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Couples trying for a baby should have sex before 7.30am on spring mornings if they want to become pregnant,research suggests.
Scientists claim sperm have their own 24-hour internal clock – and they are most potent in the early hours of the morning.
Swiss researchers also discovered that sperm is of its healthiest size and shape to fertilise an egg during March, April and May.
University Hospital Zurich experts analysed semen samples from 7,068 men, aged between 25 and 40, undergoing fertility treatment.
The samples were cross-checked for sperm concentration, total sperm count, progressive motility and normal morphology.
Lead author Dr Brigitte Leeners said: ‘Male semen quality varies with both circadian and circannual rhythms.
‘Collection of semen in the early morning, where semen quality was highest, can be used to improve natural fertility.’
However, she added that the findings could also be used to boost the chances of conceiving through fertility treatments.
Semen collected in the spring months of March, April and May had the greatest concentration of sperm. Significant decreases were noted in the summer.
While samples collected in the early morning, before 7.30am, showed the highest levels of concentration and normal morphology.
The study, published in the scientific journal Chronobiology International, comes amid a rise of infertility.
As many as one in seven couples has difficulty conceiving, according to the NHS. And studies suggest poor semen quality is a factor in up to half of cases.
Fertility experts across the world today welcomed the study of sperm samples, but warned its findings should be treated with caution.
Dr Hana Visnova, medical director of the IVF Cube fertility clinic in Prague, told MailOnline: ‘Sperm collection is clearly a vital aspect of any IVF procedure.
‘So if we can maximise the potency of a sample then that’s to be encouraged.
‘Yet the evidence relating to the best times of the day, or the year, for that to take place is conflicting and controversial.’
She added men with a low sperm count should not bank on having morning sex, and may still need medical help to achieve their dreams of having kids.
World health officials class low sperm count to be anything fewer than 15 million sperm per millilitre of semen.
It comes as recent reports have revealed sperm counts in men worldwide have declined by half over the past 50 years.