Compiled By Michael A. Verespej More than half of the U.S. workforce -- 51% -- says that sleepiness on the job interferes with the amount of work they get done. A survey conducted by the National Sleep Foundation also found that 27% of working adults admit to being sleepy at work two or more days per week. What's more, these sleepy workers estimate that the quality and quantity of their work is diminished by about 30% when they are sleepy. The cause of the sleepiness? One-third of all adults sleep six-and-one-half hours or less a night in order to "accomplish more."