Dr. Richard Davis is an associate professor of ophthalmology at UNC Chapel Hill and a researcher at the N.C. Translational and Clinical Sciences Institute. Here he explains what causes – and what is contained within – the tears that frequently surface during the holidays. Questions and answers have been edited.
Q. Why do people cry when they are sad?
A. It is difficult to answer exactly why tears form when a person is struck by emotion. It is much more intuitive to understand that when your eyes are dry, they tear. Or when something gets in your eye, you cry. Crying is typically an emotional response to situations that may be happy or sad. On a physiological level, we know that the connections between emotion and crying are governed by specialized almond-shaped structures deep within the brain called the amygdala, which play a role in emotional states such as depression and alcoholism.
On a more general level, crying makes people feel better and, therefore, may represent a release or letting go of more extreme or stressful emotional states. Crying may also occur as a result of shared experiences of good things or bad, such as a birthday, graduation or loss of a loved one. Interestingly, the chemical makeup of emotional tears is different from tears caused when the surface of the eye is simply irritated. Some scientists believe that these chemicals, which have built up during times of stress and are released in emotional tears, explain why people can feel better after crying.
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Q. Are there different types of tears?
A. In addition to emotional tears, there are basic or basal tears that are responsible for lubricating the eye. Another type is reflex tears, which appear when the surface of the eye is irritated. Examples of such irritants include excessive rubbing, a foreign body, or underlying medical conditions such as dry eye disease. Although counterintuitive, dry eye disease may cause excessive tearing because of a decrease in lubricating tears and a subsequent increase in reflex tearing.
Tears are formed by tiny glands that surround the eye. These glands produce tears in three layers. First, there is an innermost mucous layer that coats the surface of the cornea, a windowlike covering to the eye. The middle layer is made up of primarily water and electrolytes. The outer, oily layer helps hold the tears together and prevent evaporation. All layers work together to provide a lubricating function for the surface of the eye.
Q. Is there a physiological reason for why crying comes easier to some?
A. Some people have dry eye disease, a chronic disease that affects 5 percent to 30 percent of the population, particularly older adults. In dry eye disease, the actual volume of tears decreases, which can make crying difficult.