Despite more reports of people feeling tired or depressed during the darker days of fall and winter, weather has very little impact on these changing moods.
A 2008 study conducted by Jaap Denissen about the effects of weather on daily mood found that weather fluctuations accounted for very little variance in people's day-to-day mood.
This was a surprising discovery since there are so many observable changes in human behavior associated with the transition into fall and winter.
What Denissen's research did show, however, was that the association between sunlight and tiredness was significant. The less sunlight people were exposed to, the more they exhibited depression-like symptoms.
As the days get shorter, people may experience more feelings of fatigue during the day, difficulty rising in the morning when it is still dark outside and craving more carbohydrate-rich foods leading to weight gain, Kelly Rohan, Ph.D., Professor of Psychology, University of Vermont said.
"It is probably the short photoperiod, i.e., day-length from dawn to dusk, not rain or any weather variable that is the environmental trigger of these winter symptoms," Rohan said.
The amount of sunlight people are exposed to can have a large impact on mood because it affects the amount of vitamin D people absorb.
"Vitamin D, which is produced in skin exposed to the hormone of sunlight, has been found to change serotonin levels in the brain, which could account for changes in mood," according to Denissen's research.
Serotonin is a hormone that plays an important role in regulating mood. Lower levels of serotonin could correspond with depression-like symptoms. The less exposure you have to the sun, the lower your vitamin D and serotonin levels will be.
"Therefore, lower levels of vitamin D could be responsible for increases in negative affect and tiredness," according to Denissen's research.
The effects of shorter periods of daylight are not the same for everyone.
"Seasonality, or the degree to which mood and behaviors vary across the seasons, is on a continuum and is normally distributed in the general population," Rohan said.
Since the continuum is normally distributed, it is expected to see a certain amount of change in the behaviors and moods of people as the seasons change. However, sometimes these changing seasons can cause an unusually intense reaction.
"At the extreme along the continuum of seasonality is full-blown winter seasonal affective disorder (SAD), a syndrome involving recurrent depressive episodes during the fall and winter months with periods of remission in the spring and summer," Rohan said.
The most commonly reported symptoms of SAD are very similar to regular non-seasonal depression conditions including: significant fatigue, loss of interest in activities, sleeping more than usual, craving and eating more starches and sweets and gaining at least 5 percent of body weight according to the APA.
These symptoms of SAD are more commonly reported among women than men. The reason for this trend remains unclear to researchers.
"No one knows whether women's higher reportage rate of adverse symptoms means that women simply admit to SAD symptoms more than men, or whether women actually have more SAD symptoms than men," Mary Gregerson, Ph.D. said.
SAD symptoms are also more common in higher latitudes.
"Survey studies in the U.S have found that winter SAD prevalence increases with latitude in the general population and ranges from 1 percent in Florida to 10 percent in Alaska," Rohan said.
Higher latitudes are believed to have more extreme weather changes, which would explain the higher incidence of winter SAD symptoms, Gregerson said.
There is no one-size-fits-all treatment available for SAD, but there are multiple methods that have proven to be effective.
The most widely and extensively investigated treatment for SAD is light therapy, i.e., daily exposure to bright artificial light during the symptomatic months, according to the APA.
"Full spectrum light therapy provides documented relief for adverse effects of winter SAD," Gregerson said.
If light therapy fails, antidepressant medications are widely regarded as the second line of treatment according to the APA.
By Accuweather.com Staff Writer Rachelle Gaynor