When it begins getting dark around 4:30 p.m. and the freezing wind starts howling so furiously that most Yalies decide to stay bundled up inside, Lizzy Regis ’05 undergoes a transformation.

Regis, who hails from Trinidad and Tobago — where the temperature rarely falls below 70 degrees at any point during the year — has been forced to endure the infamously bitter New England winter for the past four years.

“Initially, the weather was a total shock,” Regis said. “When winter starts, there’s a total difference in my personality, a drastic change in my mood.”

Regis is certainly not alone in having these symptoms of winter depression. Dr. Paul Desan ’94 MED, director of psychiatric consulting services at Yale-New Haven Hospital, is a specialist in seasonal affective disorder, which affects some 11 million Americans. Desan explained that 90 percent of the population experiences a decrease in their energy level and mood during the winter, often characterized by a gain in weight and an increased amount of sleep.

For Regis, this personality reversal manifests itself in a number of ways. She calls herself a big clothes and fashion person, yet in the winter she reverts to wearing mostly sweatpants instead of her usual jeans. At one point last winter, she wore sweatpants for a month and a half straight. The change in season also affects Regis’ social habits.

“It takes a lot of convincing for me to hang out with and go visit other friends [in the winter],” she said.

Yet, while most people’s moods, like Regis’, are dampened by the darkness of winter, only about 5 percent of people meet the major criteria for winter depression, Desan said. Many Yale students, though not necessarily diagnosed as having SAD, attest to the doldrums of winter and the lowliness they feel.

Katie Matlack ’06, of San Diego, said the adjustment to winters in New Haven was a shock.

“There’s less daylight hours and less time to be outside doing things,” Matlack said. “I’m always wearing the same depressing jacket, and so is everyone else. In California, there’s just more variety in clothing and in nature.”

Haven Reininger ’08, from Los Angeles, noted that he is generally more somber now that it is winter and that he tends to sleep more.

“It’s dark at 5 [p.m.] now, and I need the sunlight,” Reininger said. “It’s a little off-putting to have it be so dark all the time. I’ve been driven to drink a little more.”

Desan explained that SAD is more prevalent in more northern areas. It is extremely rare for a Floridian to have symptoms of SAD, whereas 10 percent of people living along the Canadian border might be diagnosed with the disorder. And though Yalies from warm-weather climates seem most affected by the shorter days of winter, Desan said that there is no definitive evidence that people who move to higher latitudes are more susceptible to SAD than people who are have been living there their whole lives. He did mention, however, that people from California, for example, may be more likely to realize the effects of the darkness of winter on their moods.

“A large number of people who see this [SAD] happen every year are so used to it happening that they don’t seek help,” Desan said, referring to those people who have lived in New England cities their entire lives.

Desan added, though, that while winter weather changes may bring depression, people who are afflicted with SAD generally experience upswings in their moods come spring.

“In the spring, there’s a definite change,” Regis said. “I want to go out more, party more.”

Because Regis plans to stay at Yale for graduate school, she is thinking about getting her condition diagnosed. Though Desan noted that 5 percent of the population have full-blown SAD, about three times that amount have what is known as subsyndromal SAD. A psychiatrist would not tell someone with subsyndromal SAD that he or she had major depression but might recommend that the issue be addressed with treatment, Desan explained.

While there is still much research being done to try and find the most effective treatment for those inflicted with SAD, the use of bright lights early in the morning or the prescriptions of antidepressants are commonly used to combat the disorder. Lack of light — not a decrease in temperature — is the relevant factor in causing SAD.

“It’s very hard to tell what procedure is going to work best for each person,” Desan said.

Desan, who stressed that anyone with symptoms should always seek professional help, also warned that there are many purported treatment devices for sale on the Internet, which are dangerous and should be avoided at all costs.

“There are all kinds of treatment devices on the Internet that are not effective,” Desan said, mentioning a supposedly disease-curing light which in fact emits dangerous UV radiation. “There are cheats, villains and frauds to trap the unwary.”