Twenty-four male subjects performed a visual vigilance task under various temperature conditions (70°, 82° and 90°F, Effective Temperature, -°FET). They determined for themselves when they desired a rest from monitoring. Those subjects selecting ‘ long ’ sessions had both poorer performance scores and less accurate time estimates than their peers. The subjects in general did a poor job of relating their own thermal discomfort, drowsiness, boredom, estimates of job performance and preference for the task to their actual performance score and/or time on duty. No significant temperature effects were noted.