Long winter nights got you down? If you’re having trouble getting in the holiday spirit and these emotions feel more persistent as the season progresses, it may be time to talk to your doctor. For some people, this time of year triggers a type of winter depression called seasonal affective disorder (SAD).

SAD causes people to feel irritable, depressed, sleepy and lethargic when the days are shorter. The jury is still out on what causes SAD, but experts have many theories about why the colder season can be so bleak for some. One theory involves simply taking a look in the mirror—your eye color may affect your likelihood of developing SAD.

Often described as the “window to the soul,” eyes are also the windows where light beams enter our body so we can see. Eye color in people varies from light grey/blue to very dark brown depending on how much of the pigment melanin you have. Darker pigmentation means your eye absorbs more light before it reaches the light-sensing portion of the eye. The thin tissue that lines the very back of the eye is called the retina. Different kinds of nerve cells in the retina transmit visual information to the brain. A type of light-sensing nerve cell, called photoreceptors, express a special light-absorbing pigment that tells the brain about the level of light brightness it’s receiving. The part of the brain that receives this information may influence mood, behavior and our biological clock that controls our sleep/wake cycle.

People with light eyes are typically more sensitive to light. Therefore, less light absorption is needed for the pigment-containing photoreceptors to activate these mood, behavior and sleep-controlling centers of the brain. In other words, people with light eyes may not need as much light to maintain a stable mood. A recent study found that people with blue or other light-colored eyes reported less seasonal moodiness than people with brown or dark-colored eyes, which supports this theory.

Short periods of exposure to intense light first thing in the morning may be an effective treatment for SAD. However, if the winter blues become too much to handle, reach out to your primary care physician or a mental health professional to learn more about your treatment options.

Sadie Dierschke is a PhD candidate in the biomedical sciences program at Penn State College of Medicine. Her studies in the lab of Michael Dennis, PhD, include understanding how translational control of retinal gene expression contributes to the development of diabetic retinopathy. ​