When lifelong and longtime Seattleites talk about the weather, we tend to stick with the usual topics. We chat about the last time we had this much/little rain/snow/sun. We gripe about our fellow compatriots who, mind bogglingly, choose to ease down I-5 at 35mph because there's some orb in the sky that's shining some bright stuff in their face. We even make Twitter accounts that track whether the mountain is out.

Then there's that long-cherished weather topic that's done more for thawing the Seattle Freeze than any forced social gathering ever could: the Big Dark. After all, who doesn't love to whinge about turning back the clock, talk shop about seasonal affective disorder, and debate our need for a vitamin D supplement? Admit it—you're a sucker for a happy light comparison.

Part of the conversational allure perhaps is the staggering number of unknowns. Seasonal affective disorder, or SAD, is a variant of a major depressive episode, but it's unclear why certain people experience it and why it can come and go from year to year.

The National Institute of Mental Health notes that SAD is more common in northern locales, but Dr. John Gonsoulin, a psychiatrist at Virginia Mason Franciscan Health, says "the research doesn't necessarily say that because you live in the Northwest, you're more likely to have it." He cites one study that found those living in Iceland were less likely to develop SAD than folks on the U.S.'s East Coast, suggesting a genetic component.

Some theories about the disorder revolve around a transporter gene that can affect our brain's regulation of serotonin, a hormone that helps control our mood. 

So what does that have to do with our Big Dark? Gonsoulin says it may come down to our circadian rhythm, an internal clock that's affected by things like body temperature and light exposure.

According to SAD hibernation theorists, the lengthening of dark hours during winter leads to increased melatonin (a hormone that helps control your sleep-wake cycle)—and as a result, an increased desire to sleep and be less active. The lack of daylight when you get up in the morning can also play a role, since it's supposed to kickstart your clock and impacts your alertness, wakefulness, and focus.

Here's the bad news for vitamin D evangelists: You may hear about it helping with SAD symptoms, but Gonsoulin says there isn't a clear medical relationship between the two. "I hear about it, but it's certainly not one of the hypothesized mechanisms."

That said, he gets the conclusive leap; vitamin D gets low during the winter months, and a true deficiency can affect things like your bone density and cardiovascular systems. If you still want to pop a D supplement, Gonsoulin says around 6oo–1,000 IUs a day should do it, but talk to your doctor first because there is such a thing as taking too much vitamin D.

The other home remedy filling our Big Dark debates—light therapy or "happy lights"—happens to be legit. If you've been diagnosed with seasonal affective disorder before, Gonsoulin says to start light therapy around four to six weeks before the typical onset of symptoms, usually in late fall for winter SAD or early spring for summer SAD.

Ideally, he notes, you'd get around 30 minutes of outdoor light within an hour of waking up. Granted this isn't always feasible considering the aforementioned Big Dark. Next best is 30 minutes near a window during the start of your day. And finally, around 30 minutes of happy light time, about 10,000 lux, as soon as you wake up. As with all medical things, it's best to talk with your doctor first.

Before you know it, those 5pm (and later!) sunsets will be back, and we'll be on to griping about the next big weather occurrence. Like spring's first 60-degrees-shirt's-off-guy sighting.