I've read that poor sleep causes circles under one's eyes.
Also, I've read that they are caused by dehydration.
Is it safe to assume that the way poor sleep causes the circles is by causing dehydration?
If so, how does it happen: why does poor sleep cause dehydration, and why does dehydration cause circles under one's eyes?
If not, then how does poor sleep cause circles under one's eyes?
Dark circles under the eyes are very literally representative of infraorbital (or periorbital/periocular) hyperpigmentation. I was able to find literature pretty quickly on this subject dating all the way back to the '60s:
"Dark Circles Under the Eyes in Children." Meyer B. Marks. Clinical Pediatrics. 1966.
Dark discolorations or shadows under the eyes as seen in some children are often ascribed to fatigue or sleeplessness. With older children and even in adults a popular notion is to attribute these to chronic eyestrain. With an adolescent girl at menarche the mother may associate them with menstruation. Many of the bluish-black discolorations in the lower orbitopalpebral grooves of children – termed "allergic shiners" – result from long- standing perennial nasal allergy. They are rarely seen in the first year of life unless the infant had nasal obstruction with accompanying mouth-breathing from birth. Children with uncomplicated seasonal allergic rhinitis do not have allergic shiners.
This early paper (which mentions allergies as the cause of these dark circles) alludes to sleeplessness or fatigue as being another cause of infraorbital hyperpigmentation. Skipping several decades of research and progress, there are a number of useful, modern review papers on this subject (emphasis mine):
Dark rings under the eyes are defined as bilateral, round, homogeneous pigment macules on the infraorbital regions. There is no doubt that they are worsened by general fatigue, especially lack of sleep. This idea is corroborated by the daily fluctuation of the lesions intensity, according to the patient status. For this reason, they have been regarded as a mere physiologic phenomenon. Dark circles are more pronounced in certain ethnic groups and are also frequently seen in multiple members of the same family. These hereditary observations raise a question: are there any anatomic or histological characteristics in these populations that could give us a reasonable etiologic explanation? Histological characteristics of infraorbital darkening suggest that they are caused by multiple etiologic factors that include dermal melanin deposition, postinflammatory hyperpigmentation secondary to atopic or allergic contact dermatitis, periorbital edema, superficial location of vasculature, and shadowing due to skin laxity.
Despite its prevalence and cosmetic importance, there are few published studies in the scientific literature about dark circles. Even a good definition of this condition is lacking. We think the term infraorbital ring-shaped melanosis proposed by Watanabe et al. does not encompass its etiology in a global manner. As there is neither a general understanding about dark circles pathogenesis nor a consensus about the major responsible features, treatments are chosen in a simplified way, rendering suboptimal results most of the time. It is important to identify the specific anatomic problem of each patient in order to individualize treatment.
This gets closer to a direct answer and offers several possible etiologies (bolded) for infraorbital hyperpigmentation. These are further clarified in the context of sleep by publications from this year:
Conclusion: Periocular darkening was predominantly not due to pigment, but rather due to cutaneous laxity and vascular visibility through thin skin. Most of them with pigment had it in the dermis. Clinical dermal pigmentation correlated well with histology, unlike epidermal pigmentation. Iron and amyloid were not significant as etiological factors in our patients.
Periocular dark circles may appear because of various anatomic factors such as architecture of facial ligaments, the bony facial structure, midface soft tissue including the prominence of the orbicularis oculi muscle. Because of ageing, there is a loss of facial fat leading to inflexible ligaments causing hollowing effect on orbital rim. There is worsening of shadowing duetohollowness,which is mainly seen in the tear trough area in inferomedial orbit. The thin eyelid skin contributes to the prominence of the underlying soft tissue and subcutaneous vascular network and the orbicularis oculi muscle, due to which the overlying skin appears dark.
Visible pigmentary changes in periorbital area may be due to extravasated hemoglobin and its breakdown products bilirubin and biliverdin. A variety of pathologic and age-related processes result in the increased permeability of the local vasculature resulting in these pigmentary changes. The lower eyelid tissues may have an increased tendency to accumulate fluid due to local processes such as atopy as well as systemic fluid retention and can be limited inferiorly by the orbital rim because of the cutaneous ligaments. This fluid often takes on a purplish color due to the prominent role of the orbicularis muscle in the lower eyelid. Medical disorders including disorders of liver, heart, thyroid or kidney, hereditary blood disorders, vitamin K deficiency.
For a more complete analysis of the different contributing factors to infraorbital dark circles, check out this review from 2016:
Infraorbital dark circles are caused by a variety of anatomic features, with contributions from the skin, subcutaneous tissues, orbicularis muscle, vasculature and ligamentous architecture of the lower eyelid and cheek. A detailed understanding of the regional anatomy is crucial to the management of dark circles, which encompasses a wide variety of modalities.
