You get a dark circle around your iris when you're not sleeping well, or you have a defficiency in your diet, right?
A dark circle around the iris?
Do you mean dark circles under your eyes, like below the lower lids? With fatigue or allergies, the tiny venules dilate underneath the very thin skin, giving a dark cast to the tissue under the lower lids. It is seen more often in allergic individuals because the blood would normally drain through the nasal mucosa. The congested mucosa impedes blood flow. This is known as an "allergic shiner."
Dark circles around the iris are cause for a visit to the ophthalmologist.