Ambulatory 24-hour BP monitoring (ABPM) is considered gold standard for evaluation of hypertension, an important risk factor for cardiovascular disease and death. While it gives many parameters of useful information, it is important to minimize data inaccuracies by defining daytime & night-time periods correctly. Aim & Objectives: To determine whether defining diurnal periods by actual self-reported Sleep-Time versus a preset fixed-time produce different estimates of night-time ambulatory blood pressure, in postgraduate medical college student population of our Institute. Methods & Materials: In this observational study, medical students doing their postgraduate course in our Institute were chosen for evaluation of their BP with a 24- hour ambulatory BP monitoring. When returning the monitor, participants reported actual sleep/wake times and this was compared with preset Night-Time (22:00-07:00 hours) for interpretation of the data. Results: The study included 34 young PG doctors (18 males and 16 females), mean age 27.6 (range 23-32) years. The preset Night-Time SBP was 115 ±13 mmHg versus Sleep-Time SBP 111 ±13 mmHg. The mean difference was 4.5±4, (p < 0.0001 statistically significant). The preset Night-Time DBP was 71±10 mmHg versus Sleep-Time DBP 65±9 mmHg. The mean difference was 5±6 (p< 0.0001). Conclusions: When evaluating hypertension ABPM, it is better to diligently use self-reported actual sleep period rather than preset Night-Time for accurate estimate of nocturnal blood pressure, so that potentially hazardous overtreatment can be prevented