They work up to 140 hours a week, day and night, often in hectic 36-hour shifts with little or no sleep. Their patients' lives sometimes depend on their being able to pinch themselves awake enough to make critical, complex decisions.

They are the nation's doctors in training, living through the year-long initiation rite called internship.

Each July, in hospitals here and across the country, the fresh interns arrive to make their first life-or-death decisions as doctors.

Intership is a requirement, but it is more than that. It is part of the price of admission to the medical fraternity. Senior physicians say it hones character, builds endurance and tests dedication.

But there is evidence that the exhaustion of intership can cause fatal medical mistakes, produce at least temporary psychiatric problems for the interns themselves and drive young physicians out of the front-line medical jobs the nation needs into specialties with more humane hours.

Despite a variety of educational and financial justifications offered for the system over the years, the medical internship system is coming under increasing criticism, and some physicians are saying it makes no sense to make people work when they are too tired to function effectively in a crisis.

Experienced physicians argue that the system, which originated a century ago as the result of an architectural quirk at Johns Hopkins Hospital in Baltimore, is ideal for training new doctors and staffing hospitals. But many of the same physicians tell their families and friends, "Don't get sick in July," when the interns are raw and even more exhausted than usual.

Dr. James Ramey, who teaches at George Washington University Medical School, says interns just do not work as well after a night without sleep.

He compared a sleep-deprived intern walking into an intensive-care unit to "an airline pilot who's been working 36 hours, coming in for a landing in his DC10." Confronted with a series of important medical decisions that have to be made at once, Ramey said, exhausted interns are unlikely to call for help.

"If it's a snap decision, they make it," he said. "They can't recognize that they can't make it well."

Ramey said he always double-checks his hospitalized patients' test results because the interns who care for them tend to forget important details after being up all night.

Internship is the first and toughest year of residency, when medical school graduates work in hospitals under the supervision of established physicians before going out on their own. Residency often lasts three to five years, depending on the specialty, but doctors can be licensed to practice independently after completing their interships.

The typical intern works every weekday and spends every third night on duty in the hospital, admitting new patients and handling emergencies. This means the intern works a 36-hour shift at least twice a week, and although he or she may get some sleep during that period, it is common in busy hospitals or on bad nights to grab two hours of sleep or less.

"At four or five o'clock in the morning you ache," said Dr. James McAnally, the chief resident at Georgetown University Hospital, as he recalled his internship. "You're hungry, and there's no place to eat anything.You're irritated, and you want sleep."

None of the many interns and residents interviewed said they could recall making serious medical errors because of fatigue. But all admitted there were many times when they were insensitive to their patients' emotional needs, or even exploded in anger.

But researchers at the respected Columbia Presbyterian Hospital in New York have suggested the consequences of practicing medicine without sleep may be more ominous.

In a study of 14 interns, the researchers examined the effect of sleep deprivation on one's ability to perform a crucial medical task: reading electrocardiograms.

They found that after 32 hours without sleep interns made twice as many errors in spotting abnormal rhythms in the heartbeat than when they were rested. That is a potentially life-threatening mistake.

One intern's errors in the 1971 study increased from seven to 23.

Almost all the subjects in that study said they experienced difficulty in concentrating, depression and extreme irritability when they were exhausted. Several had loss of memory and a tendency to laugh at patients' problems.

One said he constantly wrote notes in the wrong patients' charts.

At Georgetown, where the schedule was recently modified to try to give interns more sleep, the program director has been known to order a resident off the ward for a night if he noticed unusual irritability or other signs of severe fatigue.

Although he believes working nights is essential training, Dr. Milton Corn, Georgetown's associate chief of medicine, said he worries about a tired intern's ability to weigh decisions, such as whether to aggressively treat a critically ill patient.

"The fatigued physician is more likely to make an error in compassion or in ethics than he is in actual clinical care," Corn said. But his concern is unusual among program administrators.

Corn and his colleagues trained at a time when residency demanded even longer hours, and many physicians who went through it a decade or more ago consider the lives of present-day interns easy compared to their own training.

Dr. Ramey of George Washington cited two psychological defenses that keep older physicians from recognizing flaws in the training systems: One is that the stress of intership is so great that most doctors suppress the memory of it once they finish.

The other is that interns immediately assume a supervisory role over the group that follows them.He said they then "identify with the aggressor," viewing stress as necessary initiation.

"It's a self-perpetuating system," agreed a resident at Walter Reed Hospital. "I got beaten to death as an intern, and now I'm beating my interns to death."

The system originated at Johns Hopkins Hospital in the late 19th century.

According to Dr. Thomas B. Turner, archivist and former dean of Hopkins Medical School, it was something of an accident. Dr. John Shaw Billings, who designed the hospital in the 1870s, was intrigued by the Hopkins concept of a medical school where students would learn by working with patients. He added a few sleeping rooms for students to his hospital plan.

But when the hospital opened in 1889, four years before the medical school, graduates of other medical schools asked to come to the new hospital for additional training and lived in the rooms for a year.

That, said Turner, was the beginning of the intership system.

When Turner was at Hopkins after World War I, he recalled the requirements of residency were similar to those of the priesthood.

Residents were given only a small allowance, no salary, and they lived in the hospital or in row houses connected to the hospital by a switchboard so they could be responsible for patients day and night.

Residents were not permitted to marry, he said, until 1920, when "the chief resident on one service wanted to marry the professor's daughter.

"That was the first break in the dam."

But easing of the rules has been a slow, evolutionary process.

When Corn trained at Hopkins in the 1950s, residents were on duty every other night. And although he said he remembers his intership "with a great deal of pleasure, I also resent it.

"I feel it took an important year out of my life. I was married and had a child. No one will ever persuade me a fatigued physician is a better physician."

Since 1969, there have been sporadic efforts to unionize residents, the hospitals' so-called "house staff," and there have been a few strikes for better hours, pay and working conditions.

Interns generally make between $12,000 and $15,000 a year, which works out to about about $2 or $3 an hour.

Eric Silfen, who did his intership at Washington Hospital Center last year, said it was "the worst year of my life."

Silfen said he compulsively stayed late at the hospital to make sure his patients were all right, and by February he had lapsed into a deep depression.

"I was in the intensive-care unit," he said. "It was the middle of winter. Patients were dying, and I couldn't help anybody. I felt like quitting."

By the end of the year, after being assigned to a clinic, Silfen said he was scheduling healthy patients for his clinic because he knew they would not show up and was transferring any patient who "looked the least bit sick" to the intensive-care unit rather than caring for them himself.

His interest in medicine had succumbed to exhaustion and cynicism.

Silfen has since been revitalized by a residency in emergency medicine, a field that combines exciting medical problems with regular hours.

Residents generally called internship an important learning experience, but several of those interviewed said it had driven them or their colleagues into "9-to-5" specialties like radiology or dermatology.

Silfen and his friends dreamed up what they called a "dough-to-work ratio" to define the good life.

One student charted all the branches of medicine on a graph to compare the money earned with the hours worked. The chart showed that some specialties, such as surgery, paid well but required many hours, whereas others -- dermatology, cardiology, emergency medicine -- paid almost as well but allowed more free time.

At the bottom of the scale, lowest in pay and highest in hours, were the specialties that health planners agree are most needed in the United States: internal medicine, pediatrics and family practice.

The inventor of the "dough-to-work ratio," Silfen said, is now practicing dermatology.

Some hospitals have tried to humanize interns' schedules in the interest of patient care and physician morale.

Under the "night float" system in effect at Strong Memorial Hospital in Rochester, N.Y., a fresh intern comes on duty at 10 p.m. to handle many of the crises that keep interns awake at other hospitals.

Other doctors still work late, but the system does cut down on fatigue, said Dr. Donald Bordley, chief resident at Strong Memorial.

Providing technicians to draw blood and insert intravenous needles is another way to insure some sleep for doctors on duty, although it doesn't eliminate spending every third night in the hospital.

A physician familiar with Georgetown Univeristy Hospital, George Washington University Hospital and Washington Hospital Center said the hospital center residents get the most rest because technicians' services are available.

But hospital administrators balk at increasing technical support because of the cost, saying that hiring residents is cheaper than hiring nurses or physicians' assistants, because the residents can work so many hours.

The trend, however, is against adding any medical personnel to hospital staffs at a time when there is considerable pressure to hold down skyrocketing health costs and perform as much work as possible in outpatient facilities.

Dr. Robert J. Van Houten, associate dean at George Washington University Medical School, characterized the prevailing view:

"Everybody says everybody else ought to pay, not us (the hospitals, the schools, the patients, the insurance companies or the government). And there is nobody else."

But while the system persists, the practice of medicine will continue to be affected for those who can't forget the year they lived on little sleep.

Dr. Silfen, whose residency still requires some long hours, has made himself a promise.

"When I finished this," he vows, "I will never work a night again in my life."