Background: Substantial evidence now indicates that exposure to visible light at night can be linked to a wide spectrum of disorders ranging from obesity to cancer. More specifically, it has been shown that exposure to short wavelengths in the blue region at night is associated with adverse health effects such as sleep problems. Objectives: This study aimed at investigating if exposure to blue light emitted from common smartphones in an environment with dim light at night alters human reaction time. Methods: Visual reaction time (VRT) of 267 male and female university students were recorded using a simple blind computerassisted test. Volunteer university students, who provided their informed consent were randomly divided into two groups of control (N = 126 students) and intervention (N = 141 students). All participants were asked to go to bed at 23:00. Participants in the intervention group were asked to use their smartphones from 23:00 to 24:00 (watching a natural life documentary movie for 60 minutes), while the control group only stayed in bed under low lighting condition, i.e. dim light. Just before starting the experiment and after 60 minutes of smartphone use, reaction time was recorded in both groups. Results: The mean reaction times in the intervention and the control groups before the experiment (23:00) did not show a statistically difference (P = 0.449). The reaction time in the intervention group significantly increased from 412.64 ± 105.60 msec at 23:00 to 441.66 ± 125.78 msec at 24:00 (P = 0.0368) while in the control group, there was no statistically significant difference between the mean reaction times at 23:00 and 24:00. Conclusions: To the best of the authors' knowledge, this is the first study, which showed that exposure to blue-rich visible light emitted from widely used smartphones increases visual reaction time, which would eventually result in a delay in human responses to different hazards. These findings indicate that people, such as night shift or on call workers, who need to react to stresses rapidly should avoid using their smartphones in a dim light at night.