SAD, Photoinducability, Skeleton Photoperiods, and the Winter Festivals of Lights.

Seasonal Affective Disorder, generally known as SAD, or winter depression, is the most noticeable expression in humans of photoinducability, the changing of physiology or behavior by day length (or more particularly, photoperiod, the number of hours of light in a day). Sufferers of SAD, which have occasionally included myself, are generally in good mental health much of the year, but if photoperiod gets too short, they find themselves depressed. When photoperiod increases again, the depression recedes. In Alaska, where days can get very short for extended periods, about a tenth of the population suffers from clinically significant SAD.

That much is general knowledge. It is also widely known that many other species use photoperiod to time many aspects of the biology, including breeding, migration, food choice and hibernation. If the days are getting long, it is time for breeding, or whatever else one does in the spring. If they are getting very short, it is time for winter activities, such as sleeping in one's hole. Photoperiod, and the change therein, is a highly predictable indicator of what day of the year it is at any particular location.

This week, in a lecture by Dr. George Bentley, I learned a bit about how animals measure photoperiod, and this changed my thinking about SAD and the winter festival of lights that appears as so many different holidays in so many different religions.

One can imagine various methods an animal's brain could use to figure out how long the day is. An animal could have a solar powered kitchen timer in its brain. Sun comes up, timer starts. Sun goes down, timer stops. Where the timer stops is how long the day was. But there are problems with this model. For example, what if an animal goes in its burrow for much of a day? The timer would stop running. Another model, and one that seems to be what many animals do, is to have a timer that also started running at dawn, but doesn't stop every time the animal is in the dark. This timer measures the distance between dawn, the first light the animal sees that day, and dusk, or the last bright light the animals sees that day.

The ramifications of this are not immediately obvious until you hear how can be used. Animals in the lab are often kept in specific light:dark schedules (12:12 or 14:10) to see what happens to their physiology or behavior under different light regimes. In many species, when photoperiod is increased, the animal responds as though it is spring, building bigger gonads and such. But because the animal's physiology is not actually measuring the number of hours of light, but the number of hours between first light and last light, one can induce the same effect using what is called a skeleton photoperiod. A one second flash of light at 5AM and another at 7PM, with total darkness the rest of the day, is 14 hours between first light and last light. Move hamsters into these conditions, and their gonads start growing, as thought they had had 14 continuous hours of light.

After the lecture, I asked Dr. Bentley if anyone had tried a skeleton photoperiod on human volunteers, to see if it would be effective in combating SAD and similar conditions. If the human brain could be tricked into thinking the days were long by a properly timed, but short and convenient, blast of light late in the day, this would be both extremely useful and biologically fascinating. He said that while SAD was often treated with light therapy, he was not aware of any study looking at whether a short exposure to light carefully timed late in the day could have any effect. Then someone walked by, in the hallway and said to someone else, "Happy Hanukkah, are you going to light a candle tonight?"

I think you see where I am going with this. Hanukkah candles are traditionally lit an hour or two after sundown, when it has become truly dark out. In many parts of the world, Jerusalem say, this equates to 12 to 13 hours after dawn. Celebrants are directed to meditate on the candles, and the joy of Hannukah. The traditional candles burn out after about a half hour.

Festivals this time of year often involve fixating on lights in dark evenings. Hinduism, Sikhism, Jainism, Christianity, the Persians, the Chinese, the old Germanic and Norse tribes and so on all had comparable winter festivals of light. Of course it has long been thought that the winter holidays are a great way to fight the winter "blahs" with light. Cheerful light. Evening light. Photoinducing, skeleton photoperiodic light?

In clinical trials of current treatments for SAD, an hour or so of very bright light at almost any time of day has a significant ameliorative effect, among those who stick with it. But many patients drop out, citing the inconvenience and boredom of sitting and staring toward (but not directly at) a big lamp for an hour a day. I can't help but wonder if the festivals of lights point to a better way, not only making the exposure to light a celebration rather than a chore, but also giving guidance as to what time of day to observe those lights and fixate on them. Do it in the early evening, when dusk would be in the summer, and maybe, just maybe, you make some part of your brain think it is summer.

Light regime change begins at home.