A pilot study exploring the impact of daylight gradient on occupant in underground space

The absence of daylight in underground spaces significantly affects the well-being. The negative impacts of fully static artificial lighting in deep underground spaces have been confirmed. However, the effects of the mixed lighting environment (natural light and artificial lighting) in shallow underground spaces remain unclear, specifically the impact of transitioning from 100 % static artificial lighting to 100 % natural light on individuals. In this study, a quasi-experimental approach using differences-in-differences method, was employed to assess the impact of varying daylight gradients on occupants' emotional states, cognitive evaluation, and task performance. Eight actual spaces across four underground sites were selected with varying proportions of daylight respectively (87 %, 12 %, 7 %, 0 %). It was found that, as the proportion of daylight decreases, task performance, emotional states, and cognitive evaluation are affected negatively. Comparing with task performance and emotional states, cognitive evaluation is affected in the condition of lower proportion of daylight. •Four daylight gradients (87 %, 12 %, 7 %, 0 %) were tested in underground spaces. •An on-site quasi-experiment was conducted using a Differences-in-Differences (DID) approach. •Occupants experienced negative effects as daylight proportions decreased. •Task performance and emotion deteriorate first and then cognitive evaluation, with decreasing daylight proportion.