To determine the prevalence of night eating (NE) in a large population-based of Swedish men and women and to assess the relation of NE to body mass index (BMI), sleep-related problems and binge eating.

All twins in Sweden born between 1959 and 1985.

Those who answered "yes" to one or more of the following questions (analyzed as a subset):

• Have you ever worked during the night time?
• Are you currently on maternity/paternity leave?
• Are you currently a student?

Recruitment

All twins in Sweden born between 1959 and 1985 (42,582 individuals aged 20-47 years) were invited to complete a web-based questionnaire (STAGE) conducted by the Swedish Twin Register at the Karolinska Institute, Stockholm. 

Design

Cross-sectional study 

Blinding used

Not used 

Intervention

Not applicable 

Statistical Analysis

• All prevalence estimates were age-standardized separately for men and women and the corresponding confidence intervals were adjusted for within-pair correlation using the cluster option 
• Odds ratios (OR) were estimated by logistic and linear regression models using generalized estimating equations (GEE) to account for within twin-pair correlations
• All regression models were age-adjusted
• Wilcoxon's non-parametric rank sum test was used to test differences in BMI distributions between groups with and without night eating. 

Timing of Measurements

Questionnaires completed in 2005/2006 

Dependent Variables

• Weight status: Underweight, normal weight, overweight, obese 
• Sleep-related problems: Difficulties falling asleep, not enough sleep on awakening, disturbed or restless sleep.

Independent Variables

• Broad night eating defined as awakenings with food intake during the night at least once a week and/or 25% or more of daily food intake after the evening meal
• Narrow night eating defined as awakenings with food intake at least once a week and/or 50% or more of daily food intake after the evening meal.

Control Variables

• Age
• Height
• Weight
• BMI.

• Initial N: 21,741
• Attrition (final N): 14,499
• Age: 20-47 years
• Ethnicity: Swedish
• Other relevant demographics: Not reported
• Anthropometrics:

• Location: Sweden.

Key Findings

Prevalence of Night Eating

Prevalence of Broadly and Narrowly Defined Night Eating

• The prevalence of broadly defined NE was 4.6% in men and 3.4% in women
• The age-standardized risk ratio (RR) among men compared to women was 1.4 (P<0.001)
• The highest prevalence of broad NE was 2.47% in obese men and 2.8% in obese women compared with their normal weight same-sex counterparts
• The prevalence of broad NE by age categories and sex was:

• The prevalence of narrowly defined NE was 2.5% in men and 1.7% in women in the main study population
• It was 4.6% in obese men and 4.5% in obese women
• The age-standardized RR among men compared with women was 1.46 (P<0.001).

BMI and Night Eating

• The mean BMI was higher in men with broad NE than in men without NE. This was also true for women. The mean differences were as follows:

• Among both men and women the prevalence of overweight and obesity was higher among those with broad NE than without NE
• The BMI distributions differed significantly between those with and without broad NE, both among men (P<0.001) and women (P<0.001)
• The risk of obesity was about twice as high in men (OR=1.98, 95% CI: 1.46; 2.69) and women (OR=2.36, 95% CI: 1.73; 3.22) with broad NE compared with same sexed individuals without NE.
• The mean BMI values were also higher in men and women with narrow NE than those without these symptoms
• The mean differences were 0.7kg/m² (95%CI, .3; 1.2) for men and 1.2 kg/m² (95% CI .6; 1.9) for women
• The risk for obesity was increased in men (OR=2.03, 95% CI: 1.37; 3) and in women (OR=2.87, 95% CI; 1.92; 4.27) with narrow NE compared with their counterparts without NE 
• Furthermore, the Wilcoxon's rank sum test showed that the BMI distribution in individuals with and without narrow NE were significantly different (P<0.001 in both men and women).

Prevalence of Binge Eating

• Of all men and women in the main study population 0.3% (95% CI: .2; .4) and 2.7% (95% CI: 2.4; 3) reported past or present symptoms of binge eating 
• The prevalence of binge eating in women with broad NE was 8.9% and in women with narrow NE 11.8%
• In men, the prevalence of binge eating was 0.9% in those with broad NE and 1.7% in those with narrow NE
• The risk of binge eating was elevated both in men with broad NE (OR=3.44, 95%CI:  1.17; 10.12) and narrow NE (OR=4.81, 95%CI: 2.44; 5.18) compared to men without NE  
• The risk of binge eating was also higher in with broad NE (OR=3.56 with 95% CI: 2.44; 5.8) and narrow NE (OR=4.81 with 95% CI: 3.10; 7.48) compared to women without NE.

Sleep Problems and Night Eating

• The non-response rate to questions about sleep-related problems were higher compared to the questions on night eating and anthropometry because sleep-related problems were presented in an optional part of the questionnaire  
• Only about 29% of all study participants in the cohort answered these questions. Amongst responders, men and women with NE had higher frequencies of the three different aspects of sleep-related problems than those without NE  
• The risk of having difficulty falling asleep was three to four times higher in men and women with broad and narrow NE compared with individuals without NE 
• Risks were also elevated about 1.5 to three times for feeling that they needed more sleep on awakening and for disturbed or restless sleep (about 2.5 to 3.5 times).

Other Findings 

• The main study population included subsets of students, individuals who were currently on maternity leave, leave with small children and individuals who answered affirmative to the question: "have you ever worked during the night time?"
• Since there individuals (N=7,242) might have had irregular circadian life habits for reasons unrelated to NE the analysis was repeated in the 14,449 remaining individuals
• In this subset, prevalence of NE was 4.3% in men and 3.3% in women
• Among obese men and women, the prevalence was 7.2% and 6.4%, respectively
• These results are almost identical to the findings from the main study population and as were all other results.

This study of NE in a large population-based Swedish twin sample yielded several intriguing findings:

• In contrast to previous investigations, this large epidemiological twin study suggests that NE is more common in men than in women
• A rather weak and positive association between age and NE in men as well in women was found
• A strong association was observed 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, but in in consistent with other studies
• Positive associations between NE and binge eating with binge 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 (five to six times) in men and women with narrowly defined NE
• Finally, the 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.

Other studies had mixed results regarding NE weight gain, overweight and obesity. This study found strong statistical associations between obesity and night eating and the 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, a tendency was observed 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 the study, no contribution can be made to the important question about the direction of potential causal relation between obesity and NE.

The results showing an increased risk for binge eating in both men and women with NE compared to individuals without NE are in accordance with several studies based in clinical samples. It should be noted that available data did not allow for the application of full DMS-IV-TR criteria of binge eating for at least two days during the past six 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 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, the 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 allowed exploration of 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.

The authors note the following limitations:

• The primary limitation is the definition of NE. Lack of consistent definitions complicates the comparisons between studies. Concessions have to be made to balance quality and depth of information with participate burden and fatigue. This study was able to assess the prevalence of night eating syndrome (NES) according criteria of 25% of daily food intake after evening meal and/or nocturnal ingestion of food three or more times per week. Using these definitions, broad and narrow NE were assessed. Caution must be used to distinguish between the full syndrome NES and the symptoms of NE that were reported. 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.
• Another limitation is how night eating is assessed. The first question asked how often participants get up to eat. If this question was interpreted literally, night eating may have been underestimated if individuals had woken up for other reasons and then decided to eat after they were already awake.
• The study is also limited by the self-reported nature of the data as many obese individuals underreport their weight
• The rather high overall non-response rate and the higher non-response rate on the optional sleep section 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. However, the main study population included subsets of students, individuals currently on maternity leave and individuals who worked during night time. When group were excluded and analyses were repeated, essentially the same results were found which indicates that potential irregular circadian habits in these groups may not have introduced any important bias in the results.