Prenatal programming of the circadian and limbic systems might play a role in the odds of developing lifetime depression, a longitudinal study of almost 161,000 women shows.

“Our results could add support to an emerging hypothesis that perinatal photoperiod may influence depression risk,” wrote Elizabeth E. Devore of Brigham and Women’s Hospital, Boston, and her associates. “If replicated, ... these results could translate into safe and inexpensive light-related interventions for mothers and babies.”

In the study, which was published in the Journal of Psychiatric Research, Ms. Devore and her associates examined the influence of daylight exposure during maternal pregnancy and lifetime depression risk in the resulting offspring. They found that increased exposure to daylight during maternal pregnancy correlated with reduced lifetime risk of depression. Of greatest significance was exposure during the second trimester, a critical period during which neuronal generation, migration, and organization take place, wrote Ms. Devore, who also is affiliated with Harvard Medical School, Boston, and her associates.

The effects of daylight exposure were considered modest within the study population, but the authors emphasized that the finding would have much “larger effects at the population level,” given the occurrence of depression in the general population. They added that their findings reinforce a growing consensus that perinatal exposure to daylight could have the ability to influence the risk of developing a mood disorder.

The investigators accessed the Nurses’ Health Study (NHS) and the NHS II, established in 1976 and 1989, respectively, to assess risk factors for chronic conditions in female nurses. Both studies biennially surveyed demographic data on health, lifestyle, and medication use through mailed questionnaires. The first group was composed of 121,701 women aged 30-55 years; the second included 116,430 women aged 25-42 years. Altogether, 160,737 women born full-term were included in the study; 20,912 were excluded from the original survey group for not reporting depression status, as well as an additional 43,325 for not reporting their state of birth.

From data collected regarding participants’ day and state of birth, the researchers were able to estimate total length of daylight exposure during pregnancy using mathematical equations published by the National Oceanic and Atmospheric Administration.

Longitudinal coordinates pinpointing the center of population density for a participant’s birth state were used to identify the location of each participant during gestation. Using those assumptions, the authors were able to establish the two key data points evaluated for the study: total daylight exposure during pregnancy gestation, which was calculated by adding the lengths of all 280 days of the pregnancy, and extreme differences in daylight exposure that might have occurred throughout the pregnancy, which was measured by subtracting the longest and shortest day lengths during gestation.

The investigators paid particular attention to reported levels of depression, evidence of suicide, and personal characteristics and lifestyle factors, such as race, hair color, and early-life socioeconomic factors, including parents’ homeownership at the time of offspring birth; birth weight; history of having been breastfed; and parental occupation throughout the participant’s childhood.

Participants did not begin reporting antidepressant use for the first time until 1996; history of clinician-reported diagnoses of depression began in 2000, Ms. Devore and her associates reported.

Total daylight exposure during pregnancy was found to have “a borderline significant association with odds of lifetime depression,” but the trend was not convincing qualitatively, “and individual estimates across quintiles of exposure” were not considered to be statistically significant. In fact, the authors found that a larger difference between minimum and maximum daylight exposure throughout pregnancy significantly lowered lifetime risk of depression. Women with the largest differences in minimum/maximum daylight exposure during gestation had a 12% lower risk of depression in the NHS population. That reduced risk increased to 15% with the NHS II group. When both cohorts were combined, the reduced risk of depression was 13%.

When evaluating the role that daylight exposure plays with regard to trimester of pregnancy, the authors did note an association for the first trimester, but the association was much stronger for the second trimester; no association was found for the third trimester.

In terms of the effects of daylight exposure on incidence of suicide, no significant associations were found.

Because birth latitude and birth season were of key interest in this study, their relative contribution to total daylight exposure and extreme differences in exposure were considered. Citing observations from the National Health and Nutrition Examination Survey (NHANES), the authors noted that those born at higher latitudes were found to have significantly lower risk of depression. In this study, the authors found that women born in northern latitudes were found to have a 7% risk for lifetime depression, compared with women born in middle latitudes. Conversely, women born in southern latitudes had a 15% risk of depression. No association was found between birth season and incidence of depression, regardless of how season was defined.

The investigators cited several limitations. One is that they did not collect behavioral factors such as the time women spent outdoors. “Our method of exposure calculation relied on the assumption that participants’ mothers were exposed to sunlight from sunrise to sunset,” Ms. Devore and her associates wrote. This way of assessing exposure might have biased their results.

Nevertheless, they said, more studies are needed to examine the role that birth latitude and birth season might play with regard to depression.

The research for this study was supported by the National Institute of Mental Health and the Centers for Disease Control and Prevention/National Institute for Occupational Safety and Health. Additional infrastructure support for the Nurses’ Health Studies was provided by the National Cancer Institute.

The authors declared no conflicts of interest. Ms. Devore has reported receiving consulting fees from Epi Excellence and Bohn Epidemiology.

SOURCE: Devore EE et al. J. Psychiatric Res. 2018. 104(08):e20180225.