The usage of artificial electrical lightning has led to an increase in artificial light at night (ALAN), and now, ALAN covers more than 80% of the world’s inhabited areas [1], which has become a global concern and has recently been asserted as a source of environmental pollution by the American Medical Association. During the last decade, the consequences of ALAN in an ecological aspect, in particular, its effects on avian species, have received great interest [2]. ALAN exposure disrupts the clock-regulated daily patterns of physiology and behaviour and suppresses nocturnal melatonin production [3,4]. Like most organisms, birds have evolved to keep time with the 24 h environmental light–dark cycle, which is regulated by endogenous circadian clock(s). Daily timings and temporal niches are important for fitness and survival. ALAN exposures have been reported to be associated with serious ecological and health consequences, such as disruption of immune, metabolic, reproductive, and cognitive functions in both birds and mammals [5,6].

Presence of ALAN is a disturbance of the natural habitat, with bright intensity of light close to the light source and low intensity of light at greater distances at night-time, and evidence shows comparable effects of both no-night environment (24 h constant light, LL) and dLAN (dim light at night) in laboratory experiments. There is accumulating evidence that exposure to ALAN regulates and suppresses daily rhythm activity, melatonin, and sleep in birds. Circadian clock, melatonin, and sleep regulate avian cognitive performance. Many studies in wild birds have shown the adverse effects of ALAN on several territorial behaviours, including reproduction, singing, migration, and sleep [7,8,9,10,11]. However, field studies have suggested ALAN increases foraging opportunities in Domestic Pigeon (Columba livia domestica) by promoting nocturnal activity and feeding [12] and night foraging in Southern Lapwing (Vanellus chilensis) [13]. It is also reported that ALAN allows the Northern Mockingbird (Mimus polyglottos) to feed nestlings after dark [14]. Thus, ALAN changes the circadian phase of the feeding behaviour. Further ALAN increases foraging opportunities in nocturnal predatory birds, such as European Nightjars (Caprimulgus europaeus) and Burrowing Owl (Athene cunicularia), by increasing the abundance of the prey near the light source [15,16]. Thus, modifying the nest-site selection, this might increase vulnerability for the highly specialized birds [13]. Additionally, several laboratory studies have demonstrated the negative effects of ALAN (LL and dLAN) on learning, memory, mood, and exploration [17,18,19]. Although there is limited research on this topic, most of the insights on the adverse effects of ALAN on cognitive functions are from behavioural studies. Nevertheless, these results raise intriguing question about the molecular underpinning of the ALAN-induced negative consequences on brain functions. Despite clear evidence of potential consequences, how the environmental light signal is translated and induces long-term consequences remains unclear. In this review, I have first outlined the consequences of the ALAN circadian system and sleep, then mainly focused on the effects of ALAN on brain functions and known underlying mechanisms. Thereafter, I have highlighted the possible involvement of melatonin and epigenetic modifications in ALAN-induced responses. The overarching mechanism(s) are general and apply to many different species; however, here, we focused on bird species and discussed the importance of the avian system for better understanding of the subject.