A study was conducted among 10 operators of 18 and 112 call center of Marseille city. These operators are firemen responsible for the management of calls. Management of calls includes taking calls, filling about a call via an interface in parallel with information acquisition on the situation, the diagnosis of the situation, the mobilization of appropriated resources (tanker, emergency vehicle, etc) or referral to another service. This call centre operates continuously, and operators must be constantly on site, being confronted thus shiftwork schedules.The aim of this study was to determine the influence of organisational factors and operators’ perception of work situation on alertness and tension levels. Indeed, factors such as time-of-day, accumulated fatigue, or work activity have an important effect on these variables. Thus, alertness level is determined by circadian and homeostatic components. Circadian component corresponds to biological rhythm characterised by an alertness increase throughout the day to reach a maximum at 18-19 hr, and an alertness decrease during the night with a minimum at 3-4 hr. Homeostatic component corresponds to accumulated fatigue during wakefulness. According to Thayer’s multi-dimensional theory, tension increase could compensate alertness decrease to maintain acceptable performance. Questionnaires were filled by operators every 4 hours during 24 hours. These questionnaires allowed estimating self-reported alertness and tension levels (Thayer’s adjective checklist), and perception of work situation (physic demand, psychological demand, work life/private life interferences, decisional latitude, professional social support, and resources available). In parallel, continuous observations were made when operators were on shift in order to obtain an estimation of work amount. Results reveal a modulation of work amount by time-of-day on physical and psychological demands (significant effect only at the morning) and show a time-of-day effect on alertness and tension with lower levels at night that during the day. Analyses suggest that this is not the task that is problematic (alertness decrease or tension increase) but the context in which it is performed. Task can be stimulating. Otherwise, different patterns appear depending on work activity characterising the period of work considered and suggesting that operators implement control strategies when work amount is high that would explain low effect of perception of work situation in these periods.