MEDICAL INSIGHTS: Seasonal affective disorder and sunlight
There are specialized cells in the eye that send signals to areas in the brain that influence wether you feel sad or happy. The shorter days of winter meaning days with less daylight may cause mood changes of a depressive nature. These cells are able to detect the shorted duration of sunlight and send signals to the brain which make you feel depressed.
These feelings could be related to what is now known as seasonal affective disorder. Jerome Shanes, a neuroscientist at Brown University, has found evidence of this type of brain circuit in humans. Mood changes affect one in five people and light therapy is sometimes appropriate treatment. Therefore your eyes are influencing your brain to cause mood changes.
It has been thought that exposure to light only affected the rods and cones of the retina. However, another group of scientists have identified previously undetected light-sensitive cells which contain a substance known as melanosporin. These receptors are not part of the visual system and reduced light exposure seems to trigger depression.
Light seems to have a strong biological effect on us unrelated to vision. Intrinsically photosensitive retinal ganglion cells/iPRGCs may regulate mood by light exposure. Adult humans were scanned after varying exposure to daylight in an MRI machine and certain areas of the brain seemed to be receiving signals from photoreceptors and these are the same areas that are involved in feelings of depression. In someone ways you may need need light to be happy.
In the northern hemisphere the winter solstice usually occurs around Dec. 21, which therefore is shortest day of the year. The northern pole is maximally tilted away from the sun whereas the south pole is tilted toward the sun in December.
The further north of the equator, the less daylight you have during this time of year. Fairbanks, Alaska has only about three hours and 43 minutes of daylight on Dec. 21, and there is essentially no daylight north of the arctic circle.
In the southern hemisphere, however, Dec. 21 is the longest day of the year. A good friend and colleague just returned from a trip to Cape Town, South Africa. She was there on Dec. 21 and returned to Aiken with a sunburn as well as a happy smile on her face.