Women who undergo fertility treatment during the summer are twice as likely to become pregnant as when they try in winter, British researchers have found.
Longer daylight hours appear to improve the chances of successful treatment, according to the study.
The research was presented at the annual conference of the American Society for Reproductive Medicine (ASRM) yesterday.
Dr Simon Wood and a team from the Countess of Chester Hospital in Ellesmere Port looked at more than 3,000 cycles of fertility treatment that were carried out over a four-year period.
They studied women who had undergone cycles in both summer months of April to September and the winter season from October to March.
The pregnancy rate during the summer was 15.7 per cent compared to 7.5 per cent in the winter.
During the summer cycles, the women also needed lower doses of the drugs designed to stimulate their ovaries prior to having embryos transferred.
Dr Wood and his team concluded: "The results of this study appear to demonstrate the benefit of increased daylight length on outcomes of IVF cycles.
"Further research is required to confirm these findings and to identify the possible mechanism of action for the effect of extended daylight on reproduction."He suggested that one of the reasons may be evolutionary; it is known that mammals are more likely to conceive during the summer because it means their offspring will be born in the spring, when food is more plentiful, the weather warmer and they have a higher chance of survival.
This process is known as photoperiodism, and is linked to the hormone melatonin.
It is not known precisely how the hormone controls fertility, but melatonin regulates sleeping and waking cycles.
It is produced during the night and is suppressed during the day.
Experts had previously thought that women undergoing fertility treatment were immune to the impact of photoperiodism because the strong drugs they are given to stimulate their ovaries to override Nature.
They believed that melatonin acted through the pituitary gland, which is "switched off" during IVF cycles,However, researchers have now found that melatonin is found in receptors throughout the reproductive system and may be more important than they thought.
Helen Kendrew, of the British Fertility Society, said: "I am surprised.
"These findings are interesting and need more investigation."Previous research has shown that women are also more likely to become pregnant naturally during the summer months and that vitamin D, derived from sunlight, can improve sperm quality.
Reacting to the latest research, fertility expert Dr David Ball, a laboratory director from Minnesota, said: "It's interesting that during the summer months, the women required fewer drugs, which does seem to suggest it's a natural reaction.
"We know that mammals are more likely to survive if they are born in the spring, although some people like to think we have evolved beyond this kind of thing."However, he added: "People should not delay treatment because they think they will have a better chance in the summer, particularly older women.
"Age will trump trying to manipulate daylight every time and women should act sooner rather than later."A separate study, also presented at the conference, found that sperm quality is also better during the summer months.
Researchers from Mount Sinai Hospital in Toronto, Canada, looked at more than 5,000 sperm samples given by men coming forward for infertility assessments.
They found that DNA damage to sperm was significantly higher in samples given during the winter than those given in spring and autumn.
Sperm concentrations were also higher during the samples given in the spring, suggesting that the seasons can also affect the fertility of men as well as women.
The researchers concluded: "Our results provide evidence that semen quality in humans is subject to seasonal variations.
"Further studies are required to explain these findings."