That makes sense since lack of sunlight is one of the major causes of SAD. "We rely on sunlight to regulate our natural circadian rhythm and to help regulate the release of important chemicals in our brain," says Clifton Berwise, a clinical psychologist with Modern Health. Lack of sunlight can mean lower levels of serotonin, which regulates symptoms of depression, and higher levels of melatonin, which can make us feel sleepy. An analysis between a city's weather and the positivity of their driver feedback found that riders who get even 30 more minutes of daylight in February—like those in Jacksonville and Austin—are less susceptible to SAD.
But you also might be more vulnerable depending on when you get your sunlight. In particular, later sunsets correlate to better moods, even if the overall amount of sunlight is the same over the course of the day. For instance, while Indianapolis and Philadelphia both get a minimum of 8.75 hours of light in February, the sentiment holds stable in Indianapolis—where the sun sets around 6:30 p.m.––but drops 3% in Philadelphia, where the sun sets before 5:55.
Temperature also makes a difference. This analysis suggests that SAD seems to hit hardest in cities that plunge below 40 degrees in February.