Blood pressure (BP) during siesta declines to levels similar to those of night time sleep. The objective of the study was to assess the effect of siesta on 24-h ambulatory BP (ABP) data. Two different approaches were employed for the definition of day and night periods: (1) actual patient reported day and night intervals (ACT) with siesta period analysed as a third time period; and (2) arbitrary day and night time intervals (ARB) with the presence of siesta being ignored. A total of 203 24-h ABP recordings were analysed, with a siesta during ABP monitoring reported in 154 of them. Mean siesta BP was very close to ACT night time BP. Among recordings with a siesta, ACT daytime BP was higher and night time BP lower than the corresponding ARB BPs (P < 0.001). The magnitude of night time BP drop was greater with ACT intervals, resulting in a lower percentage of non-dippers (P < 0.001). Among 49 recordings without a siesta, differences between ACT and ARB BPs were less pronounced for daytime but not for night time. Differences in the magnitude of nocturnal BP drop between ACT and ARB periods, although statistically significant, did not affect the prevalence of non-dippers. In conclusion, analysis of 24-h BP profiles by using ARB instead of ACT day and night intervals results in underestimation of the nocturnal BP drop and overestimation of the proportion of non-dippers. This bias is more pronounced in patients who take a siesta during ABP monitoring.