Two of my friends have Seasonal Affective Disorder. One has been diagnosed by a doctor and sits next to harshly bright lamp for twenty minutes every morning before getting on the subway to go to work, which, in my mind, promptly cancels any benefit derived from the lamp. My second pal has, after several winters of low energy, oversleeping, and depressive feelings, diagnosed herself and pursued a strategy of working out regularly, eating lots of fruit (because vitamin C is captured sunshine), and is in the process of painting her room a vivid shade of yellow that calls to mind an egg yolk aglow with nuclear radiation.
Seasonal Affective Disorder, or SAD (surely the most opportunistic acronym ever), is not a joke. According to Wikipedia:

SAD can be a serious disorder and may require hospitalization. There is also potential risk of suicide in some patients experiencing SAD. One study reports 6-35% of sufferers required hospitalization during one period of illness. The symptoms of SAD mimic those of dysthymia or clinical depression.

SAD can also exacerbate or bring on bi-polar symptoms, and in places like Alaska or Scandinavia with extended winter and low light, SAD is considered something of a public health issue.
That said, I am skeptical of the popularization of SAD.

I believe the malady people experience is real and the science is straightforward, if inconclusive (online discussions about this abound – google if you’re curious). I understand the winter blues because I’ve had them, and who hasn’t? I’m sluggish, I tend to want more sleep and feel less motivated. I’m less social. But if I’m honest about it, I’m not more sad during the winter. Some of the most difficult times in my life, mental health-and-well-being-wise, have been during the summer
I fear SAD is often misapplied in the same manner as ADHD, muddling an essential, serious diagnosis by over-using it as a nebulous, catchall term for normal (and, I daresay acceptable) levels of suffering. As happens with this kind of thing, the generalization of SAD has spawned an industry of cures, from herbal “natural remedies” to the pricey sun lamps my friend usees. Some may help. And some are part of our proud national tradition of snake-oil remedies.
Emotions (and mood), motivation, and self-perception are complex, interdependent psychic forces that are determined by more than the level of seratonin uptake in the brain as it relates to sunlight. Slapping a medical label on your sadness and sitting under a bright lamp may, for many people, be a clever dodge around emotions better dealt with on the cushion.
In myself, I’ve noticed that feelings of low motivation, fatigue, and sadness occur with about the same frequency during the winter months as during the summer months, but they can be magnified and feel as endless as the night that creeps in at 4:00 PM. There is no green breeze to cheer me up, bring me out of myself, and remember that the world is a great big, beautiful place and that I should talk to my friends about a trip to California or Japan or maybe Baltimore. Winter can be a dark, inward-looking time.
In short, maybe it’s more about my perception than my seratonin levels.
As with any strong, disruptive mental-emotional energy, meditation can help take the edge off. It might sound counter-intuitive, but if I sit for 25 or 30 minutes in a dim room after a day of the winter blues, and let my mind settle a bit, I rise with a sense of renewed energy.
Now, is it as much renewed energy as practicing in a field of tall grass at Karme Choling on a bright July day? Probably not, but that’s just the normal cycle of things – clinical diagnosis and abrasive sun lamp not needed.