The pressure to succeed can take its toll on some interns, who have only about eight weeks of summer holidays to prove themselves to potential employers.
Some dare not leave the office before their superiors.
"There's the sense of face time and, even if you don't have any urgent work, you are required to stay late. The culture feeds itself," said a former intern at Merrill Lynch who secured a job, but quit after a year with work-related repetitive strain injury.
A former investment-banking intern at Goldman Sachs Group, who declined to be identified, said he worked an average of 18 hours a day, seven days a week, without a single day off last summer.
He remembered one particular week in which he pulled three consecutive all-nighters and left the office only to change clothes, shower and sleep once - for two hours - in his own bed.
He said: "I was basically taking power naps at my desk and in bathroom stalls (the rest of the time)."