Our pupils change size in response to light levels in our environment, but it seems that just thinking about something bright or dark can have the same effect
Source:
https://www.newscientist.com/article/2221634-just-thinking-about-bright-objects-changes-the-size-of-your-pupils/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home