Maybe he had slept for three hours, or maybe it was four.

At 5 o’clock on a Wednesday morning, Michael McDonough is up, with enough time to get dressed and ready before he takes the Green Line to Northeastern. But McDonough doesn’t take classes here or hold a job nearby. He’s going into his sophomore year at Suffolk, and at the age of 23, is the oldest in his class by a few of years. Northeastern is just where he trains, three days a week, with other members of the local Reserve Officer Training Corps Program (ROTC).

Passers-by can’t see McDonough through the windows that span the Marino Center gym, but inside, he’s running and lifting weights alongside young cadets. Like McDonough, the cadets – undergraduate students from around the city – are pushing through their regimented morning workout. But there is a fundamental difference between them and the man who is a few years their senior. The cadets haven’t been where he’s been. In fact, that’s what they’re striving toward: to do what he has done, in the name of their country.

McDonough has been to the war in Iraq and this dogged workout is just one way in which his life as a soldier has merged with his life as a college student.

Thirty percent of the soldiers enrolled in the US Army today fall between the ages of 21 and 24, according to the US Army website. That means thousands of young veterans will have the chance to make the same transition McDonough has – from combat to campus – thanks to the promise of the GI Bill, which, since World War II, has guaranteed secondary education to veterans.

While college offers some of the same hallmarks to veterans as it does civilians, like a sense of intellectual community, coursework and extracurricular opportunities, the social rite of passage is cast in a different light for those who have seen the front lines. They aren’t only set aside by age – most are several years older than their peers because they spent their “college years” serving their country – but by perspective.

“They’ve had responsibilities most of their peers won’t have until they’re in their 40s, if ever,” said Cathal Nolan, executive director of the International History Institute at Boston University, and author of two volumes of the Encyclopedia of World Wars. “They’re different. Their experience has made them different from their peers. They’ve had to grow up fast, in a way that most college students do not.”

Throughout the history of American wars, young people have consistently filled the front lines, Nolan said. But what it means to come home can’t be pinpointed for any generation – it’s as varied as each veteran’s personality, and what he or she experienced at war.

Three and a half years ago, McDonough enlisted in the Army. In fall 2007, he enlisted at Suffolk as an international affairs major. These two paths are inextricably married for the Hull native, who said he can’t help but see his daily routine as a student through the lens of an active duty infantryman, forcing him to push the limits of physical and mental strain for a year.

“I’m at the point, after combat deployment, that it’s a lot easier to get up for classes,” he said. “To not complain about having to go to school for two hours, as opposed to going on a 10-hour patrol.”

When McDonough’s morning workout is done, he has three classes downtown, which will last, with hardly any break, until 4 p.m. But he can’t complain, he said. He’s just thankful he can tell Wednesdays from Thursdays, he said. In Samara, Iraq, the days blended together so seamlessly, it was like living in the film Groundhog’s Day, he said – like living one extended, exhausting moment.

“One group of guys would be shooting up a building in the middle of the city, and you’d go to deal with it. You’d come back a few hours later, and you’d sleep for an hour and, all of a sudden, something would blow up on the other side of the city and you’d get up and go deal with that,” he said.

Nowadays, McDonough’s responsibilities are practically effortless, he said. He has a week to tackle five hours of homework; he used to patrol for 12 hours a day. He has to study later for upcoming exams; he used to kick down doors and raid homes. Every day for a year, he said, he had to confront some form of small arms fire, attacks by mortars or rocket-propelled grenades. At Suffolk, he just has to hand in his work on time.

In some ways, McDonough is an anomaly – a self-proclaimed “adrenaline junky” who said he thrived on the adventures of war and hit few bumps along the road back home. But for other young veterans returning to the states, the proverbial light at the end of the tunnel can itself be an unexpected challenge. And for those who are college-bound, integrating onto a college campus adds a new dynamic. School could be a supportive environment for the transition into civilian society, or a strange place full of peers and professors with whom veterans can’t relate.

“Many find it difficult to discuss the events of their deployment with others whom they feel haven’t had similar life experiences,” David Benedek, an associate professor of psychiatry at the Uniformed Services University in Bethesda, Md., said in an e-mail to The News. “Some find thinking or talking about their experiences anxiety-provoking.”

For Jonathan Janiec, a veteran who graduated from Northeastern last semester, his vivid memories are worst at night, even though it’s gotten better.

The sounds that wake him are usually explosive and always imagined: remnants of the afternoon his company was driving down a dusty road in Al Fallujah when three artillery shells blew up behind a nearby berm and knocked him out cold, he said.

Since September 2003, more than 59,800 veterans have reported suffering from post traumatic stress disorder (PTSD) as a result of their military participation, said Peggy Willoughby, a spokesperson for the National Center for PTSD. She said 70 percent of those veterans, nearly 42,000, are between the ages of 20 and 29. Although Janiec has not been diagnosed with PTSD, he said he may be one of them.

“Most of that age range is in combat units,” Willoughby said. “The more trauma you’re exposed to, the more prone you are to develop a reaction.”

Janiec was hands-on in combat, a gunner who rode almost every day in the turret of a Humvee – first through Iskandaria, Iraq, then through the Al Fallujah region where the accident occurred. In his year abroad, Janiec said he experienced about 30 explosions, caused by improvised devices, which were hidden inside guardrails and trees or camouflaged on the side of the road. After all that, he said he feels relatively lucky for the sporadic nightmares.

“It’s not something that haunts me,” he said. “I don’t wake up in the morning and that’s the first thing I think of. I don’t think it’s something that hinders anything I do. And I’ve shown much better symptoms than some of my friends. Some others haven’t dealt with it so well.”

Janiec said if it weren’t for school, with its structure and class schedule, and Northeastern’s ROTC community, he might have fared differently.

“Several of the cadre have been to Iraq and Afghanistan, and they’ve been on several deployments,” he said. “So I’ve had the ability to talk to them, to discuss things, and that makes the transition a lot easier.”

But outside the ROTC community, Janiec, an international affairs major, said he’s run up against a thick wall of stereotypes that keeps him from forging friendships with more than a handful of civilian students, who are willing to embrace his lifestyle.

“People have preconceived notions about the military … about soldiers,” he said.

Some people had him pegged from the start, he said, remembering the time a professor excused him ahead of time for what she assumed would be a sub-par paper, because she said she knew the military stifles creativity and self-expression.

“How academia views the military is almost with an undertone of, I don’t want to say hostility, because it’s not hostility, but they don’t really know anything about it. What they say is because it’s on the news or in the movies, books,” Janiec said.

Since September 11, the stigma that historically shrouded returning veterans, particularly those who fought in the Vietnam War, has mostly lost its place in American society to be replaced by a sense of reverence, Nolan said. Still, he said the sense of isolation that Janiec said follows him from class to class is not uncommon among young veterans who tend to realize they are naturally separated from their civilian peers once the thrill of coming home is gone.

In response, a number of grassroots groups, like the Student Veterans Association (SVA), found on some college campuses, have cropped up in the past few years to unite geographically disparate veterans, who are often students, in need of social support.

“Veterans don’t share a lot of common characteristics with their peers, who tend to be, by and large, 18-year-olds that tried all their lives to be where they are [at college],” said Luke Stalcup, who finished graduate school at Columbia this year. Stalcup was part of the first group of American soldiers deployed to Iraq in November 2002, and spent eight or nine months as an Explosive Ordinance Disposal team leader, defusing roadside bombs and minefields.

“We carry a kind of dual-identity,” he said.

The national chapter of SVA was created in January from a handful of smaller campus groups with a common goal: to provide a lattice of emotional and social support for other young people in their particular predicaments. Nearly four months later, SVA has more than 150 satellites, including groups at the University of Massachusetts Amherst and Boston College.

“On the aggregate, for the broad middle of veterans, [they transition] better if they’re dealing with other veterans who have a shared experience,” Stalcup said. “It is common sense, but it’s also sound in a clinical sense, because one of the primary treatments for things like PTSD is group therapy. We’re just group therapy at large. We’re not actually going out and talking about our feelings, but that’s basically the gist.”

Benedek also said that seeking out strength in numbers is an affective means for young veterans to dispel feelings of isolation, whether they are connecting with other veterans or counseling professionals.

For Stalcup, college itself might be the greatest tool for young veterans trying to ease their way back into the civilian world, he said.

“The kind of structure and social and goal-oriented process of getting a degree – having something to work for, and having structured social interactions – is a good thing for veterans transitioning back into society,” he said. “Veterans are very well equipped to deal with requirements and demands that universities put on them. School is about deadlines, and those deadlines create a structure for your life.”