I’m not proud to reveal that I occasionally blacked out after drinking binges during college and in the first few years after graduation. Waking up and not remembering parts of the previous night was always embarrassing and sometimes frightening. Did I do anything I would regret?

Thankfully, I never found myself answering a mortified “yes” to that question, even though there were plenty of cringe-worthy moments I don’t remember that I learned about later.

What I do remember after those experiences is that I always felt different than my friends; I assumed there was something about me, maybe my family history of alcoholism, that caused me to black out while they didn’t. But blackouts just weren’t something that we discussed. I was embarrassed, and I didn’t think they could relate.

I’m glad we’re talking about them now, though, thanks in part to a powerful best-selling memoir by Sarah Hepola titled “Blackout: Remembering the Things I Drank to Forget.”

Hepola, a personal essays editor at Salon who experienced blackouts during her 25 years of drinking, assumed everyone knew what they were. So much so, in fact, that when her father suggested she needed to explain them in her book, she thought he was mistaken.

“I, such a snarky little teenager at heart, was just like, ‘Dad, you don’t understand. It’s not the 1950s anymore. Everybody knows what a blackout is,’ ” she told me during a recent interview. “Well, he was right and I was wrong.”

Her realization came after she had lunch with a well-respected magazine writer friend, talked about her book and told her what a blackout was.

“She was like, ‘Wait a minute … Are you telling me that people have amnesia while they’re drinking?’ … She had no idea this was going on and it occurred to me that every time I or anybody else was saying ‘blackout,’ she had no idea what that was.”

Blackouts ‘frighteningly’ common

Blackouts are periods of amnesia about things a person did or places a person went while intoxicated, according to the National Institute on Alcohol Abuse and Alcoholism. Blackouts are not the same as passing out while intoxicated, and a drunk person and others around him or her might not realize they’re happening. For most people, the sign of a blackout is waking up wondering, “What happened?”

“They’re very common, frighteningly so,” especially among college students who drink alcohol, said Aaron White, PhD, senior adviser to the director of the National Institute on Alcohol Abuse and Alcoholism, and one of the country’s leading experts on blackouts.

Back in 2002, White, then at Duke University, conducted a study involving nearly 800 students at the university who had consumed alcohol at some point in their lives. Some 51% reported having at least one alcohol-induced blackout.

And for the students who drank during the two weeks before the survey was conducted, 9% of men and 9% of women said they blacked out. Many had driven a car, had sexual intercourse, vandalized property or engaged in other risky behaviors during a blackout, according to the study. Several studies have found that students are more likely to sustain alcohol-related injuries during blackouts.

In another study White did, involving a 2009 survey of 4,500 students during the summer between high school and college, 12% of men and 12% of women who drank during the previous two weeks said they had a blackout.

“Studies tell us at least half of young people who drink experience a blackout before they graduate college,” White said.

“What’s really concerning is that, in a two-week period, about one in 10 college students who drink say there are parts of a night they don’t remember.”

Blackouts: You can’t easily tell when they’re happening

In a blackout, you could be doing mundane things like brushing your teeth, walking home or talking to a friend. Or, you might carry out more emotionally charged or risky behaviors such as having sex. Whatever it is, while that’s happening, your brain is unable to create memories for those events.

Hepola writes about coming out of a blackout in a Paris hotel room while she was having sex with a man that she had no memory of meeting.

“I was not asleep,” she said. “It’s almost like your mind goes online again after being kicked offline … and I come out of this blackout and I’m on top of this guy and I’m having sex with him, and I don’t know where he came from, and it’s the strangest thing that had ever happened to me.”

The man Hepola was having sex with most likely had no idea she was in a blackout, White said.

“Even for spouses of hardcore alcoholics, they report usually not being able to tell when their spouse is in a blackout,” he said.

A person might have this look in the eye where they don’t appear to be fully there, or might repeat the same story over and over again, White said, but for the most part, it’s challenging to know when someone is blacking out.

“So you could be talking to somebody and having a conversation with them about something that happened the day before or a month ago or a year ago and everything seems fine,” White said, “but while you’re having that conversation, the information about that conversation is falling into a void.

“Unless you give somebody a memory test, you’re probably not going to know.”

Implications for consent during sex

In her book, Hepola writes about an extreme example when she performed on stage before hundreds of people during a comedy event while she was in a blackout. She has no memory of her performance, but people later told her she was hilarious.

Now think about all the other things she could have been doing at the moment, or what others could have done to her.

“Those people in the audience did not know I was in a blackout and I did not know I was in a blackout and so that’s a very important thing to acknowledge when we think about what constitutes valid consent,” she said. “The legal standard for consent is that if you’re incapacitated you can’t consent, but what is incapacitation and does blackout fall under that category?”

Mixing alcohol and sex is a complicated issue “because you can do pretty much anything in a blackout that you can do when you’re drunk and not in a blackout, and that often means making bad choices,” said White, of the National Institute on Alcohol Abuse and Alcoholism.

Hepola asks, was there actual valid consent? Should a person be held responsible if neither can remember?

There isn’t a clear answer. While the legal system tries to sort that out, Hepola said she’s just thankful that we are now talking about how drunk is too drunk to have sex.

“I drank for 25 years, and I don’t remember anybody ever asking me about consent to sex. I just don’t remember that,” she said. “It was just kind of this idea of ‘Oh, you’re handling it’, ‘Oh, that’s a funny story,’ or ‘Isn’t it crazy what happens when we drink?’

“Nobody ever said, ‘Were you too drunk to consent?’ That just wasn’t asked. Now that’s a conversation that we’re having.”

Risk factors for blackouts

A few important things to note about blackouts: First, you don’t have to be an alcoholic to have one.

In fact, up until research in the 1990s and early in the 2000s, the thinking had been that blackouts only happen to alcoholics. Research by White and others involving college students proved that even people who would not be classified as alcoholics could and did experience blackouts.

Second, there are ways to avoid blackouts. “There is sort of a recipe if you want to black out,” White said. “You basically drink in a way that gets alcohol into your brain fast and so that could be by doing shots, chugging beer, but also by skipping meals. So avoiding blackouts involves doing the opposite. Have food in your stomach, pace yourself, limit the amount you drink.”

Blackouts can happen to anybody, White said, but women tend to be at higher risk for them, in part because they are more likely than men to skip meals, and also because they tend to choose beverages that have a higher concentration of alcohol, such as wine or spirits, versus beer.

Women also tend to be smaller on average than men, have less water floating around in their bodies and have less of an enzyme in the gut that breaks down the alcohol before it’s absorbed, he said. No more than one drink per day for women and no more than two drinks per day for men is considered drinking in moderation.

“Because of those differences, it doesn’t take as much alcohol for a typical female to get enough alcohol in the brain to black out.””

There is still so much we don’t know about blackouts. For instance, why do some people get them and some people don’t? There is some genetic component to blackouts, White said, pointing to studies involving twins; if one twin blacks out, the other tends to be prone to them, as well.

We don’t know the why, White said, or what in the brain causes them, or whether it could have long-term repercussions.

Where’s the worry about blackouts?

White puts it this way: Consider if you could go to a gas station and come out with something that could produce amnesia or if there were something you could buy off the shelf and it was capable of shutting down the memory circuits. Wouldn’t we be “deeply concerned” about that?

“Alcohol is safe in moderation but too much too fast can shut off the memory making parts of the brain and cause anterograde amnesia. What else can you buy anywhere, legally, that can cause such a profound neurological deficit?” he said.

Hepola hopes her book will help raise awareness about blackouts, and make people realize they are a lot more common than people might think.

In her case, she not only spent more than two decades dealing with blackouts. She also was battling an alcohol addiction — an addiction that covered up her fears of judgment and inadequacy.

Five years ago, at the age of 35, she called her mother and told her she needed to quit drinking.

“That was a big moment for me … it was like I had to destroy all my hiding places because once I told my mom, then I couldn’t take it back. Once I told my mom how bad it was, that’s a little bit of a ‘no backsies’ moment,” she said.

She called her mom every night for 30 days and eventually fully embraced Alcoholics Anonymous, something she scoffed at earlier in her drinking life.

Through readings and events around the country, she’s now committed to trying to help other young women and men who may be struggling with alcohol, and putting a spotlight on blackouts and how dangerous they can be.

“People need to know that they exist,” said Hepola, who now lives in her hometown of Dallas. “This is such a widely used and available substance. We have kind of turned a blind eye to the fact that kids abuse it. It’s a rite of passage for them, but I feel like there’s just not at all an awareness that there are … consequences.

“The truth is a lot of kids are blacking out and we don’t even know what it is.”

Editor’s note: Kelly Wallace is CNN’s digital correspondent and editor-at-large covering family, career and life.