Okay...my.02.
I don't question that SAD is out there, living in Seattle you certainly hear a lot about it and for me it's almost a daily thing because I work at Seattle Lighting ( a large firm that retails residential and commercial fixtures as well as many different types of light bulbs. I have a degree in Interior Design, an ALA certificate and have worked in lighting for more than 10 years.
I mention this because I know a fair bit about various light bulbs and what they do, so before anybody else gets the idea to go out and waste their hard earned $$$ on "Full Spectrum lighting" in an attempt to cure SAD - read a little.
The term "Full Spectrum" is complete Bull$#!*, invented by someone in the attempt to sell more light bulbs, the visable color spectrum (or full spectrum if you like) can be seen with ANY light source, be it a candle, incandescent, halogen or fluorescent bulb. Don't belive me? Try a simple test, get an artists color wheel or one each of the house paint chips from the hardware store. hold them under any of the light sources and you will see all the colors......the whole visable spectrum! What will change as you try the various light sources is how each will accentuate or diminish colors, in general you'll find.
Incandescent bulbs (think Thomas Edison) will be very warm yellow and work hard in the red to yellow portion of the spectrum while dulling blue, purple violet. ( "Daylight" incandescents are an industry gimmick to try to change this by coating the inside of the bulb blue or pink, they are more expensive and produce less lumens (measurable light) because of the coating. Don't waste your $$$ on these.)
120 volt halogen types operate at a higher temperature (that means the filiment glow a brighter, whiter color) and thus produce notably more light ( at less wattage)and tend to be more even handed with color tho not perfect. For general household use these are good for many fixtures especially recess cans and track lighing.
12 volt halogen types especially MR 16's, produce a very bright white light, very good for color, fixtures employing these all have a transformer to drop the incoming voltage from 120 to 12 . These bulbs produce about twice as many lumens as an incandescent type and are used commonly in art galleries and museums as well as high end kitchens.
Fluorescent type bulbs ( there are many, but I'll limit my comments to compact fluorescents) are getting alot of hype right now and while they do use less electricity than incandescents they are definately NOT the great light source you're lead to belive, they work very hard in the blue portion of of the viable spectrum and are typically weak in the red to yellow part. CFL's work well in applications where they are left on for a long perod of time, like your front porch outdoor fixture, why? Likely you don't care about the generally poor color rendering or the corkscrew shape. Also you'll get much better bulb life if the they aren't turned on/off for short duration, which wears the ballast out and causes the bulb to fail. Further, no fluorescent bulb, regardless of how it's shaped will push the light it produces out in the fashion that a PAR halogen will, as a bulb technology it simply doesn't work! So don't waste your $$$ on fluorescent bulbs shaped like reflector types!
Fluorescent bulbs can however produce light of the same color temperature as sunlight (5000 degress Kelvin on a sunny summer day in the Northern Hemishpere) when specific combinations of gases are used in the tube. One company, Ott has produced a bulb of very quality that actually induces photosynthesis in plants. Will it help with SAD? Questionable at best, exercise, diet and getting outside even on a rainy day are certainly viable alternatives.
Hope that helps a little, when it comes to buying light bulbs my best advice is : Buyer Beware! There are far more crappy products out there right now than ever before!
