Blackouts happen when your brain temporarily loses the ability to form new memories or when blood flow to your brain drops sharply enough that you lose consciousness. The most common cause is alcohol, but low blood sugar, sudden drops in blood pressure, seizures, and fainting spells can all produce blackouts too. The type of blackout you experience depends on what’s causing it, and understanding the difference matters because the word “blackout” actually describes two very different things: losing your memory while staying awake, or losing consciousness entirely.
Alcohol Blackouts: Awake but Not Recording
An alcohol-induced blackout is not the same as passing out. During a blackout, you’re conscious and interacting with people, but your brain stops creating new long-term memories. You might carry on conversations, walk around, or make decisions, and the next day have zero recollection of any of it. To anyone watching, you may seem functional (or at least as functional as a very intoxicated person can seem). But your memory system has gone offline.
This happens because alcohol disrupts the hippocampus, the part of your brain responsible for converting short-term experiences into lasting memories. Alcohol activates an enzyme that essentially shuts down the receptors your hippocampus needs to lock in new information. Without that process, called long-term potentiation, experiences pass through your brain like water through a sieve. You’re perceiving events in real time, but nothing gets saved.
There are two distinct types. A “fragmentary blackout,” sometimes called a brownout, leaves you with patchy, incomplete memories that may come back when someone reminds you of what happened. In these cases, your brain formed weak memory traces that need a cue to resurface. An “en bloc” blackout is far more severe: it has a clear starting point, and no amount of prompting will bring those memories back. They were never stored in the first place.
How Much Alcohol It Takes
Blackouts typically begin around a blood alcohol concentration (BAC) of 0.14%, though most occur closer to 0.20%. At a BAC of 0.22%, there’s roughly a 50/50 chance of experiencing either a fragmentary or en bloc blackout. En bloc blackouts become a coin flip at around 0.31%. For context, the legal driving limit in most U.S. states is 0.08%, so blackout territory is two to four times that threshold.
How quickly you drink matters as much as how much. Chugging drinks or drinking on an empty stomach spikes your BAC rapidly, and that rapid rise is a major trigger. Two people who reach the same BAC may have very different outcomes if one got there slowly over several hours and the other downed shots in quick succession.
Why Some People Black Out More Easily
Not everyone who gets very drunk blacks out, and genetics play a significant role. Twin studies estimate that about 43% of the variation in whether someone experiences blackouts comes from genetic factors. Even after researchers accounted for how often people got drunk, genetics still independently predicted blackout susceptibility, accounting for about 21% of the remaining variation. In other words, some people’s brains are simply more vulnerable to alcohol’s effects on memory, regardless of how much or how often they drink.
People with a naturally low sensitivity to alcohol’s effects are at higher risk. If you don’t feel very drunk even at high BAC levels, you’re more likely to keep drinking past the point where your hippocampus shuts down. Sensitivity to alcohol is itself heritable, with some estimates suggesting genetics account for 60% of the variation.
Fainting and Blood Pressure Drops
The other major category of blackout involves actually losing consciousness, even briefly. Vasovagal syncope is the most common form. It happens when your nervous system overreacts to a trigger, causing your heart rate and blood pressure to plummet simultaneously. Blood pools in your legs, your brain gets starved of blood flow, and you faint. Common triggers include standing for long periods, seeing blood, having blood drawn, extreme heat, intense emotional distress, and straining on the toilet.
Orthostatic hypotension is a related cause. This is the lightheaded, vision-narrowing feeling you get when you stand up too fast. It’s defined as a drop of at least 20 points in systolic blood pressure (the top number) or 10 points in diastolic pressure (the bottom number) within two to five minutes of standing. Dehydration, certain medications, and prolonged bed rest all make this worse. Most episodes are brief and harmless, but frequent occurrences can signal an underlying problem with how your body regulates blood pressure.
Low Blood Sugar
Hypoglycemia, or low blood sugar, can cause blackouts when levels drop severely. Blood glucose below 55 mg/dL is considered dangerously low and can trigger extreme confusion, slurred speech, seizures, fainting, and loss of consciousness. This is most common in people with diabetes who take insulin or certain oral medications, but it can also happen after prolonged fasting, heavy exercise without adequate fuel, or excessive alcohol consumption (which impairs the liver’s ability to release stored glucose).
The symptoms tend to build gradually: shakiness, sweating, and irritability at mildly low levels, progressing to confusion and disorientation before consciousness is actually lost. If you recognize the early signs, fast-acting carbohydrates like juice or glucose tablets can reverse the slide before it becomes dangerous.
Seizures
Seizures cause blackouts by disrupting normal electrical activity across the brain. The type most easily mistaken for “zoning out” is an absence seizure, which causes brief, sudden lapses in consciousness lasting just a few seconds. Someone having an absence seizure may stare blankly, blink rapidly, or make repetitive chewing or hand movements, then snap back with no memory of what just happened.
Focal impaired awareness seizures (previously called complex partial seizures) also cause blackout-like episodes. During one, a person may look confused, pick at their clothing, or smack their lips, and they typically can’t respond to questions or directions for several minutes. Generalized motor seizures are more dramatic, involving loss of consciousness, falling, and muscle jerking or stiffening. In all cases, the person has no memory of the seizure itself.
How to Tell What Caused Your Blackout
The key distinction is whether you lost consciousness or just lost memory. If you were told you were walking, talking, and interacting but remember nothing, that points toward an alcohol-related memory blackout. If you physically collapsed or went unresponsive, you’re looking at fainting, a seizure, a blood sugar crash, or a blood pressure drop.
Context helps narrow it further. Blackouts that happen after drinking are straightforward to identify. Fainting after standing up quickly or seeing blood suggests vasovagal syncope or orthostatic hypotension. Brief staring spells that happen without any obvious trigger, especially if they’re recurring, raise the possibility of seizures. And blackouts accompanied by shakiness, sweating, and hunger before the episode point toward low blood sugar, particularly if you have diabetes or hadn’t eaten in many hours.
A single alcohol blackout is a warning sign that you drank past the point your brain could handle. Repeated blackouts from any cause, whether alcohol-related amnesia or unexplained fainting, signal something that warrants medical evaluation. Recurring episodes can indicate a pattern of dangerous drinking, an undiagnosed seizure disorder, a cardiac rhythm problem, or a metabolic condition that needs treatment.