Objective: To study whether two widely used methods of analysis of ambulatory blood pressure monitoring (ABPM), which calculate means for fixed periods, provide accurate estimates of blood pressure during the true waking and sleeping periods. Design and methods: Ninety-five ABPM recordings were retrospectively analysed. Mean systolic and diastolic blood pressures during waking and sleeping were calculated in three ways: the waking and sleeping period according to the diary of the patient (diary method); the waking period from 0700 to 2200 h and the sleeping period from 2200 to 0700 h (9h); and the waking period from 1000 to 2300 h and the sleeping period from 0100 to 0700 h (6 h). Results: Systolic and diastolic blood pressures during the waking period were not different among the three methods of analysis. Compared with the diary method, the 9h sleeping period method overestimated systolic blood pressure during sleep by 3.74 mmHg [99% confidence interval (Cl) 2.70-4.77] and diastolic blood pressure by 2.44 mmHg (99% Cl 1.75-3.14). In contrast, there was no significant difference between the diary method and the 6h sleeping period method. Conclusions: The 6h sleeping period method gave accurate estimates of blood pressure during sleep. The use of the long (9 h) sleeping period method should be avoided in studies in which sleeping-waking differences will be part of the analysis.