Among nurses working successive 12-hour shifts, 39 percent had moderate lapses of attention on a vigilance test and seven percent had frequent lapses. Factors associated with lapses included sleep prior to shift, caffeine use and fatigue levels. Average total sleep time between 12-hours shifts was only 5.5 hours, and night-shift nurses averaged only about 5.2 hours of sleep. The study involved 80 registered nurses who were working three successive 12-hour shifts.