Fertility treatment is more likely to be successful in the summer months, researchers have found.
More daylight appears to boost IVF success rates
The team from Liverpool and Chester said the trend was probably linked to the same biological process that means other mammals give birth in spring.
They told the British Fertility Society Conference the women needed far fewer drugs to help them ovulate during months with more daylight.
The rate of successful pregnancies was also higher after summer treatment.
The researchers from the Countess of Chester Hospital and Liverpool Women's Hospital looked at 3,000 IVF cycles carried out between 1997 and 2001.
They found significant differences between success rates for procedures carried out in the months with the most daylight hours - May to September - compared to those with the least - November to February.
Women were found to need more gonadotrophins -hormones used to stimulate the ovary to produce eggs - in the winter months.
Eggs were of the same quality and could be harvested as successfully whatever time of year a woman was treated.
But only 16% of IVF treatments resulted in a successful pregnancy in months with less natural light compared to 20% in the summer.
The researchers, led by Dr Simon Wood from the Countess of Chester Hospital, suggest the hormone melatonin may be responsible.
Melatonin levels naturally cycle in response to light and dark.
All mammals have physical responses to this cycling, such as the natural sleep/wake patterns.
Until recently it was thought that melatonin acted only through the pituitary gland in the brain.
This gland is "switched off" during IVF treatment because it controls a woman's own hormone production, preventing any activity which could interfere with treatment which artificially stimulates the ovaries to release an egg.
However, the researchers say there are also melatonin receptors throughout the reproductive system.
They suggest the hormone may act directly on reproductive tissues to make women more fertile in lighter months.
Dr Wood said: "Although we may not think that our bodies function differently from month to month, our work indicates that human fertility is still influenced by these primitive mechanisms common to all mammals."
He told BBC News Online the pattern was the same one seen in nature.
"If you look at the animal world, it's biologically better for offspring to be born in spring, so they then get six to eight months winter comes to develop.
"It's a biological advantage."
Dr Wood said the same pattern had historically been seen in natural human reproduction, but it was less obvious now because people had more control over deciding when they tried for a baby, and could use contraception to prevent conception.
He said melatonin may be just part of the picture, and more research was needed to explain exactly the seasonal variations further.
And he added: "I would certainly tell patients embarking on treatment about the findings, but I would not advise them to deliberately avoid treatment in winter."
Fertility expert Dr Mohammed Taranissi, who runs the Assisted Reproduction and Gynaecology Centre in London, said: "It has been observed that there is a seasonal variation, but it has been thought it might be linked to warmer temperatures in summer months."
He added: "Patients might have a better chance of a successful pregnancy in summer, but it doesn't mean that people can't get pregnant except in those months."
But Lauretta Krishna, whose second IVF treatment was successful, said: "I think, without a doubt, we would have waited and gone for it if there was a time it might have been more successful.
"I don't know anyone who's gone through IVF that would not have wanted to have it during the period where it's even possibly more effective than at a different time of year."