Men trying to have a baby could increase their chances of getting their partner pregnant if they go to bed before 10.30 at night.
Men with an early bedtime are almost four times more likely to have good-quality sperm, a study has found.
Scientists asked men what time they had typically gone to bed over the past month, and if a doctor had found them to have normal or low-quality sperm.
Those who went to bed before 10.30pm were almost four times more likely to have normal sperm than men whose bedtime was 11.30pm or later.
When most men trying to start a family have an early start each day for work, experts say getting enough sleep is crucial.
Evidence suggests sleep deprivation can cause the immune system to overreact and attack sperm, while going to bed too late may see couples run out of time to have sex.
Professor Hans Jakob Ingerslev, co-author of the study from Aarhus University, said: ‘These were men who had been trying to have a baby for about two years and an early bedtime was likely to be important because it allowed them to get more sleep.
‘The reasons could be psychological as sleep-deprived men suffer mores stress and that can have an impact on fertility.’
Dr Raj Mathur, a consultant gynaecologist at Manchester University NHS Foundation Trust, who was not involved in the study, said: ‘The study suggests men who are trying for a baby should be going to bed early and getting a decent amount of sleep.
‘They should think about what is keeping them up and try to change their habits, whether it’s binge-watching Netflix or sending emails.
Going to bed earlier with their partner may also increase their opportunity for sex, improving their chances of success further.’
Researchers found men who went to bed ‘early’ before 10.30pm were 2.75 times more likely to have normal sperm than those with a more ‘regular’ bedtime between 11pm and 11.29pm.
Men who slept for between seven-and-a-half and eight hours more often had normal sperm, boosting their ability to have a baby naturally.
The results suggest they were six times more likely to have good-quality sperm than those who slept less than seven hours a night on average.
The Danish study, presented as a poster at the annual conference of the European Society of Human Reproduction and Embryology (ESHRE) in Vienna, asked men how many hours of sleep they got, as opposed to how many hours they spent in bed.
Study participants, who were aged 34 on average, were also given a score on sleep quality, based on their total answers in a questionnaire.
Sleep quality was poor, for example, if they had to get up during the night to use the bathroom or struggled to get to sleep within half an hour.
However this overall sleep quality score was not found to have a link with sperm quality, which the men and their partners both reported based on their test results from a fertility clinic.
The authors state that their study was small and further research is needed.
But Dr Christopher Barratt, professor of reproductive medicine at the University of Dundee, said: ‘Telling men who are trying to conceive to get more sleep is a very sensible message, although this is a small study.
‘If you don’t get enough sleep, that has an impact on the metabolism, which will have a negative effect on sperm quality.
‘It is pretty basic advice but a lack of sleep is closely tied to feeling stressed and stressed men are also less likely to have sex once a day, as we recommend when trying for a baby.’