1 minute Biology: Dark circles, the symbol of sleepless nights

Written by Crystal Jang

‘Dark circles’ are often used as an indicator for not getting sufficient sleep. There are numerous causes for dark circles, including genetics, ageing, or allergies, but the most commonly known source is lack of sleep. But how exactly do they work? It is indeed a strange concept, but the science behind it is quite simple. 

Role of sleep

To fully understand what goes on in the body when we do not get enough sleep, we would first have to know what happens when we do sleep. Sleep is a major part of the body’s nourishment, as it partakes significantly in muscle recovery and replenishment. Therefore, when one stays awake for a prolonged time, muscles start to fatigue. Muscles primarily recover during deep sleep, also known as the slow-wave-sleep (SWS) stage. During that stage, the body secretes multiple growth hormones, aiding tissue growth and regeneration. 

Effect of sleep deprivation

Due to the insufficient sleep, therefore insufficient recovery, muscles heighten the demand for blood supply. The vital components in blood, such as oxygen and glucose, assist metabolic reactions, which is the reason for the elevated demand. These requirements are met through a process called ‘vasodilation’: ‘vaso’, meaning vessel, and 'dilation', meaning to widen. Straightforwardly, vasodilation widens the blood vessels in order to provide more nutrients faster. 

How dark circles are formed 

The skin around our eyes, especially the eyelids, is known to be the thinnest in the human body. Therefore, when vasodilation occurs, the increased volume of blood becomes apparent near our eyes. Furthermore, the reason for dark circles being a dark shade of purple is because the vessels visible under our eyes are mostly veins, meaning that they’ll always carry deoxygenated blood, which is purple, back to the heart. 

Wrap-Up

Dark circles are one visible feature of sleep deprivation. However, it is worth noting that not all ‘dark circles’ are the same or caused by lack of sleep. Areas under our eyes are not only extremely sensitive but also vary between individuals in aspects such as skin thickness, how pale they are, or the bone structure. So, it is best not to assume or comment on one’s ‘dark circles’, as it could be something that is not easily fixed by just sleeping more. 

References:

Dattilo M, Antunes HK, Medeiros A, Mônico Neto M, Souza HS, Tufik S, de Mello MT. Sleep and muscle recovery: endocrinological and molecular basis for a new and promising hypothesis. Med Hypotheses. 2011 Aug;77(2):220-2. doi: 10.1016/j.mehy.2011.04.017. Epub 2011 May 7. PMID: 21550729. 

Koehling, L. (2020, August 11). Workout recovery: Sleep, eat, move, plan. Johns Hopkins University Student Well-Being.

Healthline. (2018, November 2). Vasodilation: Your blood vessels opening. Healthline.