Corresponding autor e-mail: valdez.pa@gmail.com Human performance is modulated by circadian rhythms. Efficiency in many tasks increases during daytime and decreases at nighttime and first hours in the morning. It is possible that the observed changes in performance are produced by circadian variations 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 variations in the indices of sustained attention. Participants were 9 undergraduate female students, (mean age 18.0 yr, SD=1.31, range 16-21 yr), that attended school from 07:00-13:30 h, Monday to Friday. They were recorded in a constant routine protocol during 28 h; in this protocol feeding, environmental temperature, motor activity and room illumination were controlled. Rectal temperature was recorded each minute, whereas indices of sustained attention were assessed each hour through a continuous performance task (CPT). This task required them to press 1 to any number (except “9”) appearing at the center of the computer screen, to press 2 when a “9” appeared, and to press 3 when a “4” appeared after the “9”. The CPT had 27 blocks with 20 stimuli each. Three indices of sustained attention were obtained: general stability (standard deviation of correct responses and reaction time within the blocks), time on task stability (linear regression of correct responses and reaction time throughout the blocks) and short-term stability (hit runs and error runs). Rectal temperature showed circadian variations. Subjective sleepiness and tiredness increased progressively during the recording session, but tiredness did not show circadian variations. All indices of sustained attention declined through the 28 h recording period. But only two indices showed circadian variations: general stability and short-term stability. Time on task stability declined through the recording period (with time awake), but did not show circadian variations. In conclusion, the indices of sustained attention are crucial for circadian and homeostatic variations. Circadian variations in general stability and short-term stability can modulate variations in the execution of many tasks. Whereas a decline in time on task stability can modulate the execution of many task with time awake. Publication History Article published online: 16 June 2026 © 2009. Brazilian Sleep Academy. This is an open access article published by Thieme under the terms of the Creative Commons Attribution-NonDerivative-NonCommercial License, permitting copying and reproduction so long as the original work is given appropriate credit. Contents may not be used for commercial purposes, or adapted, remixed, transformed or built upon. (https://creativecommons.org/licenses/by-nc-nd/4.0/) Thieme Revinter Publicações Ltda. Rua Rego Freitas, 175, loja 1, República, São Paulo, SP, CEP 01220-010, Brazil
P. Valdez (2009) studied this question.