Superman earliest comic book appearances always portrayed him as a dashing, heroic figure. Joe Shuster drew the character as always in command and never surprised. As a result, he eyes were always shadowed or squinting. Did Superman even have white eyeballs?
Around the time of Superman #40 (March 1946) I started noticing more illustrations of Superman with his eyes wide open, particularly on the covers. This trend humanizes the character and may be a result of more humorous stories such as this “villain” team-up of Mr. Mxyztplk and Susie Tompkins or it may just be a result of trying to challenge the character in new ways and not always show him as completely in control.
Here’s another glimpse of Superman Eyes Wide Open from World’s Finest #22 (also March 1946):