The most important factor to remember when looking at this large study is that correlation does not equal causation. Although the findings that the nurses who worked in rotating shifts were more likely to have irregular periods and with that also had a statistically reduced chance of getting pregnant, this does not mean the irregular periods were in fact caused by their work patterns. The researchers from the National Institute for Occupational Safety and Health were convinced that their findings made the link more clear, however. The new study didn't examine those nurses who did night shifts only, but those who worked nights on occasions and during the day on other occasions, in accordance with a rotating schedule.
Those who worked the most rotating shifts had an irregular period rate of 12 percent, compared to nine percent of those who did no rotating shift work in the past two years. That does not sound like a very significant difference, but looking at the number of women they included in their study, it's actually quite a lot. Lead researcher Christina Lawson said that they were still unsure what might might be causing irregular cycles for women who work in rotating shifts. "One possibility could be that exposure to light at night alters melatonin production," she said. And her advice? She told women working in rotating shifts to look after themselves and catch up on their sleep when they could.