London - Women who work night shifts may find it harder to become pregnant, experts say.
Shift work and inconsistent working patterns could be linked to lowered fertility, the study led by Harvard Medical School found.
The US researchers said women with physically demanding jobs that involve lots of heavy lifting may also struggle to conceive.
The scientists suspect this is linked to hormone levels which fluctuate according to the wake-sleep cycle and physical exertion. Scientists looked at 473 women, aged 35 on average, undergoing fertility treatment at Massachusetts General Hospital.
They found the number of "mature" eggs – those capable of turning into a healthy embryo – was 24 percent lower in those who regularly worked night shifts. And it was 14 percent lower for women whose jobs involved heavy lifting.
Turning an "immature" egg into a ‘mature’ egg is the natural action by which the body prepares an egg for ovulation. This reduction would not mean a woman could not conceive, but might mean it takes longer or may require IVF treatment.
Experts believe lifestyle factors such as not receiving enough sunlight or too much physical activity may interfere. The researchers, whose work is in journal Occupational And Environmental Medicine, said their study was observational – so no firm conclusions can be drawn about cause and effect.
Professor Darren Griffin, of Kent University, warned the conclusions were "perhaps a little over-stated" since the study had not directly examined the mechanisms at play.