For hundreds, if not thousands of years, people have noticed that their mood is worse in the fall and winter than it is in the spring and summer.  This has been attributed to many things over the years, pre-Christmas blues, post-Christmas blues, being cooped up, cabin fever etc. Ironically, it was staring us in the face all of this time and no one saw it.  People get depressed in the winter for biological reasons.

It was only in the late 1970s that researchers started to ask whether winter depression (now known as seasonal affective disorder) could be due to low levels of light exposure.  Again, in retrospect it should have been obvious. We all complain about how early it gets dark this time of year. We all know that after 4 or 5 days of cloudy weather we feel less cheerful, and when the sun finally comes out, everyone feels better and comments about it.  We can even see it in our language. We use words like “gloomy,” “dark,” and “sunny personality” to refer to our emotional states.

When the scientists finally looked into it more deeply, they found that light exposure has a lot to do with how we feel.  In one survey in the Washington DC area, 92% of respondents reported that they felt worse during the fall and winter than they did during the spring and summer.  About 6% of them had full-blown depression symptoms. And that was south of New England; the problem gets worse the further north you go, likely because the days get shorter.

This actually offers a clue.  We are tropical animals (just look at our lack of fur!).  In the tropics, the days don’t get very long or short; this gets more pronounced closer to the poles you get.  Go far enough, and you have 24 hours of daylight or night. So we are exposing ourselves to more variation in the length of the day than we are programmed for by nature.

Further, we are exposing ourselves to a lot less sunlight.  The sun is stronger in the tropics, and we in the Northeast live in a place that is significantly cloudier.  Even worse, we spend most of our days indoors, at least most of us. The amount of light even in a brightly lit room or office can be 1/10 or even 1/100 of what you would be receiving outdoors.  Think about the patch of sunlight on your floor by the window, and how much brighter it is then a spot was next to it.

We now understand a lot about the underlying biology. Buried deep in the most primitive part of our brain lies a structure called the hypothalamus.  Many of our basic bodily functions are controlled by the hypothalamus: eating, sleeping, heart rate, etc. In one part of the hypothalamus is our main biological clock.   All animals use this to monitor the length of the day so they know what season it is. This guides them as to what they should be doing. Is it time to start burying nuts? Is it time to build the nest?  Is it time to mate?

We have learned that about 10% of the cells in your retina are not wired to the vision part of the brain.  Instead, they are wired to the clock in the hypothalamus. Furthermore, they are specifically sensitive to blue light.  Why is this? This a daylight detection system: if the sky is blue it is day. If it is not blue it is night.

What we have also learned is that providing bright artificial light can treat seasonal affective disorder.  The light has to be very very bright, brighter than even a well-lit room. So we have developed light boxes, small desktop devices that can be easily bought online.  Many of the new or ones have blue LEDs in them. White light is okay; it contains blue light along with all the other colors of the rainbow, but only the blue is used. The light box is best used in the morning for about 20 minutes.  You do not stare directly into it. It can be used at the breakfast table or even at work.  

Seasonal affective disorder can also be treated with antidepressants, which seemed to work about as well as they do for non-seasonal depression.  Some people have year-round depression, but it then worsens when the days get short. Some of those people increase the dose of the medications for several months. Of course, getting outside and getting natural sunlight, especially if it involves exercise, works as well or better.  

By the spring, the days start getting longer, and people emerge from their winter blues to greet the coming of the light.