Obstructive sleep apnoea (OSA) is prevalent in 4% of middle-aged men and 2% of middle-aged women [1]. It is caused by the collapse of the pharynx during sleep, which leads to airway occlusion and hypoxia, and is reversed when pharyngeal muscle tone suddenly increases, usually in coincidence with an arousal [2–4]. Sleep apnoeas are repetitive, with severely affected patients having hundreds of respiratory events and arousals every night. Furthermore, sleep apnoeas are associated with sharp, short-lasting blood pressure augmentations at their termination, and with some reduction during their course [5–7]; as an effect, blood pressure during sleep in OSA is highly variable and has slightly higher mean values than during the preceding quiet wakefulness. This is well demonstrated by beat-to-beat measurements, and is different from that occurring in normal subjects, who decrease their blood pressure as they fall asleep [8]. In addition to nocturnal haemodynamic effects, according to epidemiological studies, OSA coexists with systemic hypertension in approximately 50% subjects, and this association is independent of obesity and other risk factors [9–12]. In addition, OSA may be a frequent cause of drug-resistant hypertension [13], and of a ‘non-dipping’ nocturnal blood pressure behaviour [14]. Studies investigating the potential pathophysiological mechanisms involved in hypertension development in OSA have proposed the involvement of increased sympathetic activity [15], baroreflex impairment [16], endothelial dysfunction [17–18], vascular inflammation [19,20], altered blood rheology [21] and early atherosclerosis development [22–24]. Nocturnal ventilatory treatment by continuous positive airway pressure (CPAP) is the most effectively established treatment for OSA. In this issue of the journal, Chin et al. [25] explore several aspects related to the effects of prolonged CPAP treatment on morning office blood pressure in patients affected by OSA. Although several studies have investigated the effects of OSA treatment on blood pressure, and most of them have shown that blood pressure may be lowered by CPAP treatment, there is still much to clarify. The time course of blood pressure improvement after CPAP treatment is initiated and the possible difference between CPAP effects on diurnal and nocturnal blood pressure are not clear. The time required for blood pressure to decrease may be different when an individual is asleep or awake. An immediate decrease in nocturnal blood pressure could be expected when CPAP is applied, as a result of elimination of apnoea-associated haemodynamic effects. The time course of blood pressure improvement with OSA treatment when awake is likely to be slower than the time course of blood pressure when asleep. Experimental studies in normal subjects showed that, after short-term exposure to repetitive voluntary apnoeas, sympathetic activity remains high for several minutes, and blood pressure is rapidly reversed to normal as the normal breathing pattern is resumed [26,27]. This suggests that an increase of diurnal blood pressure in patients chronically exposed to apnoeas is not due to a carry-over effect of repetitive apnoeas in the immediately preceding night. However, chronic reduction of nocturnal sympathetic activity by apnoea prevention may be followed by diurnal sympathetic tone reduction with a subsequent decrease in blood pressure. Sympathetic tone [28] and baroreflex gain [16,29,30] change slowly after treatment initiation, whereas diurnal sympathetic activity is not reduced by short-term apnoea treatment [28,31]; similarly baroreflex activity does not improve with acute CPAP application [30]. However, some acute contribution of CPAP in decreasing diurnal blood pressure could also be hypothesized, taking into account that an increased nitric oxide availability is evident after acute OSA treatment [32]. As regards nocturnal blood pressure in OSA, it should be noted that its high variability reduces the reliability of intermittent blood pressure monitoring, particularly when measurement intervals as long as 30 min are used [33]. Among the available studies investigating the effects of CPAP on blood pressure in OSA, only a few were based on beat-by-beat measurements [16,29,30,34–41]. Although the method of investigation may not be the most important source of differences among the results reported in the literature, it may comprise an important contribution to their disagreements. Studies concerning the effects of CPAP on diurnal and nocturnal blood pressure have produced different results. The effects of the duration of treatment on blood pressure were found to vary among studies. A few investigations studied the effects for the first few nights of CPAP application: most [34,37,42,43], but not all of them [36,44], observed a decrease in blood pressure. Most other studies investigated the effects of CPAP after weeks or months of treatment. A few studies demonstrated effects that were limited to nocturnal values [45,46], but how the nocturnal blood pressure decrease could be attributed to long-term treatment or to an immediately occurring CPAP effect is not clear. Other studies reported both diurnal and nocturnal blood pressure reduction [34,41,47–50], which could be in agreement with a progressive slow effect of treatment. A final group of studies, in addition to a nocturnal decrease, reported a reduction in blood pressure mainly in the early morning hours [40,51], in agreement with a possible carry-over effect of acute apnoea elimination. Only one study investigated both the acute (1–3 nights) and long-term (4–6 months) effects of CPAP, separately for nocturnal and diurnal values, and found a significant decrease in both of them at the first evaluation and, subsequently, a further but not statistically significant decrease [42]. Some clues about differences between acute and chronic effects of CPAP may be derived by studies concerning the effects of acute CPAP withdrawal after chronic treatment. In one of the few studies not demonstrating any reduction in blood pressure after CPAP, blood pressure was measured before beginning treatment and immediately after its withdrawal [52]. However, more recent studies have compared blood pressure during CPAP application after chronic treatment, and after 1 night [53] or 1 week [48] of CPAP withdrawal, finding only minor differences between the two nights. Moreover, blood pressure reactivity to apnoeas decreased after chronic CPAP treatment [39], whereas an increase in baroreflex gain occurring after chronic CPAP treatment could attenuate apnoea-induced blood pressure variations [16,29]; a similar baroreflex improvement was not observed during acute CPAP application [30]. Therefore, nocturnal blood pressure improvement after chronic CPAP may be caused not only by acute apnoea prevention, but also by the beneficial effects of prolonged treatment. The study by Chin et al. [25] does not clarify the lowest treatment duration associated with a blood pressure reduction because it did not investigate the very acute effects of CPAP. However, it deals with an aspect that has received very little attention to date, as pointed out in a recent meta-analysis [54], namely, the possible long-term persistence of blood pressure reduction with CPAP treatment. In particular, it found that office morning blood pressure recorded after a 1-month treatment is maintained at a constant level for up to approximately 3 years [25], which is a follow-up duration that had not been reached previously. In addition to the period of treatment, it is important to consider the effects of compliance to treatment. Use of CPAP is variable among patients, and the lowest compliance to treatment necessary to obtain some effect on blood pressure is unknown. In a previous study, a significant CPAP effect on blood pressure reduction was observed only in patients using CPAP more than 5 h per night [49]. According to Chin et al. [25], 3 h would be enough for a significant effect, whereas the effect of a lower compliance remains dubious because patients with low compliance were mostly normotensive and, as such, less prone to decreasing their blood pressure [25]. Another important point concerns why some patients undoubtedly decrease their blood pressure in response to CPAP, but others do not. Who are these patients? Apart from few studies demonstrating no differences related to pre-treatment blood pressure level [47,49], almost all studies, including the one by Chin et al. [25], agree that a blood pressure decrease is related to its baseline values and occurs only in hypertensives [41,42,55–57]. Besides, some of the different responses could be related to the severity of nocturnal respiratory disorders [49,51] or to genetic factors [58], but this remains almost totally unexplored. An interaction with simultaneous pharmacological antihypertensive treatment is controversial because almost all possible alternatives have been found: from a greater CPAP effect in patients receiving drugs [49], to a similar effect [57], to a lack of effects in patients who are treated pharmacologically [56]. In patients with drug-resistant hypertension, CPAP was found to decrease blood pressure [59]. Chin et al. [25] found that the effects of CPAP were evident on both systolic and diastolic pressure in non-pharmacologically treated patients, whereas the effects were only on the diastolic pressure in the other patients [25]. Thus, effects of CPAP on systolic and diastolic blood pressure could differ. In this regard, a controversy remains between those studies that found a CPAP effect for both systolic and diastolic pressure, and those that found an effect on only one of them. To better analyse this point, studies that evaluated both systolic and diastolic pressure separately for diurnal and nocturnal hours are required. Some of them found significant falls after CPAP treatment in both systolic and diastolic blood pressure [34,43], whereas, with only one exception [51], the other studies, including that by Chin et al. [25], suggest a prevalent decrease in the systolic pressure during the night, and in the diastolic pressure during the day [35,37,38,45,47,50,55]. Based on the above, apart from the almost common agreement that CPAP treatment may beneficially influence blood pressure in patients with OSA, the more specific details of the effects of CPAP are far from clear and remain highly controversial. The different designs and quality of studies may partly account for the different results reported. The study by Chin et al. [25] adds some data concerning several controversial aspects of the effects of CPAP and, to date, it is the only one to show that the effects of CPAP on blood pressure may persist for as long as 3 years [25]. Future studies should aim to explore more specific aspects of the effects of CPAP. Hopefully, this would allow us to better understand what can be expected from treatment, including identifying those patients who may benefit from treatment, and how treament must be applied to obtain the best results.