Men should get to bed before 10.30pm to improve their chances of fathering children, scientists have said following a groundbreaking study.
The data showed that those tucked up early had nearly four times greater sperm quality compared with those who went to sleep at 11.30pm or later.
Fertility experts said they believe a lack of sleep provokes the immune system into overreacting and attacking healthy sperm.
It also puts men under physical and psychological stress.
The team at Aarhus University in the Netherlands studied 104 men with an average age of 34 over a two-year period, tracking sleep patterns and comparing them against sperm sample analysis.
The figures showed that even the difference between getting to sleep before 10.30pm and between 10.30pm and 11.30pm yielded a 2.75-time greater sperm health.
Presented at the European Society of Human Reproduction and Embryology conference in Vienna, the study contributes to a growing shift in focus towards the impact of substandard sperm in infertility, which scientists believe has been underestimated for years.
The team also assessed sleep duration against the health of sperm. They found that between seven and a half and eight hours sleep was ideal, with sperm appearing more than six times healthier than that of those who got less than seven hours.
No link was found between sperm health and sleeping for eight hours or longer.
Meanwhile, scientists have suggested that humans will be able to colonise space, after an aerobatic study indicated that sperm can thrive in zero gravity.
Ten frozen samples were put on an aircraft that performed "parabolic manoeuvres" of the kind used to accustom trainee astronauts to weightlessness. Afterwards, scientists found them almost identically healthy to control samples left on the ground.
The results suggest it will be possible for humans to procreate in space.