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Memory is not a single thing stored in one part of the brain. Psychologists usually distinguish between several kinds of memory that work together. Sensory memory holds information from our senses for a fraction of a second, just long enough for us to notice it. Short-term memory, sometimes called working memory, holds a small amount of information for a few seconds while we use it, for example, a phone number we repeat to ourselves before dialling. Long-term memory stores information for days, years or a lifetime.
The capacity of short-term memory is surprisingly limited. In a famous paper published in 1956, the psychologist George Miller suggested that most people can hold about seven items in mind at once, plus or minus two. Later research suggests the true number may be even smaller, closer to four meaningful units. One way to get around this limit is a technique called chunking: grouping individual items into larger units. It is much easier to remember the number 1945 2001 1969 as three historical years than as twelve separate digits.
Moving information from short-term to long-term memory is not automatic. Much of what we notice during the day is forgotten within hours. In the 1880s, the German researcher Hermann Ebbinghaus tested his own memory by learning lists of meaningless syllables and checking how many he could recall after different periods of time. He found that forgetting happened very quickly at first and then slowed down, a pattern now known as the forgetting curve. Within a day, he had lost a large part of what he had learnt.
Ebbinghaus also discovered something more encouraging: each time he reviewed the material, he forgot it more slowly. This finding is the basis of spaced repetition, a learning method in which information is reviewed at gradually increasing intervals, for example, after one day, then three days, then a week. Many language-learning apps today are built on this principle, and studies consistently show that it is far more effective than studying the same material many times in a single session.
Sleep also plays an important role. During deep sleep, the brain appears to replay and strengthen the memories formed during the day, while weakening less important connections. Students who stay up all night before an exam may therefore remember less than those who study for a shorter time and sleep well.
Finally, memory is not a perfect recording. Each time we recall an event, we rebuild it, and in doing so we may change details without realising it. This explains why two people can remember the same event quite differently, and why witnesses to a crime sometimes give honest but inaccurate accounts. Understanding how memory really works can help us to learn more effectively and to be more humble about what we think we remember.