a higher prevalence of academia-related dreams, whereas sexual dreams are among the most prevalent dreams for both Canadians and Germans. However, I did notice an interesting aberrancy. Chinese and Canadian students dream about “Trying again and again to do something” – a theme which is quite rare among German students. I have simple explanation for this (possibly influenced by the fact that I am German): Germans get it right the first time which is why they do not dream about repeatedly attempting the same task.

The strength of these three studies is that they used similar techniques to assess dream content and evaluated study subjects with very comparable backgrounds: Psychology students in their early twenties. This approach provides us with the unique opportunity to directly compare and contrast the dreams of people who were raised on three continents and immersed in distinct cultures and languages. However, this approach also comes with a major limitation. We cannot easily extrapolate these results to the general population. Dreams about studying and school may be common among students but they are probably rare among subjects who are currently holding a full-time job or are retired. University students are an easily accessible study population but they are not necessarily representative of the society they grow up in. Future studies which want to establish a more comprehensive cross-cultural comparison of dream content should probably attempt to enroll study subjects of varying ages, professions, educational and socio-economic backgrounds.

Despite its limitation, the currently available data on dream content comparisons across countries does suggest one important message: People all over the world have similar dreams.



Yu, Calvin Kai-Ching. “Typical dreams experienced by Chinese people.” Dreaming 18.1 (2008): 1-10.

Nielsen, Tore A., et al. “The Typical Dreams of Canadian University Students.” Dreaming 13.4 (2003): 211-235.

Schredl, Michael, et al. “Typical dreams: stability and gender differences.” The Journal of psychology 138.6 (2004): 485-494.

Note: An earlier version of this article was first published on 3Quarksdaily.

Yu, C. (2008). Typical dreams experienced by Chinese people. Dreaming, 18 (1), 1-10 DOI: 10.1037/1053-0797.18.1.1
Nielsen, T., Zadra, A., Simard, V., Saucier, S., Stenstrom, P., Smith, C., & Kuiken, D. (2003). The Typical Dreams of Canadian University Students. Dreaming, 13 (4), 211-235 DOI: 10.1023/B:DREM.0000003144.40929.0b

Schredl M, Ciric P, Götz S, & Wittmann L (2004). Typical dreams: stability and gender differences. The Journal of psychology, 138 (6), 485-94 PMID: 15612605