Abstract Introduction Late night eating has been associated with higher odds of being overweight or obese. This study aims to evaluate the relationship between late night eating and body mass index in a nationally representative sample. Methods Actigraphy was used to estimate the average bedtime, waketime, duration and midpoint of sleep in the National Health and Nutrition Examination Survey 2003-04 and 2005-06 cohorts. Given the circular nature of clock time, the average was calculated to be the point that minimized the sum of squares of differences between time points. Dietary data was collected through two detailed interviews of the participants. Nighttime calories were defined as the average amount of calories consumed between the average bedtime and the average midpoint of time-in-bed, based on the data recorded during the dietary interviews. Results Higher average nighttime caloric consumption (in units of 100 kcal) was associated with higher BMI B(95% CI): 0.062 (0.003, 0.121); this remained significant after adjustment for age, gender, and race B(95% CI): 0.084 (0.026, 0.142). Higher nighttime caloric consumption (as a percentage of total average daily calories consumption) was associated with higher BMI B(95% CI): 1.522 (0.312, 2.733). This remained significant after adjustment for age, gender, and race B(95% CI): 1.718 (0.505, 2.931). Conclusion Higher nighttime caloric consumption, both in average amount (in units of 100 kcal) and as a percentage of average daily calories consumption, was associated with higher BMI. Additional study is needed to further elucidate the relationship between nighttime eating habits and body mass index. Support NHLBI T32HL110952