###### Abstract  
Light that enters humans' eyes and impacts circadian rhythms may come from various sources, including the sun, electric lighting systems, and self-luminous displays. Occupants' activities strongly impact the light entering their eyes, which is difficult to predict and not yet well understood. This study investigated the circadian contributions of light from different sources in real building environments to better understand the variables that influence the circadian health of occupants. Spectral irradiance distributions at a position equivalent to the front of an eye of a seated occupant in various interior office spaces were collected. Daylight and electric light were measured separately, and light emitted from displays was measured when a variety of different computer tasks was performed. Circadian stimulus (_CS_) and \(\alpha\)-optic irradiance, defined by CIE DIS026/E:2018, were further calculated, and the circadian effects of light from different sources were compared. The results show that daylight has the greatest circadian effect, while electric light in spaces that were predominantly designed with conventional downward lighting has a very limited impact. The circadian effect of light from screens was considerably high. The outcomes suggest that, to optimise the circadian effects of light, connected lighting systems are needed to control light from different sources. c 2021 2021 2021 2021 2021 2021  
of light from different sources are limited. The lack of empirical data makes the design of human-centric lighting and the development of innovative products challenging. This research aims to understand the effect of light from different sources on human circadian rhythms in office settings, based on field measurements. The outcomes of this work may guide the development of innovative human-centric lighting systems. Daylight, which has a changing spectrum over the day, has a significant impact on circadian physiology and cognitive performance [7]. One study concluded that one half hour of daylight exposure could reduce sleepiness in the afternoon [8]. It is currently possible to predict the intensity and spectrum of daylight for a specific location in a building using computer simulations. However, data presented in multiple studies showed that occupants frequently use window coverings to block solar radiation in response to both visual and thermal comfort considerations [9; 10]. Those calculations and predictions rarely consider these types of interventions or occupants' activity patterns. Based on [PERSON]'s original work [11], [PERSON] developed an algorithm to predict the use of electric lighting based on observations of window blind usage patterns in actual office buildings [12]. The author acknowledged the limitations of this algorithm, which can only be applied to private and two-person offices [12]. The model only assumed that users kept the blinds either fully opened or closed, neglecting the fact that sometimes occupants partially lower window shades. Because [PERSON]'s research was focused on energy performance, only illuminance on working planes was studied, not the spectra of light [12]. A field study was conducted in four office buildings, in which the spectra and luminous flux of light sources varied [13]. As shown in a photo of the site, offices were illuminated by typical overhead fluorescent troffers. Most participants had little or no access to daylight. The intervention lighting provided by portable desk luminaires and/or newly installed overhead luminaires was added to the existing lighting to ensure that the circadian stimulus (\(CS\)) of light incident upon the occupants' eyes was greater than 0.3. When office workers were exposed to such intervention lighting for two days, their subjective vitality and alertness were increased, and their self-reported sleepiness was reduced [13]. A field study was conducted in a university dormitory in Beijing, China [14]. Wireless sensors were used in each bed to record the illuminance at the head position.