You wake up in an unfamiliar room, missing a button or two, with a few stains on your shirt that you’re hoping are food-related. The last thing you remember from the night before was downing that fourth shot of Tequila. Okay, so you blacked out. But what exactly does that mean?

What did alcohol do to my brain?

Anyone with a strong familiarity with booze has either had a blackout themselves, or knows someone who has. But not all blackouts are created equal; there are two types, “en bloc” and “fragmentary.” As their names imply, fragmentary blackouts cause the drinker to not recall moments for small periods of time, whereas en bloc refers to larger periods.

People who experience fragmentary blackouts, sometimes referred to as “brownouts,” can typically recall forgotten events once they’re reminded of them. En bloc blackouts aren’t so lucky. But both types are believed to be caused by the same thing, namely a neurophysiological, chemical disruption in the brain’s hippocampus, a region integral to memory formation.

Alcohol interferes with the receptors in the hippocampus that transmit glutamate, a compound that carries signals between neurons. During this interference, alcohol prevents some receptors from working, while activating others. This process causes the neurons to create steroids that then prevent neurons from communicating with each other properly, thus disrupting long-term potentiation (LTP), a process believed necessary for learning and memory.

In simpler terms, the effect is similar to anterograde amnesia in that the brain temporarily loses the ability to create new memories. Blackout sufferers still may be able to partake in spirited discussions or send late-night emails to former employers. What they won’t be able to do is create memories of any of it. And of course, there’s a dark side to all this. Blackouts tend to indicate a high level of intoxication, during which time drinkers don’t exhibit their best judgment, raising the risk of dangerous behavior such as having unprotected sex or driving a car.

Can blackouts be avoided?

As is the case with many drinking-related woes, having a full stomach helps. Not eating will cause your blood alcohol level to elevate more quickly. Drinking less and slower also obviously helps. Studies show a blackout’s main culprit to be a fast, dramatic spike in blood alcohol content; they usually kick in at blood alcohol levels of at least 0.15 percent. That’s roughly twice the legal limit for driving. And the trouble really begins when this level is reached quickly.

Women may have a harder time avoiding blackouts since their blood alcohol content increases more quickly than men’s. Not only do they tend to have less water in their bodies to disperse the alcohol, but they also have less gastric dehyrogenase, an enzyme that breaks down alcohol.

There also appears to be a tendency for people to revert back to blackout states once they start experiencing them, so you probably should lay off the sauce for a while after blacking out. But basically the way to avoid a blackout is either to not drink quite so much, or to at least drink it slower.

Does blacking out mean I’m an alcoholic?

Addiction expert E. M. Jellinek, who started the first significant research on blackouts in the 1940s, believed blacking out was a sure sign of dependency. More recent studies tend to indicate this isn’t always the case, as social binge drinkers are just as likely to experience blackouts as people who drink heavily on a daily basis. Basically, it all comes down to that sudden spike in blood alcohol content, though evidence suggests some people may have a genetic predisposition to blacking out.

Either way, frequent blackouts should not be taken lightly. Aside from engaging in questionable behavior that you’d likely regret if you could remember, any time you alter your basic brain function in such a significant way leaves you open to long-term damage. basic functions in the brain are significantly altered could represent long-term damage.

[NIH, LiveScience, WUSTL 1, 2, NYT, Drug Free, New Harvard Guide to Women's Health, Globe and Mail]

Image credits: Shutterstock/yuriy kulik, Wiki Commons