The use of arbitrarily predefined time intervals during the circadian cycle is common practice in 24-h studies of gastric acidity. To determine whether slight displacements of a given time interval can be responsible for relevant changes in the acidity indexes calculated from pH profiles, we analyzed pH frequency distributions, and individual median and mean pH levels in three different, commonly adopted nocturnal time windows related to 132 curves obtained with single bedtime (10 PM) doses of various H2-antagonists. The time period 12 PM-8 AM has significantly less acid values (p less than 0.001) than the other two (10 PM-6 AM and 8 PM-8 AM), and therefore identifies the drugs' actions more closely. The individual median pH levels describe the H2-antagonist effect in 93% of profiles in the former time segment, whereas the percentage of accuracy is significantly lower (p less than 0.01) in both remaining periods (73% and 67%, respectively). The corresponding percentages of the individual mean pH levels are similar, even though their decline is less evident in the 10 PM-6 AM and 8 PM-8 AM time intervals. Our findings suggest that 1) slight shiftings of arbitrarily predefined time segments are associated with relevant fluctuations of acidity indexes and, therefore, should be standardized whenever possible, in order to favor international comparisons of results; 2) time intervals should be set to fit in as closely as possible with the drug-induced alkalinization wave; 3) pH frequency distribution should always be calculated and taken into careful consideration.