Ever woken up after a big night out with no memory of how you got home, what you did or what you said?
It's called blacking out and according to Professor Paul Wallace, chief medical advisor to Drinkaware, it's surprisingly common.
"In a study on 772 college undergraduates carried out in the US in 2002, 40% of those who drank alcohol at all in the previous year reported experiencing a blackout," he told HuffPost UK.
"Those with blackouts reported learning later that they had participated in a wide range of potentially dangerous events they could not remember, including vandalism, unprotected sex and driving."
But why do we fail to remember events when we've been drinking and what are the long-term health implications?
According to Professor Wallace, alcohol has the capacity to affect our memory, even when it is consumed in lower quantities.
"Alcohol is a depressant which works in the same way as a sedative, making your brain processes slow down, with resultant impairment of memory," he said.
"Drinking heavily over long periods of time can have long-term effects on memory, so that even on days when you don't drink any alcohol, recalling what you did the day before or even where you have been earlier that day, can become difficult."
Blackouts are extreme versions of this memory loss that are common among social drinkers. We don't just forget parts of the night - our brain doesn't even create and store these new short-term memories.
"Blackouts are a frequent consequence of acute intoxication, regardless of age or whether the drinker is clinically dependent on alcohol," Professor Wallace added.
"Getting drunk happens when alcohol is consumed in excessive quantities over a short period of time. This results in a rapid rise in blood alcohol levels and as with any sedative, the result is a reduction in higher brain function.
"This includes memory loss and is frequently associated with inability to recall key details of events, or even entire events."
According to Professor David Nutt, from Department of Medicine at Imperial College London, the complete memory loss we associate with blackouts occurs because alcohol messes with the chemicals needed to lay down memory in the brain.
Alcohol blocks pro-memory neurotransmitters and enhances anti-memory neurotransmitters, which relay signals between nerve cells in order to communicate information between the brain and the body.
Alcohol also has an impact on receptor cells, which detect changes in the environment - such as the consumption of alcohol - and send these signals to nerve cells.
"Alcohol enhances the GABA-A receptor function in the brain and this causes amnesia in the same way as drugs, such as Benzodiazepines - the brain cannot lay down memories," he told HuffPost UK.
"To lay down a memory, the brain needs to increase [the neurotransmitter] glutamate and decrease [the neurotransmitter] GABA in the memory centres. Alcohol has the opposite effect on both these neurotransmitters, making it harder to lay them down.
"Also, at higher concentration alcohol blocks the NMDA glutamate receptor in the same way as anaesthetic drugs, such as ketamine, and this further impairs memory function."
Professor Wallace said while blackouts "don't necessarily cause permanent damage to brain cells", they can have serious consequences.
In the short team, those who forget their behaviour when drunk may have engaged in high-risk behaviour, putting themselves or others in danger, without even realising.
"In the long term, frequent heavy sessions can damage the brain because of alcohol's effect on chemistry," Professor Wallace added.
"Studies show that both men and women develop learning and memory problems as a result of heavy drinking. Brain imaging studies have demonstrated that alcoholics show significantly greater brain shrinkage than control subjects.
"Up to 80% of alcoholics have a deficiency in [the vitamin] thiamine, and a proportion of these people go on to develop serious brain disorders, such as Wernicke–Korsakoff syndrome. This life-threatening condition is associated with serious memory loss and other major neurological problems."
He advised that if someone is experiencing regular lapses in memory after drinking, they should reduce their alcohol consumption to within the Chief Medical Officers' low risk alcohol unit guidelines of 14 units per week or stop drinking altogether.
"This process can be challenging and heavy drinkers should seek qualified help in person and/or online," he said.
"They should also talk to their doctor to ensure that there is no other underlying cause for their symptoms."
For more information about drinking responsibly, visit drinkaware.co.uk.