The erosion of the natural nighttime by the introduction of artificial lighting is globally widespread and increasing rapidly. There is now substantial evidence of the biological impacts that this change is having on a wide array of biological phenomena, from gene expression to ecosystem functioning, and from individual organisms to communities. Attention to date has focussed almost exclusively on impacts that arise from the downward emissions that are experienced by organisms in the vicinity of artificial light sources (such as streetlights). However, when uninterrupted, or reflected at shallow angles, artificial light can carry horizontally over long distances from sources at biologically readily detectable levels. In some instances this horizontal transmission is unintentional, in others it is not (e.g. vehicle headlights). At distance, it may manifest as a point emission through to more of a lit horizon. In this presentation we will consider the nature, extent and ecological implications of horizontal artificial light emissions. In particular, we will focus on how the challenges and effects of these emissions differ from those that are more commonly considered.