What Happens Inside the Brain During Sleep?

Sleep is the natural restorative period that allows your body to repair itself, restore your energy and clear out waste products. It’s also critical to your mental health: Research shows that people who get poor sleep tend to have a higher risk of depression, anxiety and other mood disorders.

Despite its importance, it is not yet known exactly what happens inside the brain during sleep. The fact that a large portion of brain activity is suppressed during sleep makes it challenging to pinpoint specific neuronal processes involved in the experience of sleeping. But research is beginning to reveal some of the important roles that different types of sleep play in the human brain and in the body.

Many people think of sleep as a passive activity that takes the body and brain down for a break, but it’s much more complex than that. Recent research has revealed that during sleep, the brain is active and that this activity can be categorized in distinct phases.

The scientific definition of sleep is based on convergence of behavioral, motor, sensory, and physiological criteria. While these criteria are not always met in all cases (for example, some sleepwalkers may appear to be awake while asleep), they are generally satisfied in most mammals, birds, and closely related reptiles. There is less evidence that all fish and insects sleep, and in many of those instances, the available behavioral and phenomenological criteria are not satisfactorily fulfilled.

Most people are familiar with the concept of sleep cycles, characterized by NREM and REM sleep, but there is more to the story than these two phases. In fact, research has uncovered many different phases of sleep and that brain activity can be both intense and passive, depending on the stage of the cycle.

A newer view of sleep is that it is a hierarchical process with both global and local determinants. For example, the occurrence and intensity of slow waves during sleep are largely controlled by subcortical inputs, but these changes can also be modulated locally within cortical networks. Slow waves can thus serve a dual role as both an initiation and a regulation of sleep.

This fine-grained spatio-temporal description of brain dynamics during sleep is making it increasingly likely that the waking state depends on the level of activity in certain parts of the brain, as demonstrated by the lower levels of SWA over prefrontal cortex observed in sleepers who are more prone to taking risks. The interplay between the varying states of the brain during sleep appears to have profound implications for the psychological and physical wellbeing of all individuals.

The interplay between sleep and a person’s psyche and physical health is evident in the way that it correlates with a wide variety of conditions, from chronic pain to cardiovascular disease. Research has also shown that getting sufficient sleep can help improve a person’s quality of life as much as other proven pillars of health, such as exercise, a healthy diet and nonsmoking.