Influence of light exposure during early life on the age of onset of bipolar disorder

Highlights

• Large spring increase in sunlight associated with younger onset of bipolar I disorder.

• We studied the impact of hours of daylight during early life (first 6 months) on this association (N = 3215).

• More daylight in the first 3 months of early life associated with older onset.

• Human circadian system still developing during early life.

• Early life light exposure may affect vulnerability to future circadian challenges.

Abstract

Background

Environmental conditions early in life may imprint the circadian system and influence response to environmental signals later in life. We previously determined that a large springtime increase in solar insolation at the onset location was associated with a younger age of onset of bipolar disorder, especially with a family history of mood disorders. This study investigated whether the hours of daylight at the birth location affected this association.

Methods

Data collected previously at 36 collection sites from 23 countries were available for 3896 patients with bipolar I disorder, born between latitudes of 1.4 N and 70.7 N, and 1.2 S and 41.3 S. Hours of daylight variables for the birth location were added to a base model to assess the relation between the age of onset and solar insolation.

Results

More hours of daylight at the birth location during early life was associated with an older age of onset, suggesting reduced vulnerability to the future circadian challenge of the springtime increase in solar insolation at the onset location. Addition of the minimum of the average monthly hours of daylight during the first 3 months of life improved the base model, with a significant positive relationship to age of onset. Coefficients for all other variables remained stable, significant and consistent with the base model.

Conclusions

Light exposure during early life may have important consequences for those who are susceptible to bipolar disorder, especially at latitudes with little natural light in winter. This study indirectly supports the concept that early life exposure to light may affect the long term adaptability to respond to a circadian challenge later in life.

Introduction

Environmental conditions during early life may amplify individual vulnerability to psychiatric disease later in life, especially in those with a genetic susceptibility to a specific disease (Bale et al., 2010, Gluckman et al., 2008, Rutter, 2005). Multiple studies have reported an association between bipolar disorder and stressful early life events such as gestational hunger (Brown et al., 2000), gestational influenza (Machon et al., 1997, Parboosing et al., 2013), childhood abuse (Daglas et al., 2014, Etain et al., 2008, Gilman et al., 2014) and early parental loss (Mortensen et al., 2003). Early life events that may induce circadian dysfunction are of particular interest since bipolar disorder involves the disruption of many biological rhythms affecting the 24 h sleep–wake cycle, energy and alertness (Giglio et al., 2009, McClung, 2013, Murray and Harvey, 2010, Wirz-Justice, 2006). The most recognized symptoms of circadian disruption are ongoing sleep disturbances that increase prior to and during episodes (Murray and Harvey, 2010, Ng et al., 2015). However, the consequences of sleep and circadian disruption extend to include irregularity in daily routines, impaired functioning, vulnerability to stressors, and increased risk of episode recurrence (Frank et al., 2000, Giglio et al., 2010, Shen et al., 2008, Sylvia et al., 2009).

At birth, the human circadian system is still immature and developing, and the physical environment may influence its maturation by means of complex epigenetic mechanisms (Azzi et al., 2014, Brooks and Canal, 2013, Ciarleglio et al., 2011, Gluckman et al., 2005, Masri and Sassone-Corsi, 2010, Rivkees, 2007). It was postulated that exposure to light during early life imprints an individual's circadian clock, setting vulnerability to future environmental challenges to the circadian system (Ciarleglio et al., 2011, Erren et al., 2011). We previously found that the larger the springtime increase in solar insolation at the onset location, the younger the age of onset of bipolar I disorder, especially for those with a family history of mood disorders (Bauer et al., 2012, Bauer et al., 2014). Solar insolation is a measure of the electromagnetic energy from the sun that reaches a surface area on the earth in units of kWh/m²/day (kilowatt hours/square meters/day) (NASA, 2012). The purpose of the current analysis was to investigate if sunlight present at an individual's birth location would impact the challenge to the circadian system from the springtime increase in solar insolation at the onset location. The developing circadian clock in infants can be entrained, or synchronized to the earth's 24-h day/night cycle, using cycled lighting of only 200 lux (Rivkees et al., 1997, Rivkees et al., 2004). For comparison, the illumination of bright sunlight is estimated at about 100,000 lux per square meter at the earth's surface (Tiwari and Dubey, 2010). Since low intensity lighting is sufficient for entrainment, the hours of daylight at the birth location were investigated rather than the solar insolation.