Mental health researchers said that "blue Monday" occurred thanks to a combination of bad weather, debt, the need for Christmas detox and poor motivation.
The charity, Mental Health Research UK, aims to raise awareness of depression and seasonal affective disorder (SAD) with its new campaign, Blooming Monday.
It is calling on people to dress in colourful clothing to highlight the plight of those who suffer from the conditions and to raise money for research into treatments.
The charity's Dr Laura Davidson said: "This is not just about awareness of seasonal affective disorder and depression, but it's also a wellbeing campaign.
"While bright colours won't prevent SAD, which is caused by a lack of sunlight, there has been plenty of research linking mood and colour."
A survey found that 28pc of workers rose before sunrise and returned home from work after sunset during winter.
The poll of 2,000 adults found that almost one in 10 people had no natural light in their workplace.
Dr Davidson said: "Employers have a responsibility towards their employees and they need to take on board just how important natural light is to good mental health."