Artificial blue light and teenagers: Does artificial blue light exposure at night have negative health and wellbeing implications on teenagers?

Written by Maya Wilson

  Introduction

Technology and use of screens has become increasingly integrated into our lifestyles. It has become almost impossible to avoid the rate at which technology progresses and advances and its involvement in our daily lives. These advances have enabled the override of natural daily cycles of dark and light which have extended our activities into the night. As technological advancement has only come about within recent years, there is lack of research and understanding of the health and wellbeing side effects are. Negative health and wellbeing implications are a possibility due to the artificial blue light emitted from personal devices such as cell phones and laptops. In this literature review, a discussion will be made between the relationship of teenagers and their exposure to artificial  blue light emitting digital technology at night. It will provide many possible consequences due to these activities and the health and wellbeing related implications. It is important to focus on teenagers as they are first generation to be born in the ‘beginning of the digital age’ (Hilbert, 2011) and therefore are the first generation to have the most technological exposure from birth and develop possible health and wellbeing implications from an early age. Cheeseman (2012) explains that understanding the health implications of increased artificial blue light exposure on our biology is new and is an area that requires further research.

Teenage use of technology at night in the bedroom is broadly linked to poorer sleep (Gamble, 2014). The disruption of sleep cycles has an impact on mood disorders, risk of obesity, depression, cognitive dysfunction (Royal Society Te Aparangi, 2018). This can explained by the component of artificial blue light exposure during dark hours (Royal Society Te Aparangi 2018). It is a reason for concern of New Zealand teenagers, as addressed in current news publications. The Guardian (2017) found that teenagers who texted constantly into the night had the problem increase a year later. Some staying up until 3am. This suggests the problem, if not intervened, will only worsen overtime. Not only are New Zealand teenagers spending an average of 8.9 hours a day using their cell phone but these hours extend into the night (Catherall, 2016). To research the possible health and wellbeing implications that teenagers may face due to artificial blue light exposure at night, the PECOT model will be used. It is used to identify the key components that have been refined into a question for literature research (Schenider, 2013). The question formed from the PECOT model is “Does artificial blue light exposure at night have negative health and wellbeing implications on teenagers”.

  PECOT

 

PECOT Category

Information relating to question

Explanation

P (population)

Teenagers (13-19 years old)

This population was chosen as they’ve experienced the most digital exposure since an early age and may be the first generation to experience the long term health effects of exposure

E (Exposure)

Exposure to blue light emitting devices at night.

The aim is to understand and identify the health and wellbeing implications for teenagers. Research suggests that the specific exposure of blue light emitted from devices during night may have cause many health disturbances and possible long term health implications.

C (Comparison)

Teenagers who are not exposed to artificial blue light at night.

To identify health and wellbeing implications due to the use of blue light exposure at night we must compare it those who make active decisions to avoid blue light exposure at night and practice better sleep hygiene. As this is an under researched topic, it will be difficult to find specific comparisons among teenagers but comparing biological differences between exposure and non exposure will give an appropriate contrast.

O (Outcomes)

Less sleep during the vital teenage years where development occurs. Hormonal off balance, such as melatonin which affects the biological circadian rhythm and can furthermore possibly cause negative health implications. Serotonin imbalance which may correlate with mental health disorder development.

This is to understand the explicit correlation between blue light exposure and its negative health and wellbeing outcomes. It is undeniable that technology has positively affected the modern age in terms of practicality but little research has been performed on the negative health implications of its users and the long term health outcomes. There is hope to find evidence of negative health implications as can be used for development of public health knowledge and awareness.

T (Time)

N/A

Artificial blue light exposure during night (dark hours). Night begins after the evening but is still subjective. This is why ‘dark hours’ are used as we only receive natural blue light during the day, ending during the dark hours. Research is collected from the artificial blue light exposure during dark hours.

 

  Blue Light

Light is made up of electromagnetic particles that travel in waves. The shorter the wavelength; the higher the energy (Schlotter, 1997). These wavelengths are measured in nanometers and each wave represents a different colour that lies on a spectrum of wavelengths. Blue light has one of the shortest wavelengths and therefore produces high amounts of energy (Schlotter, 1997). Artificial blue light can be found in laptops, televisions, smart phones, tablets, fluorescent and LED (Light-emitting diode) lighting and other electronic devices (Heiting, 2017). Natural blue light comes from the sun which the human body requires to regulate natural sleep and wake cycles known as the circadian rhythm (Schlotter, 1997).

  Sleep and Development

A recent news article from Insight New Zealand reported that high school teachers were finding that their pupils were staying on their phones into the night and arriving to the next morning with red eyes (Cowie, 2018). This aligns with Gradisar’s (2013) studies suggesting teenagers who use their phones before bed went to sleep later, had less hours of sleep on school nights and had higher scores of sleep disturbances. It is important to understand the necessity of sleep for teenagers and what factors are affecting their behaviour towards sleep behaviour (Taras, 2005). Sleep is a natural state of unconsciousness in which an individual can be aroused from (Marieb, 1995). This natural act is shown to be an essential health component, affecting the wellbeing and quality of an individual's life (Ministry of Health, 2018). Sleep is an active period involving important processing and restoration (Jensen, 1993). An adequate amount of sleep for different age groups varies on their stage of biological development, The National Sleep Foundation (2015) recommends that teenagers between the ages between 13-18 years of age need between 8-10 hours of sleep and no less than 7 hours. One study found that the ages between 11 and 17 are the most extensive period for EEG (electroencephalogram) changes in sleep, which represent the crucial component of the brains maturation and remodelling process (Darchia, 2014). These are all restorative factors and the detrimental impact of sleep loss effects cognitive domains of the teenage brain (Tarokh, 2016). Such as attention, executive function, reward sensitivities, emotional regulation, learning, memory (Tarokh, 2016), and daytime fatigue (Chan, 2010).

When understanding the necessity of sleep for teenagers, it’s not only the quantity of sleep but also the quality. There are two distinct physiological states of sleep that dictate the quality. Firstly non-REM (Rapid Eye Movement) sleep, comprising of three stages. Stage one and two as light sleep and the third being slow wave sleep (deep sleep). Secondly is REM sleep where the functions of memory storage, learning (cognitive) abilities and balancing of mood happens (Kayaba, 2014). All vital factors for the maturation of the teenage brain (Jensen, 1993). Wams (2017) states that those exposed to artificial blue light at night took longer to fall asleep and were less alert in the morning but most importantly, spent less time in REM sleep (Wams, 2017). For the teenage maturation of the brain, this may be a crucial element that is deprived in the developmental stages (Grave, 2006).

This is important to understand as it has implications on the maturation of the teenage brain and their  physical development (Circadian Rhythm, 2019). Leading to daytime sleepiness and poorer academic performance at school (Ming, 2011). Ming (2011) states that there is a need for high school curricula to integrate ‘sleep health’ education which will adjust their sleep/wake cycles. Teenagers are a target group for sleep health education as it will set them up with knowledge to avoid unhealthy sleeping habits and future sleep disorder development in adult life (Hirshkowitz, 2015). Sleep is a vital part of physical cognitive and emotional health just as much as diet and exercise is (Hirshkowitz, 2015).

  Circadian rhythm

There are two factors that determine how an humans sleep. Firstly how long it’s been since you last slept and secondly a complex system called your circadian rhythm, a cyclical 24 hour period of human biological activity (Hastings, 1995). A teenager between 13-18 years of age should sleep 8-10 hours and be awake 16-14 hours (National Sleep Foundation, 2015). In wakeful hours, a teenagers physical and mental functions are most active and tissue cell growth and maturation occurs (Circadian Rhythm, 2019). During sleeping hours the majority of voluntary muscle activities cease, there is a decrease in metabolism, heart rate, respiration, blood pressure,  body temperature and an increase in digestion (Circadian Rhythm, 2019). All factors contributing to the teenage bodies rest, restoration and maturation (Kayaba, 2014).

Hasting (1995) explains the biological cycle of the circadian rhythm. The circadian rhythm is a cycle that is controlled by the master centre of the brain for rhythmic information and sleep pattern establishment, known as the hypothalamus. The suprachiasmatic nucleus (SCN) in the hypothalamus receives light and dark signals from the retina of the eye. Natural blue light from the sun activates photoreceptor cells in the retina to transmit signals to the SCN. Signals are further transmitted to the pineal gland, attached behind the hypothalamus and is responsible for the production of the hormone melatonin. Melatonin cyclical fluctuation is the key hormone in maintaining the normal circadian rhythm. When blue light wavelengths are detected by the retina, melatonin production is inhibited which causes wakefulness. In response to darkness at night, melatonin production is increased and the body prepares for sleep.

The issue that teenagers face is the shift in circadian rhythm due to being exposed to artificial blue light at night during dark hours where the natural increase of melatonin is supposed to be produced to prepare for sleep. Natural lighting signals are consistent within the cycles of the earth's orbit around the sun. When the use of artificial blue light activity on devices are present during late hours of the night, which many teenagers participate in, it has an association to developing morbidities. Suggested links have been found between the delayed phase of the endogenous circadian pacemaker due to exposure of artificial blue light at night and unintended biological consequences (Chang, 2015). Costa (2010)  found that it was linked to breast, advanced prostate and colorectal cancer.

  Obesity

A morbidity risk factor to recognise from night time artificial blue light exposure to teenagers is obesity. New Zealand currently has the third highest rate of adult obesity in the OECD and these rates continue to rise (Ministry of Health, 2019). The World Health Organisation describes the prevalence as an epidemic. One in three New Zealander adults over 15 years are classified as obese and 12% of children obese  (Ministry of Health, 2019). These statistics continue to increase every year (Ministry of Health, 2018). This shows an important risk factor to address in teenagers within New Zealand because obese children are more likely to be in adulthood (Ministry of Health, 2019).

Wiley (2000) explains that longer periods of light due to artificial blue light gives the brain the assumption that it is summer. It signifies that winter is on the horizon. Winter signifies famine to your internal organs and therefore during those longer days it is instinctive to crave carbohydrates to store fat for hibernation and scarcity. This fat storage is accomplished by increasing carbohydrate consumption until your body responds to all the insulin by becoming insulin resistant in muscle tissue which ensures the consumed carbohydrates are stored as adipose tissue. This prompts the liver to dump extra sugar into cholesterol production which stops the freezing of cell membranes at low temperatures.

Leptin regulates appetite and fat storage and is produced and secreted from adipose tissue (Myers, 2012). Leptin is released by melatonin and so is decreased when an individual is sleep deprived. Therefore an individual is not sated (Myers, 2012). Overtime an individual with an unregulated circadian cycle develops leptin resistance, meaning an increased appetite which causes over-nutrition and obesity due to increased carbohydrate consumption (Myers, 2012).

Cortisol is a stress hormone that increases insulin levels and drops your blood sugar levels which increases appetite for carbohydrates (Van Rossum, 2017). Cortisol is decreased when melatonin is released (Van Rossum, 2017). When an individual is exposed to artificial blue light during dark hours, normal release of melatonin is delayed and cortisol therefore does not decrease, causing increased appetite and weight gain (Van Rossum, 2017).

  Mental Health

Consistent evidence suggests the disruption of the circadian rhythm is altering the function of brain regions involved with emotion and mood regulation (Bedrosian, 2017). Depression is a mood disorder that can leave teenagers feeling hopeless, worthless and exhausted (Parker, 2013). New Zealand teenagers between ages 15-19 show one of the highest rates of suicide in the world (OECD, 2017). Depression has many potential causes but a lesser known factor is sleep deprivation as a direct result of too much artificial blue light exposure during dark hours (Safety blue, 2019). It is known that technology use has mental health correlations related to depression and anxiety from bullying on a surface level through social media, but also through a biological link. Depression is a serious public health concern, as the incidences of suicide within New Zealand teenagers are increasing (OECD, 2017).

Cortisol (stress hormone) production is reduced naturally during dark hours as it is time for the sleep cycle where physical activity is not needed (Premkumar, 2013). When these days become extended due to the exposure of artificial blue light from technology, cortisol is released into the dark hours where melatonin should be present for sleep (Premkumar, 2013). Higher cortisol levels in the evening correlate with increased scores of depression by 7% compared to those who avoid blue light during dark hours (Premkumar, 2013). It is understood then that sleep disruption a night causes an imbalance of hormones which has an implication of mood disorders such as depression (Bedrosian, 2017).

  Implications

Findings within this literature review shows that with the increase of technology development and use, there is an increased rate in health implications in teenagers. As there is a lack of research due to the modernity of the issue there is a need for a change in behaviour as the use of artificial blue light during dark hours is a deleterious state. It is important to protect teenagers from the implications as they are going to experience these throughout their life. Early intervention for teenagers gives the best chance of sleep hygiene habits and avoiding personal devices at night. National Sleep Foundation (2015) states that phones should be avoided 30 minutes to two hours before bed. The natural progression of light exposure in the evening turns blue light into warmer red light that becomes darker as the night progresses which releases melatonin for sleeping (Blue Light Aotearoa, 2018). Therefore it is important to avoid artificial blue light devices and if unavoidable there should be some strategies to limit this exposure (Blue Light Aotearoa, 2018). Software such as ‘f.lux’ is a computer application that warms the screen display according to time of day and night, reducing the effects of dark hour blue light exposure (Hoffman, 2017). Another form of blue light blocking is optical lenses that filters out blue light wavelengths (Huang, 2016). Evidence shows that teenagers wearing blue light filtering glasses 3 hours before bedtime reported better sleep quality and mood than those who didn’t (Burkhart, 2009). A recommendation for nurses is to provide health literacy within high schools to teenagers. Nurses should take every interaction with a teenager in primary care as an opportunity to assess their psychosocial as well as physical wellbeing. A key challenge in the improvement of teenagers sleep patterns is their lack of knowledge (Perfect, 2014). Adolescence is an age of maturation and development, therefore an impressionable age for healthy lifestyle habit change (Perfect, 2014). Students with a sleep pattern of less than 7 hours a night show poorer academic performance (Ming, 2011).  A high school curricula on sleep health and the effects of artificial blue light from personal devices is important for teachers too as it could improve their students alertness and academic performance. The link to obesity and mental health is also an important implication to consider due to the high prevalence in New Zealand among young people (Ministry of Health, 2018). Health promotion is a critical part of future health of the New Zealand population and targeting the obesity epidemic in teenagers would have an impact on future generations. Sleep hygiene education should include the understanding of dimmer lighting during dark hours to regulate circadian cycles, quiet activity before sleeping, and not using artificial blue light devices into the night (National Sleep Foundation, 2015). This can be beneficial for improved sleep and balanced circadian rhythm for reduction in long term health implications (National Sleep Foundation, 2015). Although blue light is not all bad as studies showing blue light therapy having a positive impact on teenagers suffering from Seasonal Affective Disorder (SAD) (Ebbert, 2016). Blue emitting lamp therapy provides more daylight hours throughout the winter to balance hormones (Ebbert, 2016). As this is a new topic to research it is difficult to find information related specifically to teenagers, therefore further research could look at the improvement that sleep hygiene education has on the behaviour of teenagers one year later. If there is a positive correlation between sleep hygiene and sleep habits then there may be evidence for high school curricula change in policy.

  Conclusion

From an evolutionary perspective, the invention of artificial blue light has revolutionised the way humans live. Creating longer days and shorter nights for efficiency and flexibility of lifestyles. Humans are naturally diurnal creatures, biologically built to sleep during dark hours and be active during light hours which explains the circadian cycle. Removing a human from its natural 24 hour circadian cycle environment has many health implications. These health implications are a recent contemporary issue due to the modernity of artificial blue light technology, therefore showing minimal data. This literature review investigated to find primarily the impact of artificial blue light exposure during dark hours on teenagers as this is the first generation to have had this exposure from birth. What was identified is that artificial blue light exposure during dark hours does have health implications on teenagers. Firstly, lack of sleep due to sleeping later in the night had an effect on the quantity and quality of ones sleep. This had an impact on the development and maturation of the teenage brain and the morning after cognitive abilities. Secondly, it was found that artificial blue light exposure at night was linked to development of morbidities such as obesity. Thirdly, development of depression was positively correlated with dark hour blue light exposure. This literature presents the need for health education and better sleep quality for normal biological human functions.

  References

Bedrosian, T. A., & Nelson, R. J. (2017). Timing of light exposure affects mood and brain circuits. Translational psychiatry, 7(1), 1017.

Burkhart, K., Phelp, J. R., (2009). Amber lenses to block blue light and improve sleep: a randomized trial. Chronobiology International. (26)8, 1602-1612.

Catherall, S. (26th April, 2016). Tweens, teens and screens: How harmful is technology to young brains? Retrieved from: https://www.nzherald.co.nz/technology/news/article.cfm?c_id=5&objectid=10653913

Chang, A., Aeschbach, D., Duffy, J. F., Czeisler, C, A. (2015). Impact of light-emitting eBooks before bed. National Academy of Sciences, 112(4) 1232-1237.

Chan, S., Debono, M. (2010). Replication of cortisol circadian rhythm: new advances in hydrocortisone replacement therapy. Therapeutic advances in endocrinology and metabolism, 1(3), 129-138.

Chaput, J., & Dutil, C. (2016). Lack of sleep as a contributor to obesity in adolescents: Impacts on eating and activity behaviors. International Journal of Behavioral Nutrition and Physical Activity, 13.

Chen, D. Huang, K., Lee, S., Wang, J. (2016) Blue Light Blocking Lenses Measuring Device. Procedia Engineering, (140), 17-29.

Cheeseman, J. F., Winniebeck, E. C., Millar, C. D., Kirkland, L. S., Sleigh, M., Pawley, M. D. M., Bloch, G., Lehmann, K., Menzel, R., Warman, G. R., (2012). General anesthesia alters time perception by phase shifting the circadian clock. National Academy of Science ,109(18), 7061-7066.

Circadian rhythm. (2019). Encyclopædia Britannica. (Accessed: 14th March 2019). Retrieved from: https://academic.eb.com/levels/collegiate/article/circadian-rhythm/82684

Costa, G., Haus, E., Stevens, R. (2010). Shift work and cancer - considerations on rationale, mechanisms, and epidemiology. Scandinavian Journal of Work, Environment & Health, 36(2), 163-79.

Cowie, T. (2nd September, 2018). Insight: Should NZ Schools Ban Mobile Phones? Insight, Retrieved from: http://www.bibme.org/citation-guide/apa/newspaper/

Darchia, N., & Cervena, K. (2014). The journey through the world of adolescent sleep. Reviews in the Neurosciences, 25(4), 585-604.

Ebbert, J, O., (2016). Blue and white light for seasonal affective disorder. Internal Medicine News. (49)14, 14.

Expert Reactions. (13 November 2018). Exposure to artificial blue light – Expert Reaction. Retrieved from: https://www.sciencemediacentre.co.nz/2018/11/13/exposure-to-artificial-blue-light-expert-reaction/

Gamble, A. L., D'Rozario, A.,L., Bartlett, D. J., Williams, S., Yu, S. B., Grunstein, R., Marshall, N. S. (2014). Adolescent sleep patterns and night-time technology use: Results of the australian broadcasting corporation's big sleep survey. PLoS One, 9(11).

Gradisar, M., Wolfson, A. R., Harvey, A. G., Hale, L., Rosenberg, R., Czeisler, C. (2013). The sleep and technology use of Americans: findings from the National Sleep Foundation's 2011 Sleep in America poll. Journal of clinical sleep medicine : JCSM : official publication of the American Academy of Sleep Medicine, 9(12), 1291–1299.

Graven, S., (2006). Sleep and Brain Development. Elsevier Saunders, 33(3), 693-706.

Hastings, M. (1995). Resetting the circadian cycle. Nature, 376(6538), 296.

Hoffman, C., (10th September, 2017) Reduce Eye Strain and Get Better Sleep by Using f.lux on Your Computer. Retrieved from: https://www.howtogeek.com/199303/reduce-eye-strain-and-get-better-sleep-by-using-f.lux-on-your-computer/

Heiting, G. (November, 2017) Blue Light: It's Both Bad And Good For You. Retrieved from: https://www.allaboutvision.com/cvs/blue-light.htm

Hilbert, M., Lopez, P. (2011). The World’s Technological Capacity to Store, Communicate, and Compute Information. Science. 332(6025), 60-65.

Hirshkowitz, M., Whiton, K., Albert, S. M., Alessi, C., Bruni, O., DonCarlos, L., Hazen, N., Herman, J., Adams-Hillard, P. J., Katz, E. S., Kheirandish-Gozal, L., Neubauer, D. N., O’Donnell, A. E., Ohayon, M., Peever, J., Rawding, R., Sachedeva, R. C., Setters, B., Vitiello, M. V., Catesby Ware, J. (2015). National Sleep Foundation’s Updated Sleep Duration Recommendations: Final Report. Sleep Health: Journal of the National Sleep Foundation, (1)4, 233-243.

Kayaba, M., Iwayama, K., Ogata, H., Seya, Y., Kiyono, K., Satoh, M., Tokuyama, (2014). The effect of nocturnal blue light exposure from light-emitting diodes on wakefulness and energy metabolism the following morning. Environmental health and preventive medicine, 19(5), 354–361.

Jensen DP, Herr KA (1993) Sleeplessness: advances in clinical nursing research. Nursing Clinics of North America. (28)2, 385-405.

Marieb E (1995) Essentials of Human Anatomy and Physiology Third edition. London, Benjamin/Cumming. Mosby (1990) Medical Nursing and Allied Health Dictionary Third edition. London, Mosby.

Martin, H. (23rd April, 2017) Screen-time before bed linked to sleep deprivation and behaviour issues. Stuff. Retrieved from: https://www.stuff.co.nz/national/health/91433710/screentime-before-bed-linked-to-sleep-deprivation-and-behaviour-issues

Ming, X., Koransky, R., Kang, V., Buchman, S., Sarris, C. E., & Wagner, G. C. (2011). Sleep insufficiency, sleep health problems and performance in high school students. Clinical medicine insights. Circulatory, respiratory and pulmonary medicine, (5) 71–79.

Ministry of Health. (9th July, 2018) Helping teenagers sleep better. Retrieved from: https://www.health.govt.nz/your-health/healthy-living/food-activity-and-sleep/sleeping/helping-teenagers-sleep-better]

Ministry of Health. (15th November, 2018) Obesity Statistics. Retrieved from: https://www.health.govt.nz/nz-health-statistics/health-statistics-and-data-sets/obesity-statistics

Ministry of Health. (22nd January, 2019) Obesity. Retrieved from: https://www.health.govt.nz/our-work/diseases-and-conditions/obesity

Myers, M. G., Heymsfield, S. B., Haft, C., Kahn, B. B., Laughlin, M., Leibel, R. L., Yanovski, J. A. (2012). Challenges and opportunities of defining clinical leptin resistance. Cell metabolism, 15(2), 150–156.

National sleep foundation; national sleep foundation brings the sleep community together for sleep health and safety (2015). Psychology & Psychiatry Journal (12)11, 164.

OECD Report: Teenage Suicide. (2017) Retrieved from: https://www.oecd.org/els/family/CO_4_4_Teenage-Suicide.pdf

Parker G. (2013). Teenage depression: some navigational points for parents and professionals. World psychiatry : official journal of the World Psychiatric Association (WPA), 12(3), 272–274.

Perfect M. M. (2014). Evidence for a school-based sleep health education program?. Journal of clinical sleep medicine : JCSM : official publication of the American Academy of Sleep Medicine, 10(7), 793–794.

Premkumar, M., Sable, T., Dhanwal, D., Dewan, R. (2012). Circadian Levels of Serum Melatonin and Cortisol in relation to Changes in Mood, Sleep, and Neurocognitive Performance, Spanning a Year of Residence in Antarctica. Neuroscience journal, 2013, 254090.

Royal Society Te Aparangi (November, 2018) Blue Light Aotearoa. Retrieved from: https://royalsociety.org.nz/assets/Uploads/Blue-light-Aotearoa-evidence-summary.pdf

Safety blue. (2019) (The link between poor sleep, blue light & depression. Retrieved from: https://safetyblueblockers.com/blogs/news/the-link-between-lack-of-sleep-blue-light-depression

Schlotter, P., Schmidt, R., & Schneider, J. (1997). Luminescence conversion of blue light emitting diodes. Applied Physics A: Materials Science & Processing, 64(4), 417.

Schneider, Z., Whitehead, D., Lobiondo-wood, G., & Haber, J. (2013). Nursing and Midwifery research methods and appraisal for evidence-based practice (4th ed.). Sydney, Australia: Elsevier.

Shapiro, C. M., Flanigan, M. J. (1993). ABC of Sleep Disorders. Function of Sleep. BMJ: British Medical Journal. (306)6874, 383.

Taras, H., & Potts-Datema, W. (2005). Sleep and student performance at school. The Journal of School Health, 75(7), 248-54.

Tarokh, L., Saletin, J. M., Carskadon, M. A. (2016). Sleep in adolescence: Physiology, cognition and mental health. Neuroscience and biobehavioral reviews (70), 182–188.

Van Rossum, E. F .C. (2017). Obesity and cortisol: New perspectives on an old theme. Obesity, 25(3), 500-501.

Wams, E. J., Woelders, T., Marring, I., van Rosmalen, L., Beersma, D., Gordijn, M., & Hut, R. A. (2017). Linking Light Exposure and Subsequent Sleep: A Field Polysomnography Study in Humans. Sleep, (40)12, 165.

Wiley, T. S., (2000) Lights Out: Sleep, Sugar, Survival. New York, United States of America: Pocket Books.