Crew fatigue was a factor. Crew fatigue continues to be a major issue in the United States, which regulations and scheduling not keeping abreast of current scientific knowledge. Complicating the political equation is the desire by scheduling and pilots to complete work in "bursts". However, the performance of crews at the end of a multi-day trip often is not optimal, especially when flights occur across multiple time zones or occur at odd hours. Note that "fatigue" is distinguished from "tiredness." Two different things.