The effect of sleeping patterns on mental distress and overweight risk under shifting light conditions in north Norway: Some findings from the Tromsø Study

Abstract

This dissertation presents the results from three studies that aimed to investigate sleep-wake rhythms and certain health risks in a subarctic community. Sleep timing and duration are influenced by outdoor light, and the darkness of winter has been found to impact the level of mental distress and sleeping problems. Sleep duration, chronotype (preference for being active during the morning or evening) and social jetlag (mismatch between the body’s biological clock and social schedules) have also been associated with body mass index and body fat distribution. Study 1 aimed to investigate how the extreme photic environment throughout the seasons would affect sleep-wake rhythms. Study 2 focused on sleeping problems and mental distress during winter, and study 3 examined the association between sleep variables and measures of overweight and obesity. The study was based on the sixth survey of the Tromsø Study (Tromsø 6) in 2007-2008 and included entire birth cohorts and random samples of the population aged 30 to 87 years, living in Tromsø. The participants completed questionnaires including self-reported sleep times, lifestyle and health. Height, weight, waist and hip circumference, and biological factors (non-fasting serum level of total cholesterol, HDL-cholesterol, LDL-cholesterol, triglycerides and glucose) were measured. The seasonal distribution of chronotypes and health variables was calculated based on the answers at the participation dates. A small (8 minutes) but significant earlier midpoint of sleep in summer compared to winter was found. The prevalence of self-reported mental distress (HSCL-10 ≥1.85) in Tromsø was lower than what has been described as average for Norway (7.4% versus 11.4%). There were no significant differences in the reporting of current mental distress depending on season but more participants reported current sleeping problems in winter than in the other seasons. The sleep length that was associated with the lowest mean BMI and the lowest mean waist circumference was 8-9 hours. Short sleepers (<6 h) had about 80% increased risk of being in the BMI ≥25 kg/m2 group and male short sleepers had a doubled risk of having a waist circumference ≥102 cm compared to 8-9 h sleepers. Other factors than daylight exposure may be more important in the regulation of sleep patterns for people in the subarctic. Furthermore, the use of stimulants (alcohol and tobacco) or excessive indoor and outdoor light may have masked the seasonal effect of variation in daylight. There were no significant seasonal differences in mental distress, but sleeping problems were most prevalent in winter. From a clinical point of view, some people in the subarctic clearly feel that they are mentally negatively affected by the darkness of winter. However, the negative impact of winter on mental distress for the adult population is not conclusive.