Students hoping to pull all-nighters during Reading Week may want to avoid the Thain Family Café.

According to several reports from student employees and a document obtained by the News that appears to be part of the café’s internal records, some managers at the café have been substituting decaf espresso for caffeinated espresso at various times since the fall — including during the entire past two weeks — when the supply of caffeinated beans ran out. But café administrators emphatically denied the veracity of those reports.

Several student employees told the News the café has not served any caffeinated espresso since April 15 and that they were instructed by café managers not to inform customers of the shortage, which affects all espresso, latte and mocha drinks, though not regular coffee.

“It’s been several weeks that they’ve been out of espresso,” said Jacqueline Gosnell ’08, a café employee who recently quit because she disagreed with what she called café management’s decision to covertly swap the beans. “I wasn’t there for the shift where they ran out and started switching it, but I came for the shift afterwards.”

According to two students employed at the café, who wished to remain anonymous, the reason for this spring’s latest shortage of caffeinated beans resulted when café manager Brian Yezierski — who is employed by University Dining Services — ordered more decaffeinated espresso than could be sold before the end of the year. After the Thain Café ran out of caffeinated espresso April 15, Yezierski allegedly decided to substitute the decaf in its place.

In a phone interview with the News on Wednesday evening, Yezierski called the allegations “absurd” and declined to comment further.

An unsigned letter received by the News last week included a supposed photocopy of a Thain Café logbook entry from Feb. 29 that reads: “We will also run out of reg. espresso and French roast most likely — secretly use decaf espresso to substitute the espresso — for the French, I don’t know — I think we’ll just have to be out.”

Another former Thain worker, who spoke on the condition of anonymity, said she accidentally discovered the substitution while on duty last month.

“Someone had ordered a shot, specifically decaf, and I saw one of the regular workers put a shot of what I thought was regular caf into it,” said the former worker. “Later, she told me they were the same.”

When reached at home Wednesday evening, Yale University Dining Services spokesperson Karen Dougherty said she was “dumbfounded” at the charges leveled by the student employees, but she declined to comment further because she was not familiar with the situation.

A University employee serving as café manager — who said managers are forbidden to talk to the media under Dining Services policy — said the café may have run out of espresso “for a few hours during the day” but that “customers were always informed.”

But Gosnell, the manager’s former employee, called that assertion “completely false.” Gosnell said several of the comment cards left by customers this semester complained that the espresso the café was serving was “really weak.”

According to Roland Griffiths, a professor of behavioral biology and neuroscience at Johns Hopkins University and an expert on the study of caffeine, caffeinated and decaffeinated espresso would have markedly different effects on people who drank them.

“Caffeine is a drug, even though it’s served in beverages like coffee,” he said. “People should know how much they’re taking and control their dose accordingly.”

Although not everyone reacts to caffeine the same way, “caffeine withdrawal is a well-documented phenomenon,” Griffiths said. “Fifty percent will exhibit symptoms, and 13 percent will be functionally impaired. It is a terrible idea to put people into caffeine withdrawal without their knowing it.”

Students studying at the tables by the Thain Café on Wednesday evening had mixed reactions to reports about the supposed switch of espressos.

“I’m a decaf drinker, and I’m appalled!” Emma Sokoloff-Rubin ’11 said.

Another student, who spoke on the condition of anonymity, said she already knew about the swap, as a Thain Café worker had recently warned her not to order a mocha if she wanted to stay alert, while cautioning her not to repeat the information to anyone else.

“It’s a miracle some of my friends got their papers written,” she added.

But others said they had not noticed any difference in the beverages they were consuming.

“I would really not believe that from my physiological reaction,” Allison Kaptur ’09 said when told of the student employees’ allegations.