The impact of study-environment on experimental outcome is mostly not realized and certainly not demonstrated. In the present study, a comparison was made between free salivary cortisol levels in healthy young men in a carefully controlled hospital setting versus a home setting. Cortisol levels during rest were increased in hospital compared to home environment: 2-fold at awakening, 3-fold at the morning peak, and 5-fold late in the evening. Early morning light increased cortisol concentrations only in the home setting, while this effect was absent in the hospital setting. The data of the present study show that study-environment has a major impact on basal hypothalamo-pituitary-adrenal (HPA)-axis activity, which is of particular relevance in future studies in which small changes in HPA-axis activity are subject of study.