Yesterday was the most unproductive day of the year, according to a survey, as workers' morale is hit by the darker evenings brought on by the clock change.

Researchers found productivity will drop by 50 per cent this week as depressed staff around the country struggle to come to terms with the dark nights closing in.

A staggering 52 per cent of workers admitted they struggled to get to grips with their work-load on Monday.

And 14 per cent said last year's slump was so bad that they were spoken to by their boss about it.

Worryingly for employers, eight per cent even admitted to phoning in sick because they were so depressed at the thought of going to work amid the shorter, darker days.

The stats emerged in a study of 2,000 workers by Promotur, The Canary Island tourism board.

On Sunday, Dr. Christian Jessen of Channel 4's Embarrassing Illnesses, said: "The Winter Blues are no joke.

"They can affect your work performance by making you unable to concentrate and carry out your normal routine, your relationship by affecting your libido and your social life by making you feel irritable and anti-social.

"Feeling low as the nights draw in and the days get shorter is something many people experience, but some will struggle with lack of motivation, tiredness and depression.

"If you find the winter months tough, particularly during December and January you may suffer from Seasonal Affective Disorder, which causes a low mood caused by the lack of light during winter.

"It's thought to affect two million people in the UK and the best treatment is to get more light.

"If these all ring bells then you may be a sufferer and should consider ways you can get more light into your life over winter."

(Agencies)