A long-term study featured in the journal Occupational and Environmental Medicine reveals that working more than two night shifts per week poses a greater risk of breast cancer.

The risk appears cumulatively higher in those who describe themselves as 'early birds' or 'morning people' than 'owls' or 'night birds'.

After discovering that shift work disrupts the body's clock (circadian rythms) and is "probably carcinogenic", the International Agency for Research on Cancer (IARC) called for more research in 2007. One in 10 people in Europe and one in five people in the US are night workers. Earlier studies in nurses who might be exposed to other cancer causing agents have excluded other potential influential factors like sunlight exposure for instance.

The researchers managed to recruit 210 of the 218 women aged between 44 and 83 years old who worked for the Danish Army between 1964 and 1999 and who suffered from breast cancer between 1990 and 2003 and who were still alive in 2005 and 2006.

The women were matched with 899 women of the same age who also worked for the Danish Army but who did not have breast cancer. 141 women with breast cancer and 551 without the disease were asked to complete a comprehensive 28-page questionnaire, which included questions related to their jobs and working patters, in addition to lifestyle questions with regard to their reproductive history, use of contraceptives, hormone replacement therapy, and sun bathing habits. The participants were also asked if they were a "morning" or "evening" person, or neither (diurnal patterns). 
 


The outcome based on 693 responses demonstrated that overall those who worked night shifts had a 40% higher risk of breast cancer than those who did not work night shifts. The risk was even higher, i.e. 50% in women who worked a minimum of three night shifts a week for a minimum of six years.

The researchers found that those who worked these shift patterns for this amount of time had an even greater likelihood if they were 'morning' persons. Their risk was almost four times higher opposed to those not working night shifts. The researchers believe that one of the reasons could be that morning types are more susceptible to body clock disruption. Whilst "Owls" were twice as likely to develop breast cancer, the overall risk was even lower for 'early birds' who did not work night shifts. The researchers state that although insufficient sunlight has been associated with the development of various cancers, they found that those who worked night shifts tended to sunbathe more often than those working during the day.

The researchers concluded that up to two night shifts per week did not influence the risk of developing breast cancer, as it may be not be long enough to disrupt the body clock, whereas frequent night shifts over a period of several years may disrupt the body clock and normal sleep patterns, in addition to compromising the production of melatonin, a cancer protecting hormone, which may be associated with developing or progression of breast cancer cells.

Written By Petra Rattue