The average person spends one-quarter to a third of their lives sleeping. But sleep is more than just a time when the body is at rest; it’s critical to physical and mental health, and plays a role in everything from energy levels to memory. In fact, getting enough quality sleep is a key preventive medicine against diseases such as heart disease and diabetes, as well as stress and obesity.

In the past, many believed that sleep was a passive state, during which the body shuts down and takes a break. But scientists now know that it isn’t quite so simple. Rather, sleep is a highly active process during which the day’s events are processed and the body restores energy.

It is also during sleep that memories are consolidated, and it is for this reason that people who do not get adequate amounts of sleep often find themselves with poorer memory. However, not everyone with a poor memory is deficient in sleep; instead they may simply lack the proper amount of early phase slow-wave sleep (deep sleep). This stage is essential for the consolidation of new memories and for transferring existing ones to long-term storage.

During sleep, the brain also makes connections, or associations, between events and emotions. This is especially important in the formation of long-term memory, which requires deep sleep to consolidate. Therefore, people who do not get sufficient quantities of sleep have a harder time recalling what happened the previous day, and can also have trouble with complex thinking, such as planning, problem-solving or decision making.

The discovery of REM sleep, or rapid eye movement sleep, revolutionized the way scientists thought about sleep. This is the stage during which we dream, and it seems more like activated wakefulness than other phases of sleep. In REM sleep, the eyes move rapidly behind closed lids and brain waves are similar to those during wakefulness.

Scientists have determined that the brain goes through a series of four or five alternating cycles of non-REM and REM sleep each night. The waking period between these cycles is controlled by the brainstem, which also controls breathing and blood pressure. The rhythm of these processes, as well as the changes in activity between non-REM and REM sleep, seem to be coordinated by chemical messengers that are released from nerve cells.

Researchers are now studying what goes on in the brain during different parts of the sleep cycle, in order to better understand how this coordination is accomplished. They are also determining why certain individuals have difficulty with sleeping, such as those who suffer from insomnia or other sleep disorders. These issues have been shown to be linked to a number of medical and behavioral conditions, such as depression, obesity, cardiovascular disease and diabetes. In fact, in some studies, the risk of these conditions is even higher in those who do not get enough sleep. This is why incorporating the American Heart Association’s “Life’s Simple 7” of healthy habits, which includes sufficient sleep duration, has been shown to reduce these risks.