With the cold snap set to extend into Monday, workers are being challenged to dress brightly to combat the most depressing day of the year.
Researchers claim that on the third Monday of January, the nation's collective well-being will to sink to an all-year low.
Mental Health Research UK (MHRUK) said that "blue Monday" occurred thanks to a combination of bad weather, debt, the need for Christmas detox and poor motivation.
The charity 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.
Dr Laura Davidson, mental health barrister and trustee of MHRUK, said: "Down with drab, dreary colours. This Monday let's brighten up Britain.
"This is not just about awareness of seasonal affective disorder and depression, but it's also a wellbeing campaign. Whilst wearing bright colours will not prevent SAD, which is caused by a lack of sunlight, there has been plenty of research linking mood and colour."
A survey conducted by the charity found that 28% of workers rose before sunrise and returned home from work after sunset during the winter months. The poll of 2,000 adults found that almost one in 10 people had no natural light in their workplace.
Dr Davidson continued: "We were shocked that 9% of those surveyed stated that their workplace had no natural light at all. Employers have a responsibility towards their employees and they need to take on board just how important natural light is to good mental health.
"Architects and planners should also take this very seriously. The UK ought not to grant planning permission for new buildings which significantly reduce natural light in favour of saving costs."
People who take part in the event are being encouraged to make a voluntary donation of £2 to MHRUK by texting BLOO22 to 70070.