Have you ever woken up after a night out with only hazy, fragmented memories of your actions? For many, this experience is not just embarrassing but deeply disconcerting, raising urgent questions about what alcohol is actually doing to our brains.
The Neurological Rollercoaster of a Drink
Alcohol's impact on the brain is uniquely broad and complex. Unlike some drugs that target a single receptor, alcohol can act on several brain systems simultaneously, explains Associate Professor Shalini Arunogiri, chair of the Royal Australian and New Zealand College of Psychiatrists’ Faculty of Addiction.
This small molecule bypasses digestion, entering the bloodstream and crossing the blood-brain barrier within minutes. Once there, it disrupts the delicate balance between two key chemical messengers: GABA, which slows brain activity, and glutamate, which stimulates it.
Dr Rachel Visontay, a postdoctoral research fellow at the Matilda Centre for Research in Mental Health and Substance Use, notes that initial drinks boost calming GABA and feelgood chemicals like dopamine. However, as consumption continues, glutamate's stimulatory effects are dampened, slowing activity across all brain regions.
"As blood-alcohol levels rise, it further affects the frontal lobe, the part of your brain associated with thinking, planning, and judgment," says Arunogiri. This impairment leads to the poor decisions and regrettable actions many associate with a heavy drinking session.
From Tipsy to Blackout: The Escalating Effects
The level of alcohol in the blood, known as Blood Alcohol Concentration (BAC), dictates the severity of impairment. Beyond 0.08g per 100ml, balance and coordination falter as the cerebellum is affected. At a BAC exceeding 0.15g, a critical shift occurs: the hippocampus, vital for memory formation, is strongly impaired.
"We’re able to still form short-term memories... but we have the failure of transferring that to long-term memory," Visontay explains. This results in the phenomenon of alcohol-induced blackouts—periods where a person is conscious and interactive but will later have no memory of events. This is distinct from passing out or losing consciousness.
At these dangerously high levels, the brain's signals to the body weaken, risking suppressed breathing. A BAC above 0.30g can induce coma or be fatal as vital functions shut down.
Why Biology, Sex, and Genetics Create a 'Perfect Storm'
Individual experiences of intoxication vary dramatically. Factors like drinking quickly, low body weight, or an empty stomach can cause BAC to spike faster. Crucially, sex is a major differentiator.
"Women drinking the same amount as men reach higher blood-alcohol levels and experience more impairment," states Dr Visontay. This holds true even for men and women of identical weight, due to biological differences. Women have less body water per kilo, meaning alcohol is less diluted in their systems. Men also metabolise alcohol more efficiently in the stomach before it enters the bloodstream.
Genetics further influence one's relationship with alcohol. Some people possess a genetic makeup that triggers a heightened dopamine response in the brain's reward centre, increasing the pleasure derived from drinking and elevating the risk of alcoholism.
The propensity for blackouts may also have a genetic component, though rapidly rising BAC is a known key risk factor. Research indicates these memory failures are common in early drinking experiences, with nearly half of adolescents who drink experiencing a blackout by age 19.
The Lasting Psychological Impact
The aftermath of heavy drinking extends beyond the physical. Waking with memory gaps is "disorientating and destabilising," says psychologist Rachel Samson, a director of the Australian Association of Psychologists. This is compounded by social fallout, such as angry messages from friends about forgotten behaviour.
Dr Kelsey Perrykkad, a cognitive scientist at Monash University, warns that discovering you acted out-of-character can be "impactful on your sense of self." These psychological shifts are mirrored by neural changes, debunking the myth that the adult brain is static. The brain changes constantly through adulthood, shaped by our experiences.
This underscores why health experts advise against any alcohol consumption for under-18s, whose brains are in a critical stage of development. For adults, while no level of drinking is completely safe, national guidelines exist to help manage risk.
"We really normalise drinking alcohol," Samson concludes. "We’re not necessarily seeing it as a drug that impacts our brain chemistry, but that is what is happening."
