Objective To assess the relationships of daytime and night-time blood pressures and the day-night pressure differences, obtained by various analytical methods, with the actual awake and asleep pressures and the awakeasleep pressure difference. Methods Ambulatory blood pressure was successfully monitored by use of the SpaceLabs 90202 device in 91 healthy young men during a weekend, when they went to bed and awoke at variable and often unusual times. The actual 'awake' and 'asleep' blood pressures were calculated on the basis of these times, noted by the subjects. The 24 h recordings were further analysed by use of two clock time-independent methods (square-wave fitting; cumulative sum analysis) and by one 'wide' (A) and one 'narrow' (B) fixed-time method (daytime: 0700-2200 h or 1000-2000h; night-time: 2200-0700h or 0000-0600 h, for methods A and B, respectively). Results In the total study population, square-wave fitting (+1.1±1.5 mmHg, mean±SD) and cumulative sum analysis (2.4 ±1.9 mmHg) overestimated the actual awake systolic blood pressure and underestimated the asleep pressure (-0.7±1.7 and -1.8±1.8mmHg, respectively). The fixed-time techniques underestimated the awake pressure (-2.6±3.1 and -0.2±3.2mmHg for methods A and B, respectively) and overestimated the asleep pressure by +4.9 ±5.5 and +2.7 ±6.1 mmHg, respectively. The actual awake-asleep pressure difference was overestimated by square-wave fitting (+1.8 ±1.9 mmHg) and more so by cumulative sum analysis (+4.2±2.5mmHg); the underestimation by the fixed time approach averaged -3.0 ±8.4 mmHg with method B and amounted to -7.5 ±8.3 mmHg with method A. Overall, the SD of the various differences between the estimated and the actual awake and asleep pressures, and consequently the limits of agreement, were larger for the fixed-time methods than for the clock time-independent techniques. In the 47 subjects who went to bed before 2400 h and awoke between 0600 and 1000h, the results of the clock time-independent methods were similar to the results in the total study population, whereas the deviations of the fixed time pressures from the actual awake and asleep blood pressures were considerably reduced. The results were similar for diastolic blood pressure. Conclusions Clock time-independent methods, particularly square-wave fitting, can predict the actual awake and asleep blood pressures and the awake-asleep pressure differences with reasonable accuracy and the results are independent of the awake-asleep pattern of the subjects. However, fixed-time methods are only reliable when the subjects go to bed and arise within well-defined periods, and yield more accurate results when the morning and evening phases are excluded from the daytime and nighttime periods