*Corresponding autor e-mail: javiertalamantes@gmail.com Introduction: Human performance depends on basic cognitive processes such as alertness. Alertness is defined as a cognitive activation that makes a person capable to produce general responses to the environment. This basic activation is necessary to support higher cognitive processes such as memory and attention. Alertness can be assessed through the readiness to respond to an alarm. The objective of this study was to analyze circadian variations in alertness. Methods: Participants were two female students, age: 17 years that attended school on a morning shift schedule. They were recorded in a constant routine protocol during 30 h; in this protocol feeding, environmental temperature, motor activity, recumbent position and room illumination were controlled. Rectal temperature was recorded each minute. Alertness and performance were recorded every 100 minutes, alertness was measured using an alarm task and performance was measured using a continuous performance task (CPT). Both tasks were recorded concurrently during 12 minutes. Participants used the dominant hand to respond to the CPT and the non dominant hand to respond to alarm task. Alarm task required to hold a key pressed during 12 minutes, if the key was released an auditive alarm appeared indicating the participant to press the key again. There were circadian variations in rectal temperature with a highest level during the afternoon and a lowest level during the early morning. Results: Performance on the alarm task showed circadian variations. During daytime the key was hold press all time (no releases), the highest frequency of key releases occurred during nighttime and early morning. Reaction time to the alarm was classified in: short (RT less than 900 ms) and large (RT more than 1 sec) responses. During night, short RT was more frequent and indicated that participants were capable to perceive the alarm and respond fast to it, while large RT were less frequent and indicated an impairment in the capability to react to the alarm. These data suggest that during the nighttime, an auditory stimulus is not capable to activate the alerting system. Large RT can be related to microsleeps. Conclusion: The results of this study may affect activities such as driving a car, as well as on many working situations during nighttime and early morning. 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
Talamantes et al. (Thu,) studied this question.