Anthropogenic noise and light pollution additively affect sleep behaviour in free-living birds in sex- and season-dependent fashions.

Rapid anthropogenic transformation of environments exposes organisms to diverse disturbance factors, including anthropogenic noise pollution and artificial light at night (ALAN). These sensory pollutants interfere with acquisition of, and response to, environmental cues and can be perceived as stressors. Noise pollution and ALAN are often experienced simultaneously, and are thus likely to jointly affect organisms, either additively or interactively. Yet, combined effects of noise pollution and ALAN remain poorly elucidated. We studied combined effects of noise pollution and ALAN on the sleep behaviour of a free-living songbird, the great tit (Parus major). Sleep is widely conserved across animal taxa and fulfils essential functions, and research has demonstrated independent effects of both noise and ALAN on sleep. We measured noise and light levels at nest boxes and used infrared video-recording to assess sleep behaviour. Results did not support interactive effects of noise and ALAN. However, noise pollution and ALAN were both independently related to variation in sleep behaviour, in sex- and season-dependent fashions. Males, but not females, woke up and left the nest box ∼20 min later in the noisiest as compared to quietest environments (range: 44.2-79.4 dB), perhaps because males are more sensitive to acoustical cues that are masked by noise. Furthermore, as the season progressed from November to early March, birds woke up and left the nest box ∼35 min earlier relative to sunrise on territories with the lowest, but not the highest, light levels (range: 0.01-8.5 lux). Thus, the seasonal difference in sleep duration was dampened on light polluted territories. These effects could arise if ALAN interferes with birds' ability to sense and respond to increasing daylength, and could have fitness ramifications. Our study suggests that noise pollution and ALAN exert additive effects on sleep behaviour, and that these effects can be sex- and season-dependent.