“I’m a light eater; when it gets light, I start eating” is a joke that has been attributed to baseball pitcher Tommy John, baseball coach Tommy Lasorda and football defensive tackle Art Donovan. A newspaper column titled “The Chuckle Box” contained the joke in February 1964—much earlier than any attribution (in print from the 1980s) to John, Lasorda or Donovan.
   
Comedienne Totie Fields (1930-1978) often told jokes about her weight. “I’m a light eater. Everytime it gets light out, I eat,” Fields was quoted in a syndicated newspaper column in March 1964. It’s probable that Fields either originated or popularized the joke.
       
“I’m on a light diet. I eat by daylight, I eat by moonlight, and sometimes, I eat by refrigerator light” is a related joke.
   
     
Wikipedia: Totie Fields
Totie Fields (May 7, 1930 – August 2, 1978) was an American comedienne.
 
22 February 1964, Capital Times (Madison, WI), pg. 7, col. 6:
FUN TIME
(The Chuckle Box)
Woman (talking to friend): I’m a very light eater.
Husband: Yes, as soon as it’s light she starts eating.
   
19 March 1964, Morning Advocate (Baton Rouge, LA), “TV Time: Totie Fields Wants to Be Second Banana on Video” by Harold Stern (Bell-McClure Syndicate),  pg. 6C, col. 2:
“I’m a light eater. Everytime it gets light out, I eat ... etc.”
     
Sports Illustrated
July 14, 1980
They Said It
Edited by Jerry Kirshenbaum
Art Donovan, former 310-pound Baltimore Colt defensive lineman, describing himself as a light eater: “As soon as it’s light, I start to eat.”
 
Google Books
It’s easier for a rich man to enter heaven than for a poor man to remain on earth
By Joseph L. Felix
Nashville, TN: T. Nelson
1981
Pg. ?:
Personally, I’m a light eater; as soon as it gets light I start eating.
   
16 March 1985, Sun Sentinel (Fort Lauderdale, FL), “Lasorda’s good—but at what?” by Mitch Albom, Sports, pg. 1C:
Listen to Tommy Lasorda—the one-liners spitting like gunfire—and you picture him on stage somewhere in the Catskills, patting his stomach and quipping, “I’m a light eater. I start eating when I see sunlight.”
     
Google Books
Baseball—a laughing matter!
By Warner Fusselle, Rick Wolff and Brian Zevnik
St. Louis, MO: Sporting News Pub. Co.
1987
Pg. 19:   
Pitcher Tommy John denied that he’s a heavy eater. “I’m a light eater,” John said. “When it gets light, I start eating.”
 
Google News Archive
24 January 1987, The Afro American, pg. 9, col. 3:
Notable Quotables
Former Baltimore Colts all-pro defensive tackle who played at 310 pounds: “Contrary to what everyone says, I’m a light eater; as soon as it’s light I start eating.”
 
13 October 1988, USA Today, “Lasorda in top form before Game 7” by Tom Weir, Sports, pg. 3C:
“I’ve become a very light eater. I start eating when there’s daylight.’‘