Our detailed exploration of NE in a large population-based Swedish twin sample yielded several intriguing findings. In contrast to previous investigations (9
), this large epidemiological twin study suggests that NE is more common in men than in women. We found a rather weak and positive association between age and NE in men as well as in women. We observed a strong association between obesity and NE with NE being approximately 2.5 times more common in obese than in normal weight individuals. This finding replicates several previous investigations (12
), but is inconsistent with other studies (7
). We also revealed positive associations between NE and binge eating, with binge eating episodes being 3.5 times more common in both men and women with broadly defined NE compared to subjects of the same gender without symptoms of NE. The observed risk for binge eating was even higher (5-6 times) in men and women with narrowly defined NE. Finally, our hypothesized association between NE and sleep-related problems was also confirmed with those with night eating being more likely to report different sleep-related problems with the strongest association observed between difficulties falling asleep in men and women with narrow NE relative to those without NE. Although perhaps unsurprising that a symptom cluster defined by nocturnal eating would be associated with disrupted sleep, our results indicate that the sleep problems faced by individuals with NE surpass those awakenings with food ingestion and include insomnia and other aspects of disturbed sleep. Despite the sleep disturbance, studies have shown the same sleep onset and offset of those with night eating and control subjects (16
). Kept together these findings suggest that NE is associated with a cluster of dysregulated behaviours in the domains of weight, appetite, and sleep.
Marshall et al. suggested that NES may be a risk factor for obesity based on two findings, namely the age difference and difference in duration of night eating between obese and non-obese persons with night eating (22
). This author found that in 52% of obese persons with night eating onset of night eating preceded onset of obesity. By contrast, a study by de Zwaan et al found that 60% of obese participants had been overweight before onset of NES (30
). However, in a true longitudinal study of night eating and weight change in middle-aged men and women Andersen et al. found that getting up at night to eat contributed to further weight gain in already obese women (19
). As already stated, we found strong statistical associations between obesity and night eating and our results provide support for the suggestion that the same association observed in many smaller clinical studies is not merely a consequence of selection bias. In women we observed a tendency to a J-shaped association of BMI with risk of broadly as well as narrowly defined NE, but the association was not statistically significant for the underweight group. However, due to the cross-sectional design of our study we are not able to contribute to the important question about the direction of potential causal relation between obesity and NE.
Our results showing increased risk for binge eating in both men and women with NE compared to individuals without NE are in accordance with several studies based on clinical samples (8
), although Allison et al. reported a lesser degree of overlap between these conditions in persons seeking bariatric surgery (10
). It should kept in mind that available data did not allow us to apply full DSM-IV-TR criteria, i.e. binge eating for at least two days during the past 6 months, with loss of control, accompanied by distress. In addition, different definitions of study samples, night eating and binge eating have been applied by various authors making comparisons less straightforward.
The results of this study must be considered within the bounds of its strengths and limitations. A considerable strength is that the study was based on questions answered by a large population-based sample of twins from throughout the country of Sweden not relying on clinical case series which inevitably introduces selection bias in estimates. Previous studies have not presented confidence intervals for their estimates making it impossible to appraise the scope of random variability. Our confidence intervals revealed good to excellent precision reflecting the large sample size of the study.
The primary limitation to be considered is the definition of NE. As noted previously, no established DSM-IV criteria for the symptoms of NE or a syndrome of night eating (NES) exist and only provisional criteria have been established. Lack of consistent definitions complicates the comparisons between studies. As is true in any large-scale population-based survey, concessions have to be made to balance quality and depth of information with participant burden and fatigue. Given the number of available questions about NE, we were unable to assess the prevalence of NES according to the provisional criteria, i.e., 25% of daily food intake after the evening meal and/or nocturnal ingestions of food three or more times per week (9
). We were able to assess broad and narrow NE as defined above and caution the reader to distinguish between the full syndrome (NES) and symptoms of NE reported here. On one hand this poses a limitation; however, it also poses advantages. Although diagnostic-level findings are meaningful, it is also important to assess eating disorders at the symptom level (31
). Understanding disordered eating at the symptom level may facilitate the refinement of phenotypes and may clarify sources of variation for specific components of eating disorder symptomatology that will be relevant to refining the diagnostic syndrome of NES (32
Another limitation is how night eating was assessed. In the first of the two night eating questions we ask the participants how often they get up to eat. If this question was interpreted literally night eating may have been underestimated if individuals had woken up for other reasons (e.g., to use the bathroom) and then decided to eat after they were already awake. Furthermore the study is limited by the self-reported nature of our data as many obese individuals underreport their weight (33
). The rather high overall non-response rate (40%) and the higher non-response rate on the optional sleep section (71%) may have resulted in some selection bias in the study group, expected to be leaner and with less deviant eating patterns and sleep-related problems than non-participants. Finally, our main study population included subsets of students, individuals who were currently on maternity leave with small children and individuals who had worked during night time. When we excluded groups and repeated our analyses on the remaining study subjects we got essentially the same results as for the full study population, indicating that potential irregular circadian habits in these groups may not have introduced any important bias in our results.
We expect the net effect to be some underestimation of NE and associated conditions. Our study was limited to twins and it is sometimes argued that twins may differ from singletons with respect to risk for various diseases. Although this may be true for some conditions closely related to exposures in fetal life and growth in infancy, for most other diseases and conditions, previous research has failed to verify differences in occurrence between singletons and multiples (34
). We are not aware of any research indicating that the prevalence of eating disorders, NES or other dysfunctional eating patterns differs between singletons and twins.
In spite of the limitations discussed above, our results highlight important aspects of the phenomenon of NE and associated symptoms. Most striking is the higher prevalence of NE in men than women and substantially elevated risk for obesity in both genders. Although this investigation cannot answer whether it is most fruitful to consider NE as an eating disorder, a sleep-related disorder or both, our results clearly show that NE is positively associated with overweight, obesity, binge eating, and sleep-related problems. The genetically-informative nature of this twin sample will allow us to explore the extent to which genetic and environmental factors contribute to the etiology of NE. Such studies may unravel the overlapping or distinct genetic factors influencing night eating, binge eating, and associated symptoms.