Cancer risk associated to shift and night work

Epidemiological studies in recent years have associated rotating shift work with increases in cancer risk: IARC has classified “shiftwork that involves circadian disruption” as a “probably carcinogenic to humans” (Group 2A), referring to breast cancer in particular, but with some evidence for prostatic, colorectal and endometrial cancer. The physiopathological mechanisms deal with light exposure at night and dis-regulation of the biological clock and connected body circadian rhythms (melatonin in particular), alterations in clock gene expression and function, and interference on neuroendocrine system and immune surveillance due to chronic sleep deprivation. Although controlled for several important confounding factors (i.e. age, menarche, parity), these studies show important limitations with respect to definition and amount of exposure to shift and night work, able to define the potential for circadian disruption, such as: rotating/fixed shifts, amount of night shifts, start and finishing time of duty periods, speed and direction (clockwise/counterclockwise) of shift rotation, and continuous/semi-continuous shift systems. The rather low odds ratios and relative risks reported, and their significant levels only after long-term exposure, may reflect the interaction with many other concurrent non-occupational risk factors, which in some cases might even mask the association. These issues need to be urgently taken into consideration as breast and prostate cancers are two of the three most prevalent cancers in the general worldwide population and the number of people involved in shift and night work are on the increase; rapid economic and productive growth in developing countries also adds urgency to the issue. In terms of preventive actions, work hygienists, ergonomists, occupational health physicians, and employers must carefully evaluate this occupational risk, in particular when assessing whether the proposed/adopted shift schedules are more or less disruptive of the circadian system, and should take into consideration the ergonomic criteria concerning the organization of the shift systems.