Circadian and homeostatic variation in sustained attention.

Human performance is modulated by circadian rhythms and homeostatic changes. Changes in efficiency in the performance of many tasks might be produced by variation in a basic cognitive process, such as sustained attention. This cognitive process is the capacity to respond efficiently to the environment during prolonged periods (from minutes to hours). There are three indices of sustained attention: general stability of efficiency, time on task stability, and short-term stability. The objective of this work was to analyze circadian and homeostatic influences on the indices of sustained attention. Participants were nine undergraduate female student volunteers (mean age 17.67 yrs, SD = 1.00, range 16-19 yrs) who attended school from 07:00-13:30 h, Monday to Friday. They were assessed while adhering to a modified 28 h constant-routine protocol during which feeding, room temperature, motor activity, and room illumination were controlled. Rectal temperature was recorded each minute, and indices of sustained attention were assessed hourly through a continuous performance task (CPT). General stability was measured as standard deviation of correct responses and reaction time, time on task stability was measured as the linear regression of correct responses and reaction time throughout the task, and short-term stability was measured as hit runs and error runs. Rectal temperature showed circadian variation; subjective somnolence and tiredness increased, while general performance and all indices of sustained attention declined throughout the 28 h recording session. General stability exhibited circadian variation, whereas time on task did not. Short-term stability showed circadian variations in short-error runs, long-error runs, and short-hit runs, but long-hit runs did not. There was a 26 sec short interval at the beginning of the task, characterized by a very high efficiency level of performance. Execution during this safe period was not affected by time awake and did not show circadian variation. Overall, these results suggest a dissociation of the effects of circadian and homeostatic factors on the indices of sustained attention. General stability, short- and long-error runs, and short-hit runs were modulated by both a homeostatic factor and a circadian effect, while long-hit runs and time on task stability were modulated only by a homeostatic factor and did not show circadian variation. There was also a 26 sec "safe period" that seems to be independent of circadian and homeostatic influences. These results suggest that performance at work deteriorates at the end of a shift due to a decrease in general stability, an increase in error runs, decrease of long-hit runs, and decline with time on task. Night shiftworkers are exposed to an additional deterioration of performance during the nighttime due to a decrease in general stability and an increase in error runs.