The present research project is a multidisciplinary and preventive study of environmental factors and, in particular, of strains associated with shift-work in general and shift-onset in particular on the workers’ health. Chronobiological studies of proposed models simulating daily variations of rhythms as the result of a circadian component (endogenous “body clock”), exogenous factors (hours awake), and their interaction (Folkard et al., 1999; Ackerstedt & Folkard, 1997). Hence, ergonomic studies based on the operators’ activity successfully simulated the circadian variations in shift-workers performance by including the factor of task requirements in a three-process model (Andorre-Gruet et al., 1999). This model was able to account for the unexpected quantitatively and qualitatively differences of operators’ activity and psychophysiological measures on shift-onset (1st hour of the shift) compared to that observed on the remaining time of the shift (Andorre & Quéinnec, 1996 ; Cariou, 2003). Elsewhere, shift-work organization is also known to interact with constraints of everyday familial and social life and, in addition, several health problems and decreased job satisfaction appear to be more frequently associated with shift-work than with work during the normal day (Marquié & Forêt, 1999; Folkard et al., 2005 ; Smith et al., 2001). More generally, the exposure to degraded physical and psychosocial working conditions affects operators’ health (Parkes, 2003), either due to high job demands associated with low resources (Karasek, 1979), or the perception of differences between the effort provided and the “reward” earned from the institution (Siegrist, 1996). To sum up, the consequences of shift-work organization on performance and health are largely documented, but from particular theoretical and practical points of view. It seems then pertinent to perform a multidisciplinary field study by combining the methodologies of ergonomics, social psychology, psychophysiology, cognitive psychology, in order to develop a model of “performance” at work on the 24-h day which may predict operators’ health and well-being. We intend to implement Andorre-Gruet et al’s model by exploring the effects of two additional dimensions, stress at work and work/family conflicts, on cognitive load on shift onset, and suggest that stress or tension perceived at work, may mediate the effects the remaining factors - task requirements, biological rhythms, hours worked, work/family conflicts - , and may thus predict physical and mental health.