It's neither rain nor snow nor heat that puts people in a bad mood. But watch out for the gloom of day as winter approaches.

That's the conclusion of three BYU researchers who researched the effect of weather on well-being and found that the number of hours between sunrise and sunset has a bigger effect than commonly presumed downers like rain and humidity.

And this is true for all people, not just those known to suffer from seasonal affective disorder, the team said in a report to be published Nov. 15.

“On a rainy day, or a more polluted day, people assume that they’d have more distress. But we didn’t see that," said Mark Beecher, a clinical professor and licensed psychologist for BYU Counseling and Psychological Services.

"We looked at solar irradiance, or the amount of sunlight that actually hits the ground. We tried to take into account cloudy days, rainy days, pollution … but they washed out. The one thing that was really significant was the amount of time between sunrise and sunset.”

Unfortunately for anyone who likes to feel good, that's on the wane, particularly in areas where Daylight Saving Time ended over the weekend. The total amount of daylight will continue to shrink until Dec. 25 when, like a Christmas present from heaven, the days start to lengthen again.

A daylight duration table on the U.S. Navy's website shows how much daylight we lose over late summer and fall.

In Salt Lake City, for example, total daylight peaked on June 26 with 15 hours and 6 seconds. The shortest span of daylight — 9 hours, 15 seconds — will occur on eight days, Dec. 17-24.

This changes, of course, depending on where you live. People in Anchorage, Alaska, will have 5 hours and 29 seconds of daylight on Christmas Eve, while people in Houston, Texas, will have 10 hours, 14 seconds, according to the Navy table.

The BYU study was the result of a casual conversation between Beecher and physics professor Lawrence Rees. During their commute to work one day, Rees asked Beecher if he had more therapy clients on stormy days. Beecher wasn't sure, but it occurred to them that they could find out by combining data sets that were available to each: Beecher's emotional health data on BYU students who sought counseling services, and Rees' data on weather conditions in Provo, Utah, where the university is located.

Assisted by BYU statistics professor Dennis Eggett and 10 students, they spent about a year crunching the numbers on things such as wind chill, rainfall, solar irradiance, wind speed and temperature. But it was the amount of daylight — or lack thereof — that stood out as a factor in whether people reported psychological stress on a 45-question survey that asked about such things as relationships, emotional and physical concerns and suicidal ideation.

Meanwhile, authors of another recent study in Denmark were reaching similar conclusions. They found that admissions for depression at psychiatric hospitals in Denmark increased by 11 percent in the two months after Daylight Saving Time ended.

"Distress associated with the sudden advancement of sunset, marking the coming of a long period of short days, may explain this finding," the authors wrote.

The time changed at 2 a.m. Sunday everywhere in the U.S. but Hawaii and Arizona, which don't participate in the time change. Even though the change gave us more light in the morning, its effect is not as beneficial since we're not as likely to be outside in those early hour, Dr. Norman Rosenthal, a Virginia psychiatrist known for his work on seasonal depression, told CNN.

Because the BYU researchers only studied the responses of people who were already in therapy, their conclusions may not apply to a general population. But numerous other studies have shown a correlation between exposure to sunlight in mood and overall health.

"It's probably best for those who are struggling with psychological/emotional difficulties to realize that they may feel worse during seasons when the days are shorter, and, as a result, to be prepared to seek additional help during those times," Beecher said.

He added that some people may find relief from therapeutic lamps that are prescribed to treat depression and seasonal affective disorder, which is acknowledged by the American Psychological Association, but was dismissed by a January 2016 study at Auburn University as "strongly rooted in folk psychology, but it is not supported by objective data."

Still, it has been proven that exposure to light activates the neurons that control the body's daily rhythms, and sunlight generates the production of Vitamin D, an essential component of health. (Some research suggests a link between Vitamin D deficiency and dementia, and some health experts believe the government's recommended levels are dangerously low.)

In addition to getting exposure to light through therapeutic lamps, Rosenthal told CNN that people should combat the psychological challenges of shorter days by planning to take a short walk after sunrise.

Other advice includes keeping your home well-lit and planning a winter vacation somewhere that is sunny.

"It's very hard to actually accept that the waning light can be having such a potent effect. And it's hidden. It's not like a broken leg, where people will open the door for you," Rosenthal said.

It's not just the absence of light that affects us; the darkness that replaces it has impact all on its own. As Beecher noted in an interview, the two are "highly correlated" and any effects of Daylight Saving Time and shorter solar days could be attributed to either.

People perceive themselves to be less safe when it is dark, and some research suggests this is true. Most crimes committed by adults occur at night, and researchers found that participants in one study were more likely to cheat and lie in darker rooms. Such concerns have led to some outlandish proposals, including a 19th-century "sun tower" that would have illuminated the city of Paris 24-7. Even without an American sun tower, the nation is so brightly lit at night that stargazers complain of light pollution.

But natural, not artificial, light is what matters to our emotional and physical health, which is why some workplaces — even hospital operating rooms — are starting to include windows and skylights into their designs.

Although sunlight must be absorbed directly — not through a window — for Vitamin D creation, that doesn't seem to be necessary for the sun's psychological benefits.