Sleep

Here is the rewritten encyclopedia entry about **Sleep**:

Sleep is a biological state where both mental and physical activity wind down. Consciousness shifts, and the brain dials back its processing of sensory information. Muscle movement drops, and a person’s connection to their surroundings fades. While a sleeping person is less responsive to stimuli than when awake, the brain remains active—far more so than in a coma or other disorders of consciousness.

The body cycles through sleep in repeating patterns, shifting between two main modes: rapid eye movement (REM) sleep and non-REM sleep. Despite its name, REM sleep involves more than just fast eye movements; it also brings a near-total paralysis of the body. Dreams—spontaneous sequences of images, ideas, emotions, and sensations—typically occur during certain sleep stages.

Most of the body’s systems enter an anabolic, or restorative, state during sleep. This helps repair the immune, nervous, skeletal, and muscular systems, and supports mood, memory, and cognitive function. Sleep also plays a major role in the endocrine and immune systems. An internal circadian clock naturally promotes sleep at night when it is dark, though daytime naps may also be beneficial—current research is exploring the value of power naps. Scientists are still actively investigating why we sleep and how it works, but it is a highly conserved behavior across the animal kingdom, likely dating back hundreds of millions of years. One leading theory is that sleep originally evolved as a way for the brain to flush out waste, including amyloid proteins.

Humans can experience a range of sleep disorders. These include dyssomnias like insomnia, hypersomnia, narcolepsy, and sleep apnea; parasomnias such as sleepwalking and REM sleep behavior disorder; bruxism (teeth grinding); and circadian rhythm sleep disorders. Artificial light has dramatically changed human sleep patterns. Outdoor lighting and the screens of smartphones and televisions emit large amounts of blue light—a type of light normally associated with daytime—which disrupts the release of melatonin, the hormone that regulates the sleep cycle.

**Physiology**

The most significant physiological changes during sleep happen in the brain. It uses much less energy than when awake, especially during non-REM sleep. In areas where activity drops, the brain restocks its supply of adenosine triphosphate (ATP), the molecule used for short-term energy storage and transport. Since the brain accounts for about 20% of the body’s energy use during quiet wakefulness, this reduction has a noticeable impact on overall energy consumption.

Sleep also raises the sensory threshold. A sleeping person perceives fewer stimuli but can still respond to loud noises or other salient events. During slow-wave sleep, the body releases bursts of growth hormone, and all sleep—even during the day—is linked to the secretion of prolactin.

Researchers monitor sleep using several key physiological methods: electroencephalography (EEG) for brain waves, electrooculography (EOG) for eye movements, and electromyography (EMG) for skeletal muscle activity. When these measurements are collected simultaneously, it is called polysomnography, often performed in a sleep laboratory. Simplified electrocardiography (EKG) tracks cardiac activity, and actigraphy records motor movements.

**Brain Waves in Sleep**

The electrical activity captured by an EEG appears as brain waves. The amplitude of these waves at specific frequencies corresponds to different points in the sleep-wake cycle—whether a person is asleep, awake, or drifting off. Alpha, beta, theta, gamma, and delta waves all appear during various sleep stages, each with a distinct frequency and amplitude. Alpha waves occur when someone is resting but fully conscious, perhaps with eyes closed and body still, beginning to slow down. Beta waves take over when a person is alert and focused on a task or concentrating; they have the highest frequency and lowest amplitude. Gamma waves appear during intense focus or concentration. Theta waves emerge while a person is awake and continue as they transition into Stage 1 and Stage 2 of sleep. Delta waves are seen in Stages 3 and 4, the deepest sleep.

**Non-REM and REM Sleep**

Sleep is broadly divided into two types: non-rapid eye movement (non-REM or NREM) sleep and rapid eye movement (REM) sleep. They are so different that physiologists consider them distinct behavioral states. Non-REM sleep occurs first, and after a transitional period, it becomes slow-wave or deep sleep. During this phase, body temperature and heart rate drop, and the brain uses less energy. REM sleep, also called paradoxical sleep, makes up a smaller portion of total sleep time. It is the primary time for dreams (or nightmares) and is marked by desynchronized, fast brain waves, eye movements, loss of muscle tone, and a suspension of homeostasis.

A typical sleep cycle—alternating between NREM and REM—averages about 90 minutes and repeats four to six times during a good night’s sleep. The American Academy of Sleep Medicine (AASM) divides NREM into three stages: N1, N2, and N3 (the last also called delta or slow-wave sleep). The normal progression is N1 → N2 → N3 → N2 → REM. REM sleep occurs as a person returns from deep sleep to Stage 2 or Stage 1.