Sleep and the Making of Memories
For much of the twentieth century, sleep was regarded by many scientists as a passive state in which the brain simply rested. Research over the past few decades has overturned this view. During sleep the brain is highly active, and a growing body of evidence suggests that some of this activity helps to stabilise memories formed during the day, a process known as consolidation.
Sleep is not uniform. Over a typical night, a sleeper passes through several cycles, each lasting roughly ninety minutes, which include light sleep, deep ‘slow-wave’ sleep and rapid eye movement (REM) sleep. Slow-wave sleep is concentrated in the first half of the night, while REM periods become longer towards morning. Researchers believe that the different stages may support different kinds of memory.
Much of the evidence comes from laboratory studies in which volunteers learn something, such as a list of word pairs or a sequence of finger movements, and are tested on it later. Those who sleep between learning and testing generally perform better than those who stay awake for the same length of time. Importantly, the advantage appears even when both groups are tested at the same time of day, which suggests that it is not simply a matter of the sleepers being less tired.
One proposed explanation is that, during slow-wave sleep, the brain ‘replays’ patterns of activity recorded during learning. Recordings from animals have shown nerve cells firing in the same order during sleep as they did while the animal explored a maze, but at a much faster speed. Whether the same process occurs in humans is harder to establish, since such recordings usually require electrodes placed inside the brain.
Sleep may also help the brain to forget. Some researchers argue that connections between nerve cells grow stronger throughout the waking day and are scaled back during sleep, so that only the most important ones remain strong. If this is correct, it would help to explain why a well-rested brain finds it easier to take in new information the following morning.
The practical implications are still debated. Students who stay up all night before an examination may remember less of what they studied, but research on short daytime naps has produced mixed results, and the ideal amount of sleep almost certainly differs from one person to another.