Melatonin is a hormone produced rhythmically by the pineal gland, normally highest at night. It has been linked to a number of important physiological functions such as influencing sleep propensity, and it is suppressed by light.    

In 2010, shift work was classified as 'probably' carcinogenic by IARC (Group 2A), specifically for breast cancer. This classification comes from 'sufficient evidence' from animal experiments and 'limited evidence' in humans demonstrating that light during the normal dark period of the day increases carcinogenicity.

Melatonin has been proposed as a potential mechanism by which EMF may cause biological effects. One proposal by Stevens et al., (1987) is that EMF may suppress melatonin production in a similar way to light at night, providing a potential mechanism for explaining increased risk of cancer.

There have been many human, animal and cellular studies investigating EMF and melatonin suppression since this hypothesis. The HPA produced a specific review of EMF, melatonin and breast cancer looking at the evidence and it has also been considered by other expert bodies such as the WHO. The summary on melatonin suppression from the HPA concluded:

"Investigations using cell, animal and humans have not given consistent or convincing evidence that EMF exposure affects melatonin production or action. However there are deficiencies in the existing research, which leave open the possibility of an effect."

and on breast cancer:

"No consistent evidence, from research using cells, animals and humans, that EMF exposure is a cause of breast cancer, nor has any mechanism for such an association been demonstrated."