The team of 44 scientists from 10 countries conducted the study at the request of the World Health Organization's (WHO) International Agency on Cancer Research (IARC) and say they found "limited" evidences of a link between cancer and people who work night shifts.

The team suggests that night shift work might raise the risk of cancer by suppressing production of melatonin, a hormone chemical involved in the circadian rhythm which are regular changes in mental and physical characteristics that occur in the course of a day.

They say there was a higher rate of breast cancer in female nurses who worked night shifts.

The study also reviewed research in which animals were exposed to light at night, disrupting the animals' circadian rhythm and found sufficient evidence of a connection between circadian rhythm disruption and cancer.

Researcher Dr. Kurt Straif, who works for the IARC in France, says shift work that involves circadian rhythm disruption is probably carcinogenic to humans.

The researchers also say that some work is worse for the health than others; people working as painters are carcinogenic, and those who work as firefighters are 'possibly carcinogenic' because of the on-the-job chemical exposure they experience.

They also say the studies were observational and do not prove cause and effect and many genetic and environmental factors affect cancer risk.

The research is published in the journal 'The Lancet'.

A report by the Human Genetics Commission is calling on the government to ensure the tests used by these companies go through a more rigorous assessment process.

Scientists have already linked a number of genes to common diseases but these interact in a complex manner and their ultimate effect is influenced by environmental factors in ways that are as yet not clearly understood.

According to Stuart Hogarth of the Institute for Science and Society at University of Nottingham these new companies have substantial financial backing, and it highlights the growing commercialisation of the gene testing industry.

Hogarth says there is no regulatory framework that can control the burgeoning field, and there is a severe danger of losing public confidence in what is a very promising and very exciting field of science.