Light, entrainment and alertness: A case study in offices MG Figueiro PhD a , B Steverson MA b , J Heerwagen PhD b , R Yucel PhD c , C Roohan BS a , L Sahin PhD a , K Kampschroer MS b and MS Rea PhD a a Lighting Research Center, Rensselaer Polytechnic Institute, Troy, NY, USA b Office of Federal High-Performance Buildings, General Services Administration, Washington DC, USA c Department of Epidemiology and Biostatistics, School of Public Health, University at Albany–State University of New York, Albany, NY, USA Received 9 August 2019; Revised 16 September 2019; Accepted 8 October 2019 It is well established that light affects both visual and non-visual systems. Laboratory studies have shown that, depending upon the time of exposure, short- wavelength light of sufficient amount and exposure duration will either entrain or disrupt the synchrony between our biological clock and our local position on Earth. Laboratory studies have also shown that light across the entire visible spectrum can enhance alertness, both day and night. Scant attention has been given to testing the effects of light on building occupants’ non-visual responses, and, consequently, lighting specifiers have been offered little guidance on the design and application of lighting for non-visual effects. The present study helps to fill that gap through field-testing of light exposures from a novel luminaire designed to promote entrainment and alertness throughout the day in actual office environ- ments. The data support the inference that light exposures, when properly applied, can promote circadian entrainment and increase alertness. 1. Introduction Exposure to a robust 24-h light–dark cycle entrains people to their local position on Earth. Laboratory studies have shown that short-wavelength (blue) light of sufficient amount and exposure duration can entrain or disrupt, depending upon the time of exposure, the timing of the biological clock that controls our 24-h behavioural and physiological cycles. The Lighting Research Center (LRC) at Rensselaer Polytechnic Institute has developed two metrics, circadian light (CL A ) and circadian stimulus (CS), 1–3 to characterize light as a stimulus to the bio- logical clock. CL A and CS are based upon the effectiveness of a 1-h night-time light expos- ure for suppressing the hormone melatonin, a well-established marker of circadian phase. 4 CL A is irradiance weighted by the spectral sensitivity of the retinal phototransduction mechanisms stimulating the biological clock, and CS is a transformation of CL A into a relative scale from response threshold (CS50.1) to response saturation (CS ¼ 0.7). Light can also elicit an acute alerting response from humans 5,6 at any time of day. 7 The lighting characteristics that promote alert- ness have been shown to be different from those promoting entrainment. A series of laboratory studies 8,9 have shown that exposure to long- wavelength (red) light can promote alertness during the mid-afternoon decline in perform- ance known as the post-lunch dip. These findings are important for daytime applications in office environments because, unlike blue Address for correspondence: MG Figueiro, Lighting Research Center, 21 Union Street, Troy, NY, USA. E-mail: figuem@rpi.edu Lighting Res. Technol. 2019; 0: 1–15 ß The Chartered Institution of Building Services Engineers 2019 10.1177/1477153519885157