The number of people who have dark circles under their eyes is huge. It is a very embarrassing condition because they make you look older than actually you are. There are many ways in which children and adults can develop this condition. When you have dark circles under your eyes, then this means that you have darkening below both eyes. This is not the same situation with swelling and redness caused by infection around only one eye or a bruising caused by a trauma. There are some people who have dark circles under their eyes because they drink too much alcohol or they have lack of sleep. But these are not only reasons why we can have dark circles under the eyes. Also there are other illnesses which can lead to dark circles under our eyes. Many people have thin skin and a combination of fair complexion which is the reason why they have this condition.
Causes: We know that when we get older, then our skin becomes thinner. If we have iron deficiency in our bodies, then this can prevent the blood from carrying sufficient oxygen to eye tissues which is a reason for the dark circles. There are many different causes why we have dark circles under our eyes. The most common cause for the dark circles under your eyes is fatigue. There are some cases in which dark circles under your eyes may be hollows under your eyes that develop as a normal part of aging or shadows cast by puffy eyelids.
- Thinning skin and loss of fat and collagen – it is a very common condition as you age – can make the reddish – blue blood vessels under your eyes more obvious
- Sun exposure, which prompts your body to produce more melanin which is a pigment that gives skin its color 
- Rubbing or scratching your eyes 
- Pigmentation irregularities – these are particular concern for people of color, especially blacks and Asians 
- Heredity – dark under – eye circles can run in families. In the most cases children can get dark circles under their eyes as result of heredity. Some children will not outgrow them while others will do. 
- Hay fever (allergic rhinitis) 
- Fatigue [2,4]
- Sleep deprivation: This is one of the most common causes for the dark circles under our eyes but also this condition can be prevented the most easily. 
- Oversleeping: This is also a common cause why many people have dark circles under their eyes.
- Contact dermatitis
- Atopic dermatitis (eczema)
- Minor trauma: If you have a minor trauma, then this can lead to appearance of a black eye.
- Allergies: If you have nasal congestion, then this can dilate your blood vessels that will drain from the area around the eyes which will cause them to darken. 
Additional causes for dark circles under the eyes:
- Excess salt in the diet: This will lead to fluid retention throughout our bodies and also it can affect our eyes. 
- Periorbital cellulitis: This is a kind of bacterial infection of the eyelids or the eyelid. If you treat this condition with antibiotics, then you should not be worried about it.
- Mononucleosis: This can lead to swollen and puffy eyes. When people who suffer from fatigue, then they feel it and their eyes are result of it. This disease can cause a yellowing of their eyes and the skin around their eyes. This condition is known as jaundice.
- Age: We know that when we get older, then we lose some of the collagen and fat surrounding our eyes. This loss in a combination with the thinning of our skin results in dark circles under eyes. 
- Excessive exposure to the sun: If you are exposed to sun a long period, then this will encourage your skin to produce more melanin. 
- Skin pigmentation, abnormalities: We know that the skin around our eyes is thinner. This is a reason why our blood vessels are more readily visible through them.
- Fluid retention: This can occur with weight gain or with the pregnancy. 
- Lifestyle: If you drink too much or smoke too much, then this can lead to dark circles under your eyes. People who use amphetamines or cocaine or who drink too much coffee can have problems with getting enough sleep. 
Above are some of the reasons for dark circles under eyes.
 Verschoore M, Gupta S, Sharma VK, Ortonne JP. Determination of melanin and haemoglobin in the skin of idiopathic cutaneous hyperchromia of the orbital region (ICHOR): A study of Indian patients. Journal of Cutaneous and Aesthetic Surgery. 2012;5(3):176–82.
 Sarkar R, Ranjan R, Garg S, et al. Periorbital hyperpigmentation: A comprehensive review. The Journal of Clinical and Aesthetic Dermatology. 2016;9(1):49–55.
 Zezima K. Putting ‘you look tired’ to rest. In The New York Times. 2008. Retrieved from www.nytimes.com/2008/06/12/fashion/12SKIN
 Sundelin T, Lekander M, Kecklund G, et al. Cues of fatigue: Effects of sleep deprivation on facial appearance. Sleep. 2013;36(9):1355–60.
 American Academy of Sleep Medicine. Study reveals the face of sleep deprivation. ScienceDaily. 2013. Retrieved from www.sciencedaily.com/releases/2013/08/130830161323.htm
 Narins RS. Why do bags form below our eyes? Scientific American. 2004. Retrieved from scientificamerican.com/article/why-do-bags-form-below-ou/
 Merinville E, Grennan GZ, Gillbro JM, et al. Influence of facial skin ageing characteristics on the perceived age in a Russian female population. International Journal of Cosmetic Science. 2015;37(1):3-8.
 Boyd K. Bags under the eyes. American Academy of Ophthalmology. 2018. Retrieved from www.aao.org/eye-health/tips-prevention/bags-under-eyes