ABSTRACT Aim of the present study is to assess whether 1.5mg of exogenous melatonin provided under modified CR in constant light (~400 lx) is capable to mimic effects of dark phase. Forty-six young adults (YA), 17–24 years old of both genders were studied under amodified CR protocol for 26 h. Initially, participants were investigated under constant light (CR-LL) and 2 weeks later under the same conditions though 1.5mg melatonin (Melaxen) was given orally at 22:30. Systolic blood pressure (SBP) and diastolic blood pressure (DBP), heart rate (HR) and body temperature (BT) were measured every 2 h. To verify the effect of constant light, formerly published results obtained under light-dark conditions (CR-LD) were reanalyzed. Administration of 1.5 mg of exogenous melatonin modified the 24 h patterns of BT and SBP within short 3.5 h time window but did not influence DBP and HR. A short-term reduction of SBP and BT for 1.5–3.5 hours was observed. The values in the CR-LL+M group were significantly lower than in CR-LL at 2:00 h. Hence, exogenous melatonin did mimic the scotophase. Though this effect was gender-specific and found only in female YA. Results of this study prompt further research to qualify and quantify dosage-, duration- and time-dependent differences of melatonin effects, to discern between short-term (acute) and long-term (chronic) melatonin administration, and to clarify its underlying mechanisms.