The biology behind alcohol-induced blackouts

A 2016 review found that approximately half of all drinkers report having experienced a blackout at some point. Some people who drink regularly even accept blackouts as just something that can happen with a big night out. You wake up the next day with gaps in your memory, unable to recall conversations you had, places you went, people you met, or things you did while drinking. A blackout can be scary, especially when someone the next day tells you that you did something dangerous or out of character.

But why do blackouts happen with alcohol, and what are the real risks?

Alcohol blackouts explained

To make sense of what happens to your memory when you drink, it helps to know a little about how memories are normally formed. When you experience something, your brain processes that information in stages.

First, sensory details are held briefly in what is known as short-term memory. If the brain decides this information is worth keeping, it is transferred into long-term storage, where it can be retrieved later. This transfer process is called memory consolidation, and it depends heavily on a small, curved structure deep in the brain called the hippocampus.

Alcohol does not erase memories that have already been stored, but it interferes with the creation of new ones. Short-term memory alcohol effects may not be obvious at first when you drink heavily, because you can still have normal conversations and follow what is happening around you.

But the process of moving those experiences into long-term storage is already starting to break down. Someone in a blackout can appear perfectly functional in the moment, but have no memory of it the next day, because the memories were never recorded in the first place.

How alcohol disrupts the hippocampus

The hippocampus is particularly sensitive to alcohol. Even moderate amounts of alcohol can begin to affect how well the hippocampus functions, but at higher doses, the impact can become severe.

Research has found that alcohol suppresses the activity of neurons in the hippocampus, particularly cells in an area called CA1 that are important for forming new memories. When these cells are suppressed, your brain loses its ability to encode what is happening into long-term storage.

The alcohol effects on the hippocampus do not happen gradually like some other effects of drinking. Once your blood alcohol concentration rises past a certain point, your hippocampus shuts down memory-making processes, but other brain functions still continue. This is why blackouts are so disorienting, because you were consciously walking, talking, and making decisions, but none of it was being recorded.

GABA, glutamate, alcohol, and the hippocampus

Alcohol also affects your brain by altering the balance between two key neurotransmitters. The first is GABA, which stands for gamma-aminobutyric acid. GABA is an inhibitory neurotransmitter, meaning it slows down brain activity. Alcohol strengthens the effects of GABA, which is why drinking makes you feel relaxed and sedated.

The second is glutamate, an excitatory neurotransmitter that is involved in learning and memory. Alcohol suppresses glutamate activity, particularly at receptors called NMDA receptors. These are essential for a process called long-term potentiation, which strengthens the connections between neurons and is crucial for forming new memories.

When alcohol disrupts both GABA and glutamate at the same time, the hippocampus becomes unable to carry out its normal memory functions. The more you drink, the more severely memory systems are disrupted, and the more likely a blackout becomes.

Why rapid drinking increases the risk

You may have noticed that not everyone who drinks heavily will experience a blackout, and not every heavy drinking session leads to one. Research consistently shows that blackout drinking risks increase when alcohol enters your bloodstream quickly. This is partly because the brain does not have time to adapt, and the hippocampus is overwhelmed before it can put any protective measures into action.

A few other things that can make this more likely include drinking on an empty stomach, consuming multiple drinks in a short period, drinking high-alcohol spirits, and even gulping your drink rather than sipping it.

Fragmentary and en bloc blackouts

Blackouts are not all the same. Researchers distinguish between two types based on how complete the memory loss is.

Fragmentary blackouts, sometimes called partial blackouts or brownouts, involve patchy memory loss. You may remember some parts of the evening but have gaps where certain events are missing. With cues or reminders, some of these memories can sometimes be recovered, which suggests your brain did record some of it, just not completely.

En bloc blackouts are more severe. In these cases, there is complete memory loss for a period of time, often several hours. No amount of prompting will bring the memories back because the brain never encoded them in the first place. Research suggests that fragmentary blackouts are more common than en bloc blackouts, though both are signs that your brain’s memory systems have been seriously affected.

Recognising the signs of alcohol blackout

One of the difficult things about blackouts is that there is no way of knowing they are going to happen. Someone experiencing a blackout can appear relatively normal, carrying on conversations and engaging in activities without any obvious sign that their brain has stopped recording. This is different from passing out, where the person loses consciousness entirely.

Most of the common signs of an alcohol blackout become apparent only afterwards. The person may have no memory of conversations they had, people they met, places they went, or things they did while drunk. They may have done things that seem out of character or made choices they would not normally make.

However, one sign that short-term memories are not being properly transferred into long-term storage is someone repeating what they said or asking the same questions over and over during the drinking session.

Factors that can increase blackout drinking risks

Several things can increase your likelihood of experiencing a blackout. A smaller body size means alcohol is less diluted in your bloodstream, leading to higher blood alcohol concentrations with fewer drinks. Studies have found that blackouts often begin when blood alcohol concentration reaches around 0.14 to 0.20 percent, but they can occur at lower levels in some people.

Drinking on an empty stomach allows alcohol to be absorbed more quickly, and mixing alcohol with other substances, particularly sedatives like benzodiazepines, can amplify the effects on memory.

Genetics also plays a role. Research suggests that some people are just more vulnerable to blackouts than others, even at similar levels of intoxication. It appears that some brains are simply more sensitive to alcohol, and that vulnerability may run in families.

Why blackouts are dangerous

As well as the memory loss itself, blackouts can carry serious risks. When you cannot form memories, your judgement is already severely impaired, even if you do not feel drunk. People in blackouts are more likely to engage in risky behaviour, from unprotected sex to driving. They are more vulnerable to harm and more easily taken advantage of by others.

Experiencing blackouts is also a warning sign. Research has found that a history of blackouts is associated with an increased risk of developing alcohol addiction. If you are experiencing blackouts a lot, you need to look carefully at your drinking habits.

Get professional help for alcohol concerns

If you are experiencing blackouts or finding it difficult to control how much you drink, it may be time to reach out for support. Bayberry offers confidential advice and medically supervised alcohol addiction treatment to help you take back control and protect your health. Contact us today to learn more about our alcohol detox and rehab programmes, and the ongoing support we provide.