Linn B. Strand

The influence of season and ambient temperature on birth outcomes: A review of the epidemiological literature

Seasonal patterns of birth outcomes, such as low birth weight, preterm birth and stillbirth, have been found around the world. As a result, there has been an increasing interest in evaluating short-term exposure to ambient temperature as a determinant of adverse birth outcomes. This paper reviews the epidemiological evidence on seasonality of birth outcomes and the impact of prenatal exposure to ambient temperature on birth outcomes. We identified 20 studies that investigated seasonality of birth outcomes, and reported statistically significant seasonal patterns. Most of the studies found peaks of preterm birth, stillbirth and low birth weight in winter, summer or both, which indicates the extremes of temperature may be an important determinant of poor birth outcomes. We identified 13 studies that investigated the influence of exposure to ambient temperature on birth weight and preterm birth (none examined stillbirth). The evidence for an adverse effect of high temperatures was stronger for birth weight than for preterm birth. More research is needed to clarify whether high temperatures have a causal effect on fetal health.

Insomnia and the risk of incident heart failure: a population study

AIMS Insomnia is highly prevalent among heart failure patients, but only a few small studies have investigated insomnia symptoms and risk of heart failure. We aimed to assess the prospective association between self-reported insomnia symptoms and the risk of incident heart failure in a large Norwegian cohort. METHODS AND RESULTS Baseline data on insomnia symptoms, including difficulty initiating sleep, difficulty maintaining sleep and having non-restorative sleep, socio-demographic variables, and health status, including established cardiovascular risk factors, were collected from 54 279 men and women 20-89 years of age who participated in the Nord-Trøndelag Health study (HUNT) between 1995 and 1997 and were free from known heart failure at baseline. The cohort was followed for incident heart failure from baseline through 2008. We used Cox proportional hazard models to assess the association of baseline insomnia symptoms with the risk of heart failure. A total of 1412 cases of heart failure occurred during a mean follow-up of 11.3 years (SD = 2.9 years), either identified at hospitals or by the National Cause of Death Registry. There was a dose-dependent association between the number of insomnia symptoms and risk of heart failure. The multi-adjusted hazard ratios were 0.96 (0.57-1.61), 1.35 (0.72-2.50), and 4.53 (1.99-10.31) for people with one, two, and three insomnia symptoms, compared with people with none of the symptoms (P for trend 0.021). CONCLUSIONS Insomnia is associated with an increased risk of incident heart failure. If our results are confirmed by others and causation is proved, evaluation of insomnia symptoms might have consequences for cardiovascular prevention.

Maternal Exposure to Ambient Temperature and the Risks of Preterm Birth and Stillbirth in Brisbane, Australia

Almost 10% of all births are preterm, and 2.2% are stillbirths. Recent research has suggested that environmental factors may be a contributory cause of these adverse birth outcomes. The authors examined the relation between ambient temperature and preterm birth and stillbirth in Brisbane, Australia, between 2005 and 2009 (n = 101,870). They used a Cox proportional hazards model with livebirth and stillbirth as competing risks. They also examined whether there were periods in pregnancy where exposure to high temperatures had a greater effect. Higher ambient temperatures in the last 4 weeks of the pregnancy increased the risk of stillbirth. The hazard ratio for stillbirth was 0.3 at 12°C relative to the reference temperature of 21°C. The temperature effect was greatest at less than 36 weeks of gestation. There was an association between higher temperature and shorter gestation, as the hazard ratio for livebirth was 0.96 at 15°C and 1.02 at 25°C. This effect was greatest at later gestational ages. These results provide strong evidence of an association between increased temperature and increased risk of stillbirth and shorter gestation.

Methodological challenges when estimating the effects of season and seasonal exposures on birth outcomes.

BackgroundMany previous studies have found seasonal patterns in birth outcomes, but with little agreement about which season poses the highest risk. Some of the heterogeneity between studies may be explained by a previously unknown bias. The bias occurs in retrospective cohorts which include all births occurring within a fixed start and end date, which means shorter pregnancies are missed at the start of the study, and longer pregnancies are missed at the end. Our objective was to show the potential size of this bias and how to avoid it.MethodsTo demonstrate the bias we simulated a retrospective birth cohort with no seasonal pattern in gestation and used a range of cohort end dates. As a real example, we used a cohort of 114,063 singleton births in Brisbane between 1 July 2005 and 30 June 2009 and examined the bias when estimating changes in gestation length associated with season (using month of conception) and a seasonal exposure (temperature). We used survival analyses with temperature as a time-dependent variable.ResultsWe found strong artificial seasonal patterns in gestation length by month of conception, which depended on the end date of the study. The bias was avoided when the day and month of the start date was just before the day and month of the end date (regardless of year), so that the longer gestations at the start of the study were balanced by the shorter gestations at the end. After removing the fixed cohort bias there was a noticeable change in the effect of temperature on gestation length. The adjusted hazard ratios were flatter at the extremes of temperature but steeper between 15 and 25°C.ConclusionsStudies using retrospective birth cohorts should account for the fixed cohort bias by removing selected births to get unbiased estimates of seasonal health effects.

Insomnia and Endothelial Function – The HUNT 3 Fitness Study

Background Insomnia is associated with increased risk of coronary heart disease (CHD), but the underlying mechanisms are not understood. To our knowledge, no previous studies have examined insomnia in relation to endothelial function, an indicator of preclinical atherosclerosis. Our aim was to assess the association of insomnia with endothelial function in a large population based study of healthy individuals. Methods A total of 4 739 participants free from known cardiovascular or pulmonary diseases, cancer, and sarcoidosis, and who were not using antihypertensive medication were included in the study. They reported how often they had experienced difficulties falling asleep at night, repeated awakenings during the night, early awakenings without being able to go back to sleep, and daytime sleepiness. Endothelial function was measured by flow mediated dilation (FMD) derived from the brachial artery. Results We found no consistent association between the insomnia symptoms and endothelial function in multiadjusted models, but individual insomnia symptoms may be related to endothelial function. Among women who reported early awakenings, endothelial function may be lower than in women without this symptom (p = 0.03). Conclusions This study provided no evidence that endothelial function, an early indicator of atherosclerosis, is an important linking factor between insomnia and CHD. Further studies are needed to explore the complex interrelation between sleep and cardiovascular pathology.

Insomnia symptoms and risk for unintentional fatal injuries-the HUNT Study

STUDY OBJECTIVES To assess the association between insomnia symptoms and risk of fatal unintentional injuries. DESIGN Population-based prospective cohort study with a mean follow-up of 14 y, linking health survey data with information on insomnia symptoms to the National Cause of Death Registry. SETTING Nord-Trøndelag County, Norway. PARTICIPANTS A total of 54,399 men and women 20-89 y of age who participated in the Nord-Trøndelag Health Study between 1995 and 1997. INTERVENTIONS N/A. MEASUREMENTS AND RESULTS There were 277 unintentional fatal injuries, including 57 fatal motor vehicle injuries during follow-up. There was a dose-dependent association between the number of insomnia symptoms and risk of unintentional fatal injuries (P for trend 0.001) and fatal motor vehicle injuries (P for trend 0.023), respectively. The proportion of unintentional fatal injuries cases that could have been prevented in the absence of difficulties initiating sleep, difficulties maintaining sleep, and having a feeling of nonrestorative sleep were 8%, 9%, and 8%, respectively. The corresponding estimates for motor vehicle injuries were 34%, 11%, and 10%. CONCLUSION Insomnia is a major contributor to both unintentional fatal injuries in general as well as fatal motor vehicle injuries. Increasing public health awareness about insomnia and identifying persons with insomnia may be important in preventing unintentional fatal injuries.

Sleep disturbances and glucose metabolism in older adults: The cardiovascular health study

OBJECTIVE We examined the associations of symptoms of sleep-disordered breathing (SDB), which was defined as loud snoring, stopping breathing for a while during sleep, and daytime sleepiness, and insomnia with glucose metabolism and incident type 2 diabetes in older adults. RESEARCH DESIGN AND METHODS Between 1989 and 1993, the Cardiovascular Health Study recruited 5,888 participants ≥65 years of age from four U.S. communities. Participants reported SDB and insomnia symptoms yearly through 1989–1994. In 1989–1990, participants underwent an oral glucose tolerance test, from which insulin secretion and insulin sensitivity were estimated. Fasting glucose levels were measured in 1989–1990 and again in 1992–1993, 1994–1995, 1996–1997, and 1998–1999, and medication use was ascertained yearly. We determined the cross-sectional associations of sleep symptoms with fasting glucose levels, 2-h glucose levels, insulin sensitivity, and insulin secretion using generalized estimated equations and linear regression models. We determined the associations of updated and averaged sleep symptoms with incident diabetes in Cox proportional hazards models. We adjusted for sociodemographics, lifestyle factors, and medical history. RESULTS Observed apnea, snoring, and daytime sleepiness were associated with higher fasting glucose levels, higher 2-h glucose levels, lower insulin sensitivity, and higher insulin secretion. The risk of the development of type 2 diabetes was positively associated with observed apnea (hazard ratio [HR] 1.84 [95% CI 1.19–2.86]), snoring (HR 1.27 [95% CI 0.95–1.71]), and daytime sleepiness (HR 1.54 [95% CI 1.13–2.12]). In contrast, we did not find consistent associations between insomnia symptoms and glucose metabolism or incident type 2 diabetes. CONCLUSIONS Easily collected symptoms of SDB are strongly associated with insulin resistance and the incidence of type 2 diabetes in older adults. Monitoring glucose metabolism in such patients may prove useful in identifying candidates for lifestyle or pharmacological therapy. Further studies are needed to determine whether insomnia symptoms affect the risk of diabetes in younger adults.

Associations of Insomnia Symptoms With Blood Pressure and Resting Heart Rate: The HUNT Study in Norway

ABSTRACT Objective: Although elevated heart rate and blood pressure might represent biologically plausible links for the association of insomnia symptoms with increased risk of cardiovascular disease (CVD), few large studies have investigated the associations of insomnia symptoms with these factors. Our aim was to investigate the associations of self-reported insomnia symptoms with systolic and diastolic blood pressure and resting heart rate in a large population-based study. Participants: Self-reported information on insomnia symptoms, including sleep initiation problems, frequent awakening and early awakenings during night, and measurements of resting heart rate and blood pressure were collected from a total of 50,806 men and women who participated in the third wave of the Nord-Trøndelag Health Study (HUNT-3) in 2006–2008. Methods: In multivariable analyses, we adjusted for sociodemographic factors, lifestyle factors, established CVD risk factors, and snoring or breathing pauses. Results: Compared to participants reporting none of the insomnia symptoms, those having all three insomnia symptoms several times a week had lower diastolic blood pressure (–0.80 [95% CI: –1.47 to –0.14] mmHg, p = 0.02), lower systolic blood (–1.69 [95% CI: –2.76 to –0.63) mmHg, p < 0.001), and higher resting heart rate (0.83 [95% CI: 0.11 to 1.55] beats/minute, p = 0.02). Conclusions: We found a modest positive association of insomnia symptoms with resting heart rate, and a modest inverse association of insomnia with blood pressure. However, the actual differences were small, and likely of less clinical importance. Prospective studies are needed to establish whether the potential link between insomnia and CVD is mediated through changes in heart rate and/or blood pressure.

Short-Term Public Health Impact of the July 22, 2011, Terrorist Attacks in Norway: A Nationwide Register-Based Study

Objectives To examine increases in several health outcomes after the July 22, 2011 terrorist attacks in Norway. Methods Retrospective analysis of nationwide registers (n = 4,953,000) where incidences of schizophrenia/psychosis hospitalizations, suicides, acute myocardial infarctions, and preterm births after the terrorist attacks were compared with corresponding periods the previous 3 years. Results Compared with the same period the preceding 3 years, the observed number of hospitalizations from schizophrenia/psychosis was 14% higher during the first 4 weeks after the terrorist attack (incidence ratio [IR] = 1.14, 95% confidence interval [CI] = 1.07–1.21). The corresponding IRs for the first 3 days and the first week were 1.26 (95% CI = 0.99–1.58) and 1.10 (95% CI = 0.96–1.24). The observed number of suicides was increased by 45% the first 4 weeks (IR = 1.45, 95% CI = 1.12–1.86), 163% the first 3 days (IR = 2.63, 95% CI = 1.15–5.20), and 105% the first week (IR = 2.05, 95% CI = 1.14–3.42). For acute myocardial infarction, there was an increase of 5% the first 4 weeks. There were also more births the 4 weeks (IR = 1.04, 95% CI = 1.01–1.07, but this increase was not seen in preterm births of less than 37 weeks of gestation (IR = 0.93, 95% CI = 0.83–1.04). Conclusions We observed a general nationwide increase of health outcomes investigated in this study the first 4 weeks after the terrorist attacks. These results may contribute to the growing body of evidence on the adverse health outcomes that may accompany national stressors.

Insomnia and the Risk of Breast Cancer: The Hunt Study.

Objective The association of insomnia with subsequent breast cancer risk is largely unknown. Therefore, we assessed whether different symptoms of insomnia and their combination are associated with incident breast cancer in a large population-based study. Methods In a prospective cohort study, 33,332 women were followed to monitor the occurrence of their first invasive breast cancer identified by the Cancer Registry of Norway. Insomnia symptoms including (1) nonrestorative sleep and (2) difficulty initiating and (3) maintaining sleep were self-reported using a study specific measure reflecting the current Diagnostic and Statistical Manual of Mental Disorders criteria. Hazard ratios and 95% confidence intervals were calculated using multiadjusted Cox proportional hazards models. Results A total of 862 incident breast cancer cases occurred during a mean follow-up of 14.7 years. No consistent association was observed between the individual insomnia symptoms and breast cancer risk. However, compared to women reporting no insomnia complaints, those who reported having all three aspects of insomnia simultaneously were at increased risk (hazard ratio, 2.38; 95% confidence interval = 1.11–5.09). Conclusion Our results suggest that having only some aspects of insomnia may not predispose someone to breast cancer. In contrast, experiencing all insomnia symptoms simultaneously might confer considerable excess risk.