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Objective Central apnoea due to immaturity of the respiratory drive constitutes the main cause of frequent and prolonged desaturations in extremely preterm (EPT) infants <28 weeks. We investigated the impact of varying the duration of apnoea before backup ventilation (BUV) on the measures of oxygenation in EPT infants during nasal continuous positive airway pressure (nCPAP) therapy.Design Single-centre randomised cross-over trial.Setting Level 3 neonatal intensive care unit.Patients 24 EPT infants on nCPAP with BUV.Main outcome measures The primary outcome was the time spent within a predefined oxygen saturation (SpO 2) target (88%–95% or ≥88% with fraction of inspired oxygen (FiO2) =0.21) during start of BUV after 4 s of apnoea duration (AD 4) or 16 s of apnoea duration (AD 16)Results The study was successfully completed in 22 children (median gestational age 24+5 weeks, birth weight 628 g, postnatal age 48 days). Mean time spent within the SpO 2 target didn’t differ between AD 4 and AD 16 (66.9% vs 67.2%, p=0.88). There were no differences in the time below or above the SpO2 target, prolonged (>30 s, >60 s, >120 s) and severe (<80%, <70%) episodes of hypoxaemias and cerebral tissue oxygenation. Mean FiO2, mean airway pressure, transcutaneous carbon dioxide pressure, heart rate and respiratory frequency did not differ while the rate of BUV was significantly higher during AD 4.Conclusion Reducing the time of apnoea until start of BUV didn’t improve the time spent within the SpO 2 target in respiratory unstable EPT infants. Our data demand intensified efforts to specify these settings of non-invasive respiratory support that better achieve this important clinical goal.Trial registration number DRKS00031911.