While several countries impose minimum light requirements for pig housing, it remains unknown whether these requirements are beneficial for pig welfare. Therefore, we aim to review the current knowledge on the effects of light on pig welfare. In this paper, we explain concepts defining light, discuss the relevance of vision for pigs and systematically review the effects of light on pig welfare. Systematic literature searches were performed in two databases to find studies about light and welfare-related topics, including behaviour, health, hormonal secretions and productivity. After screening, 63 studies were reviewed. According to literature, light is relevant in pigs' lives as they are diurnal animals and use vision in combination with other senses to, for example, locate food and interact with conspecifics. Throughout this paper, the investigated light parameters are photoperiod, intensity and spectrum. Pigs seem to have p for a certain light intensity and spectrum, but these preferences vary over production phases. Photoperiod influences feed intake and growth, especially in piglets, but no conclusion can be drawn because of contradictory results. Furthermore, pigs' activity patterns adapt to the provided light schedule and show a diurnal rhythm with higher activity during lit hours. Photoperiod also plays a role in the diurnal secretion of hormones. Cortisol secretion increases shortly before the moment of light onset, and melatonin secretion is influenced by the light and dark contrast with a nocturnal rise after light offset. Some behaviours are impacted by light intensity; for instance, dim conditions are associated with resting and bright conditions with elimination behaviour. Moreover, a few studies showed that in dimmer conditions, more negative social interactions occur, while brighter conditions lead to more positive interactions. Lastly, even though light spectrum is the least explored light parameter, several studies showed that UV B light can activate the cutaneous synthesis of vitamin D. A limitation in the current literature is that several studies tested light treatments differing in more than one light parameter, making the interpretation of each light parameter difficult. Moreover, most studies do not provide information on other light parameters not targeted by the study, particularly on light spectrum. Some clear knowledge gaps that emerged from this review are on light spectrum and on affective states of pigs in relation to light. 3
