Viewing of High Dynamic Range (HDR) video on capable displays poses many questions to our understanding of perceptual preference and vision science. One of the most fundamental aspects is the role played by light adaptation, as HDR content and displays allow for substantially increased light adaptation changes. In contrast, the traditional formats of standard dynamic range (SDR), being at best 3log10, kept the luminance ranges well within the steady state ranges of photoreceptor responses [14]. HDR video systems exceed the 3log10 luminance range, can be as high as 5log10 for professional displays, and be over a 6log10 range for laboratory research displays. In addition to the well-understood photoreceptor component of light adaptation is the pupillary component. While its light modulation is much smaller in range than the photoreceptor's adaptation range, it nevertheless has engineering consequences, and has been cited as a cause of putative discomfort with some HDR viewing. To better understand its role in light adaptation and discomfort, this study measured pupil behavior during naturalistic viewing of HDR video on a professional display, and performed various analyses.