Could this be why shoppers at malls and department stores are bathed in a constant, bright white light?
Scientists in Australia have found the higher the light levels, the worse people are with their money.
More than 2500 people were asked whether they'd take a guaranteed $5, or a 50-50 chance of $20 or nothing. The brighter the light, the more likely people were to opt for the $5 - which is mathematically the worse option.
"On the days with higher light intensity, people made worse decisions," said Associate Professor Agnieszka Tymula of the University of Sydney, who led the study.
The effect was stronger in older participants, suggesting their moods and thought patterns are more affected by weather conditions than younger people.
Strangely, when the odds of winning the higher amount of money weren't stated, the brighter the lights the more likely people were to take a gamble - especially if they were men or wealthy.
The researchers say it raises questions about the appropriateness of some treatments for conditions like seasonal affectiveness disorder - a type of depression that hits people in winter, when there is less light around.
"Light exposure… is something that we can easily manipulate not only by spending more time outdoors but also with artificial methods like the use of specially designed lamps that imitate natural light indoors," Assoc Prof Tymula wrote in journal PLOS One.
"Artificial light therapy is so successful in fighting depression, but one cannot help but wonder to what extent light therapy prescribed to depression sufferers affects their everyday decision-making."