Sarah Hepola experienced her first blackout from drinking when she was 11, at a party she attended with a female cousin she was close to. Hepola, who started stealing her parents' beer when she was about 6, chugged more than her share of suds and liquor at the bash. The next day, she was stunned and horrified when her cousin asked if she remembered removing her pants during the party.

"I thought it was really insane," says Hepola, who's now 44 and eight years sober. "I was very modest. I didn't like undressing in front of people." That wasn't the only part of the night Hepola didn't remember. Her cousin told Hepola she'd also sat on some stairs and started crying. On the stairs, Hepola lamented that everyone loved her cousin more than they loved her.

It wouldn't be the last time Hepola, author of The New York Times best-seller "Blackout: Remembering The Things I Drank to Forget," couldn't remember chunks of her life she experienced while drunk. During Hepola's early adulthood, as a running joke she'd sometimes dump beer on the head of a male friend while proclaiming something jokingly hostile like "This is for the patriarchy!" Sometimes Hepola remembered the splashing; sometimes not. At times, Hepola's friends told her she'd drunkenly lifted her shirt to flash people. More than once, she woke up after a night of heavy drinking on a friend's couch, with no memory of how she got there. "It's like the brain suffers a mechanical failure," Hepola says. "The problem with all this is that it's funny in some ways and horrible in others. And it varies vastly depending on your context and perspective."

Blackouts are periods of amnesia during which a person engages in activities like walking, talking or dumping beer on a friend's head "but doesn't create memories for these events as they transpire," according to the National Institute on Alcohol Abuse and Alcoholism. There are two type of blackouts, says Jennifer E. Merrill, a research scientist and assistant professor at the Center for Alcohol and Addiction Studies in the Department of Behavioral and Social Sciences at Brown University in Providence, Rhode Island. "En bloc" blackouts can last for hours; in these blackouts, someone may not be able to recall large chunks of time. People who experience this kind of blackout may recall the initial parts of the drinking episode, before they've ingested the amount of alcohol that would disrupt their brain's ability to record memories, Merrill says. Fragmentary blackouts, also known as "brownouts" or "grayouts," can last a few minutes at a time and may involve a hazy or spotty memory of events. This kind of blackout is comparable to a camera that records for a while, loses power and goes dark, then blips back on again. A key difference between an en bloc and fragmentary blackout is that in the latter, a person can often recall something once prompted, Merrill says. He or she may not initially be able to recall a part of the evening, but will remember if a friend reminds him or her what happened. During both kinds of blackouts, some people may become sloppy and have a hard time standing or speaking intelligibly during a blackout, while others can outwardly seem fine and engage in coherent conversations without appearing impaired.

People who suffer from alcoholism, as well as their relatives and close friends and substance misuse clinicians, are well aware of the concept, which drew attention recently with the pitched battle over President Donald Trump's nomination of federal Judge Brett Kavanaugh for a spot on the Supreme Court. Christine Blasey Ford accuses Kavanaugh of sexually assaulting her during a party in the summer of 1982, when both were teenagers. Ford testified in a Senate hearing that Kavanaugh was drunk during the incident. Kavanaugh testified that he never attacked Ford and denied having a drinking problem. He also denies allegations from two other women who accuse him of drunken sexual misconduct in high school and in college.

While Ford's account and Kavanaugh's vehement denials seem to suggest one of them is lying, some observers have discussed the theory (which Hepola wrote about in a New York Times opinion piece) that both could have testified truthfully. Under this hypothesis, Kavanaugh may not recall what happened during the night in question and believes the allegation is false.

Some people confuse alcohol-induced blackouts with passing out from excessive drinking, but they're two different things, says Reagan R. Wetherill, a research assistant professor of psychology in psychiatry at the Perelman School of Medicine at the University of Pennsylvania. Wetherill has researched blackouts. "During a blackout, a person is able to actively engage and respond to [his or her] environment," Wetherill says. "However, the brain is not creating memories for the events. Essentially, alcohol alters and impairs memory formation by suppressing activity within the hippocampus and other regions of the brain involved in the transfer of information from short-term to long-term memory." A heavy drinker can be in a blackout and not pass out, says Dr. Harshal D. Kirane, director of addiction services at Staten Island University Hospital, part of Northwell Health in Staten Island, New York. "Not all blackouts end in passing out," he says.

Blackouts are associated not just with drinking an excessive amount of alcohol, but consuming it quickly, Kirane says. There's no baseline amount of drinking that will cause someone to have a blackout, Kirane says. It varies from person to person, based on such factors as an individual's size and his or her tolerance for consuming alcohol. Genetics can play a role, because some people are genetically predisposed to experiencing blackouts, Kirane says. Once someone has experienced a blackout, he or she is more vulnerable to having future episodes of such memory loss, Kirane says. That's because biological changes occur in the brain that can lower its threshold for experiencing another blackout.

Being in a blackout can be dangerous – for the person experiencing it and for those around him or her. "It's an incredibly vulnerable place where someone won't be able to respond to potential threats or dangers, or to regulate their behaviors and emotions as they would in a sober state," Kirane says.

Not knowing how she behaved during a blackout was unsettling, Hepola says. She considered herself a "happy drunk" who freely handed out compliments while inebriated. But after a handful of drinking episodes, some people told her how rude she was the previous night. "That totally ran against how I saw myself," she says.