The Nuance

The Science of Alcohol Blackouts

Heavy drinking impacts the parts of your brain that store memories

In 2006, a 24-year-old Seattle man was arrested and charged with the murder of four people — two women and two children — who were stabbed to death in their home.

The evidence against the man was considerable and included the fact that he’d tried to burn the house down to conceal the crime. During his trial, he claimed that he’d drunk almost three fifths of vodka, blacked out, and couldn’t remember killing anyone. His attorneys argued that his inebriation and memory loss cast some doubt on his guilt.

The so-called “blackout defense” has a long history in criminal trials. While it can’t excuse criminal behavior, there are situations when blacking out is considered a mitigating factor — something that may help a defendant avoid the harshest legal punishment.

In the case of the Seattle man, the jury was not persuaded. The man was sentenced to death. However, that trial and others like it have helped instigate scientific research into the phenomenon of alcohol-induced memory loss.

The blackout research has yielded some surprising findings.

Alcohol experts usually define a blackout as the loss one’s memory for events that occurred during a drinking episode. By some estimates, more than 50% of drinkers have experienced a partial or total blackout.

These periods of memory loss were once thought to be the product of a kind of total-brain impairment or loss of function, but this doesn’t seem to be the case. Research has found that alcohol exerts its greatest effects on the hippocampus and other parts of the brain that are responsible for the storage of long-term memories. Other parts of the brain may be little affected.

In the moment — that is, while a person is living through their big night out — they may be lucid and able to recall everything that’s happened. It’s only later, usually upon waking up the next day, that they’ll discover holes or gaps in their recollections.

“In order for a memory to be formed, information from the environment — where a person is, what they are doing, who is there, etc. — is sensed or observed and put into short-term memory for further encoding into long-term memory,” explains Reagan Wetherill, PhD, an alcohol researcher and assistant professor of psychiatry at the University of Pennsylvania Perelman School of Medicine. “Alcohol can either completely block encoding information into a memory or partially block encoding.” The result of this block is a partial or total blackout.

While some people need to be falling-down drunk to black out, others may get there long before they’re visibly intoxicated.

Another misconception that research has refuted is the idea that blackouts only happen when people are excessively and obviously drunk (for example, stumbling, slurring their speech, etc.). “[I]t can be difficult or impossible to know whether or not a drinker is experiencing a blackout,” Wetherill and her coauthor wrote in a 2017 study in Alcoholism: Clinical and Experimental Research.

While some people do need to be falling-down drunk to blackout, others may get there long before they’re visibly intoxicated. “On average, we know that blackouts typically occur at a blood alcohol level of .14 or higher,” she says. According to Alcohol.org’s online BAC calculator, a 175-pound man would reach that level of intoxication if he had five beers in a 45-minute period. “But,” she adds, “[blackouts] can occur at blood alcohol levels of .07.”

Some studies have found that women are more likely to experience blackouts than men. And according to that online BAC calculator, a 125-pound woman would exceed that .07 threshold—and so could theoretically experience memory loss — after just two drinks in a 45-minute period.

Another surprise is that, in some cases, only a person’s long-term memory is impaired during a blackout. Other cognitive functions — such as planning, attention, and social skills — may be little affected.

Just about everyone recognizes that heavy alcohol use is a significant health risk. But there’s evidence that people who experience blackouts may be at greater risk for some psychological problems.

In the days following a blackout, your risk for further alcohol-related memory loss may go up.

A 2020 study of college-age drinkers in the journal Drug and Alcohol Dependence found that those who reported having blackouts were at elevated risk for symptoms of depression. More research has found that adult men and women who experience blackouts are more likely to contemplate suicide.

While it’s possible that a tendency toward blacking out may point to underlying neurological vulnerabilities — an idea that Wetherill and other experts have explored — a lot of this research is inconsistent. It’s not clear whether blacking out is a cause or a consequence of depression or other mental health challenges. Regardless, this work indicates that people who are struggling may need to be especially careful when it comes to alcohol.

While your risk for a blackout rises with your drink count, other factors also seem to matter. Not surprisingly, drinking on an empty stomach or drinking rapidly (that is, taking shots or gulping strong drinks) can spike your blood-alcohol content in ways that can quickly trigger memory loss.

Some work has also found that in the days following a blackout, your risk for further alcohol-related memory loss may go up. In other words, if you’re partying hard several days or nights in a row, you may be more likely to blackout even at lower BACs. There’s also research linking blackout drinking to cognitive impairments among older adults.

It can be unsettling — to say the least — to wake up and find that parts of your memory are missing. People sometimes talk about “drinking to forget.” In the case of blackouts, that’s literally true.