JOIN USResearchers tested the quality of sperm from 300 men chosen at random and carried out a detailed analysis of samples from a further 40 in the laboratory, the daily said.
At the same time, the level of vitamin D in their blood was measured. Almost half had an insufficient amount - linked to lack of exposure to natural sunlight or time in a solarium.
Sunlight is the major source of vitamin D, which helps regulate levels of calcium and phosphorus to generate healthy bones.
The findings, published in the journal Human Reproduction, back up previous research linking vitamin D to sperm production, and might explain why many couples seem to conceive on holiday.
Researchers discovered that almost half of the participants had an insufficient amount of vitamin D, below 50 nmol/l. The optimum level recommended by most experts is a minimum of 75 nmol/l, according to the newspaper.