Kaczmarek JL, et al. - This study was performed to determine whether the time of day and eating behaviors were correlated with the composition and function of the human gastrointestinal microbiota, including eating frequency, the percentage of energy consumed early in the day, and overnight-fast duration. Throughout the day, the human gastrointestinal microbiota composition and function varied, which could be associated with the circadian biology of the human body, the microbial community itself, or human eating behaviors. Moreover, behavioral factors, including the timing of eating and overnight-fast duration, were predictive of bacterial abundances. To determine causality of these biological and behavioral relations, longitudinal intervention studies were required.