What Happens in Your Brain and Body When You Sleep?


A good night’s sleep is vital to your mental, physical and emotional health. But while it may seem as simple as shutting your eyes and drifting off, the process of sleep is far more complex than that. That’s why researchers spend much of their waking hours trying to understand what happens in your brain and body as you rest.

When you fall asleep, a signal from a part of your brain called the hypothalamus triggers an intricate series of sleep-inducing changes. This cascade triggers nerve cells in the brain to reduce alertness and slow electrical activity, and it also affects almost every cell in your body. Over the course of a night, you move through several cycles of sleep, each characterized by different depths of sleep and specific patterns of brain waves.

These stages of sleep — referred to as non-rapid eye movement (NREM) and rapid eye movement (REM) sleep — are essential to your brain’s health, as well as to the function of other parts of your body, including your heart and digestive tract. During NREM and REM sleep, your brain is especially active, processing memories and emotions, and regulating the production of important hormones, such as growth hormone and insulin.

You may be able to survive without sleep for short periods, but you won’t thrive. And when you don’t get enough sleep, you can suffer from a wide range of medical and mental health problems. In fact, a growing body of evidence shows that sleep deprivation is linked to a number of serious illnesses, including heart disease, stroke, high blood pressure, diabetes, obesity, depression and anxiety, as well as car accidents and workplace accidents.

Researchers have uncovered many of the fundamental processes that occur during slumber, and are discovering how to control them for optimal health. But there is still a lot to learn.

NREM Sleep and Memory

One of the biggest mysteries of sleep is why we need to sleep. While scientists have found that all mammals, birds and reptiles sleep in some form, they haven’t yet determined why this is important. One major clue came in 1983, when Stanford University professor Eugene Rechtschaffen found that rats kept from sleeping began to die within two weeks. That discovery led him to a more profound realization: that sleep is a basic biological necessity for all mammals.

The prevailing view among sleep scientists is that it is an active process rather than the passive state that most people think of it as being. The definition of sleep, for example, is based on the convergence of behavioral, motor, sensory and physiological criteria that are typically satisfied during a particular behavior. These criteria usually distinguish sleep from wakefulness, but sometimes one or more of them may be absent (e.g., when you are sleepwalking). Despite these limitations, there is no doubt that the body and brain are engaged in a highly productive activity during slumber. Consequently, a large field of medicine, known as sleep disorders, has evolved.