Too little sleep could be damaging to men's fertility by slashing their sperm counts, new research shows. Scientists found sperm numbers dropped by more than a quarter in men not getting a full night's rest due to late nights, insomnia and broken sleep. Alarmingly, it even made their testicles shrink, the study found. Sleep loss has in the past been linked with increased risk of obesity, breast cancer and heart disease. Experts believe the ideal amount of rest is between seven and eight hours a night. For example, one US study showed adults getting less than six hours a night were 36 per cent more likely to suffer a heart attack and 22 per cent more at risk of a stroke than those who got a full seven or eight hours. But the latest investigation, by researchers from the University of Southern Denmark, is believed to be the first to show insufficient rest can also affect male fertility. Sperm counts have been tumbling in recent years amid fears that male fertility is being harmed by poor diet and lifestyle, or even 'gender-bending' chemicals in the environment. But the latest research, published in the American Journal of Epidemiology, suggests modern sleep patterns may also be a factor. Scientists quizzed nearly 1,000 men in their late teens or early twenties on their sleeping routines. They also took sperm samples to check for quality. The results showed men who frequently went to bed late, woke lots in the night or struggled to get to sleep in the first place, had a sperm count that was 29 per cent lower than those who had no trouble nodding off. Tests showed their testicles were also significantly smaller. In a report scientists said it's not clear why it causes sperm problems. But they stressed men who sleep less tend to have unhealthier lifestyles. They were fatter, drank more alcohol and were more often smokers. It's also possible that sleep problems damage levels of testosterone, as at least one other small study has found keeping young men awake for longer reduces the body's production of the hormone. The Danish researchers said: 'Men with a poor sleep score had poorer semen quality and smaller testicles. 'This is the first study to relate sleep disturbances to semen quality. The results may have important public health implications. 'But it remains to be seen whether improved sleeping patterns restore that quality.'