Trivia: During filming, director Jeannot Szwarc had communication problems with both Christopher Reeve and Christopher Plummer. Every time he would say Chris, both Reeves and Plummer would respond. To put an end to it, he started addressing Christopher Plummer as "Mr. Plummer" and Christopher Reeve as "Bigfoot."
Trivia: When Richard Collier leaves the men's room, bloodied from his first encounter with a straight razor, the bearded man who stares at him in the hall and declares, "Astonishing!" is screenwriter Richard Matheson in a cameo role. His screen credit at the end reads "Astonished Man." (01:01:55)
Trivia: Richard Matheson wrote this film's script based upon his novel, "Bid Time Return." The book was set in 1896 at the Hotel Del Coronado in San Diego CA. But because the "Del" couldn't close to accommodate filming, the movie was updated to 1912 and shot at Mackinac Island's Grand Hotel, which is regularly closed to guests for a few months every year.
Answer: He was no longer able to put everything out of his mind, which was a requirement to successfully time-travel. He was distraught and unable to focus enough mentally. He stopped eating, and as time went on he became weaker and weaker.