Retrospectively rated alertness during each shift for operators reporting good (open circle) and poor (filled circle) perception of adaptation to shift work . Higher scores indicate greater alertness: 7 = alert, 5 = neither alert nor sleepy, 3 = sleepy (but not fighting sleep). Error bars represent standard errors. *P < 0.05 (adjusted for age). A: afternoon shift; D: day shift; N: night shift.