Men who get too little sleep may be damaging their fertility, according to new research.
Scientists found that young men who suffered disturbed sleep had sperm counts that were more than a quarter lower than those who slept more.
Those who suffered disturbed sleep with less than six hours a night also had lower sperm quality and smaller testicles.
It is thought that poor sleep interferes with the body's ability to produce sperm.
Tina Kold Jensen, who led the study at the University of Southern Denmark, said: "Men with a high level of sleep disturbance had a 29% lower adjusted sperm concentration.
"This appears to be the first study to find associations between sleep disturbances and semen quality."
The researchers, whose work is published in the American Journal of Epidemiology, added that they hope to examine whether restoring normal sleep patterns helped to improve fertility in men.
They studied 953 young men from Denmark aged around 18-years-old and asked them to fill in questionnaire to assess their sleep.
Those who had suffered poor sleep in the past four weeks had lower sperm counts and less mobile sperm. They were also more likely to have deformed sperm.
Sleep loss has in the past been linked with increased risk of obesity, breast cancer and heart disease.