What Happens in the Brain During Sleep?

Sleep is vital for the health and well-being of humans. It plays a key role in many metabolic and regulatory functions, such as the maintenance of healthy blood pressure, glucose levels, and cholesterol, as well as the healing and repair of tissues and organs. A growing body of evidence indicates that sleep also plays important roles in memory consolidation, supporting normal immune function and healing after injury, and that it can help prevent weight gain and promote emotional stability.

In fact, without adequate sleep people have difficulty thinking, reacting, learning, and concentrating, making it difficult to make good choices or to perform well in work or school. Lack of sleep can also increase the risk of some chronic diseases and conditions, including heart disease, stroke, depression, diabetes, high blood pressure, psoriasis, Alzheimer’s disease, and cancer.

Much of what scientists know about sleep — including its stages and what happens in the brain during each stage — was learned thanks to the early sleep research done by UChicago professor Nathaniel Kleitman. His 1939 textbook, Sleep and Wakefulness, was a foundational text for the field. Kleitman founded the world’s first laboratory dedicated to sleep research and used all-night electroencephalogram (EEG) recordings to chart a sequence of brain activity during the night as he watched subjects fall asleep.

The EEG tracings during sleep show that when the body begins to prepare for sleep, the brain’s normal electrical signaling becomes irregular, with the brain’s activity shifting from rapid alpha waves to slower theta waves, and then to slow delta waves. During this phase, there are also short sequences of 11-15 Hz waves that are called sleep spindles. These rhythms appear in the thalamus and cortex, and their pattern varies between individuals. No single criterion has been identified that reliably marks the beginning of sleep.

When the brain enters REM (rapid eye movement) sleep, which occurs several times during a night’s sleep cycle, the irregular EEG patterns become even more abnormal and the body begins to act as though it were awake. That is why it is often difficult to wake someone up from this phase of sleep and they may feel drained or tired, even if they had a good night’s rest.

During this stage of sleep, the brain and body are resetting their internal clocks. The resetting is what allows us to maintain a regular, predictable wake-sleep cycle and helps regulate the circadian rhythms that govern body temperature, hunger, metabolism, growth, reproduction, and aging.

Without enough REM sleep, the body is unable to restore and repair the tissues that have been damaged during the day, which can lead to a host of health problems. Studies also show that a sufficient amount of REM sleep can help reduce the risk of cardiovascular disease by increasing good cholesterol and decreasing bad cholesterol, which decreases inflammation, and by lowering blood sugar levels and blood pressure. Getting adequate amounts of sleep is just as important for cardiovascular health as exercise, a nutritious diet, and non-smoking status.