Workers are being challenged to dress brightly at the start of next week in a bid to combat ''blue Monday''.
Some researchers claim that the third Monday of January is the most depressing day of the year.
Mental Health Research UK (MHRUK) said that thanks to a combination of bad weather, debt, the need for Christmas detox and poor motivation, the nation's collective well-being is expected to sink to an all-year low on January 20.
The charity aims to raise awareness of depression and seasonal affective disorder (SAD) with its annual 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.
A poll conducted by the charity found that one in five employees will not wear colourful outfits to work because they were considered "less professional".
The survey of 2,000 British adults also found that w omen are much more likely to wear bright attire than men, with 36% of females saying they wear bright clothes compared to 24% of men.
Dr Laura Davidson, mental health barrister and trustee of MHRUK, said: " Whilst wearing bright colours will not prevent depression, or SAD which is caused by a lack of sunlight, plenty of research links mood and colour.
"The perception that wearing dark colours in the workplace equates to professionalism is a fallacy. Clearly we need a culture change in the workplace.
"Mental Health Research UK urges employers to make a start in changing perceptions and lifting mood by taking part in Blooming Monday. We hope they'll embrace our call to brighten up for the day, and to transform the commute to work into a kaleidoscope of colour.
"Blooming Monday will encourage discussion about mental health and help to break down the stigma attached to it.
"It's a simple ask: that everyone wears their brightest clothes on one of the dullest days of the year."
People who take part in the event are being encouraged to make a voluntary donation of £2 to MHRUK by texting BLOO22 to 70070. To find out more about the campaign visit www.blooming-monday.com.