The paper shows the possibility of predicting performance decline and rise in medical staff during their work shift hours. Theoretical basis of the study is briefly presented. The research was done on 189 medical staff members in Perm. A self-report Diurnal Locus Scale was developed, exhibiting acceptable internal consistency, as well as discriminant and predictive validity. Diurnal locus mediated the effect of internality at work upon diurnal decline in medical staff performance. Limitations of the present study and research perspectives are discussed.