Extra-pair paternity occurs frequently in socially monogamous birds, but there is substantial variation in extra-pair siring success among males. Several studies have shown that siring success relates to the timing of morning activity, with the earliest active males being more successful, which suggests that early activity is important for acquiring extra-pair copulations. However, these studies are correlational, and it therefore remains unclear whether the relationship between timing and extra-pair siring success is causal. An alternative explanation is that successful extra-pair sires tend to be active earlier (e.g. because they are of high quality or in good condition), but that early activity in itself does not increase siring success. We experimentally advanced the emergence time of male blue tits by exposing them to light about half an hour before their natural emergence time. Although males that were exposed to the light treatment emerged from their roost substantially earlier than males that were exposed to a control treatment, light-treated males were not more likely to sire extra-pair offspring. Furthermore, whereas control males showed the expected relation between emergence time and siring success (although not statistically significant), there was no relation between emergence time and extra-pair siring success among light-treated males. Our results challenge the hypothesis that the timing of morning activity per se is an important factor underlying extra-pair siring success.