Sleep is a dynamic and organized biological process. Electroencephalography (EEG) is the primary method for monitoring brain activity during sleep. This technology records the brain’s electrical signals, revealing distinct patterns that indicate different sleep stages.
Understanding EEG Signals During Sleep
Electroencephalography measures the electrical activity produced by the brain’s neurons. Electrodes placed on the scalp detect these impulses, which are amplified and recorded as wave patterns, providing a window into the brain’s ongoing state. The frequency and amplitude of these brain waves vary depending on whether a person is awake or in a particular sleep stage.
Brain waves are categorized by their frequency. Beta waves (13 to 30 Hz) are prominent during alert wakefulness and active thinking. Alpha waves (8 to 12 Hz) appear when a person is awake but relaxed.
Slower theta waves (4 to 7 Hz) become apparent as sleep onset approaches. Delta waves (0.5 to 4 Hz) are characteristic of deep sleep. Changes in these waves differentiate between wakefulness and sleep stages.
The Journey Through Sleep Stages
A night’s sleep progresses through distinct stages, beginning with non-rapid eye movement (NREM) sleep and culminating in rapid eye movement (REM) sleep. A typical sleep cycle, lasting approximately 90 to 110 minutes, involves a sequential passage through NREM Stage 1, NREM Stage 2, NREM Stage 3, and then REM sleep. Most adults experience four to six such cycles over a full night of sleep.
NREM Stage 1
This stage marks the transition from wakefulness to sleep and is considered very light sleep. Brain activity slows, and theta waves become prevalent. Heart rate and breathing gradually reduce, and muscles begin to relax. People are easily awakened from this stage, sometimes reporting not having been asleep at all.
NREM Stage 2
This is a deeper, yet relatively light, stage of sleep, making up the largest portion of a night’s sleep. The EEG shows theta waves, along with sleep spindles and K-complexes. Sleep spindles are bursts of brain activity lasting 0.5 to 1.5 seconds, while K-complexes are single, high-amplitude waves that stand out from the background EEG. Heart rate and body temperature continue to decrease, and eye movements cease.
NREM Stage 3
Often referred to as slow-wave sleep or deep sleep, this is the deepest stage of NREM sleep. This stage is characterized by the dominance of high-amplitude, slow delta waves on the EEG. Physiological activity reaches its lowest point, with reduced heart rate, breathing, and muscle tone. It is most difficult to awaken someone from NREM Stage 3, and if awakened, they may experience temporary disorientation.
REM Sleep
Following NREM sleep, the brain enters REM sleep, a paradoxically active state. The EEG during REM sleep resembles that of an awake person, showing low-amplitude, mixed-frequency activity. Despite this active brain state, muscles become temporarily paralyzed, a state known as atonia, preventing individuals from acting out their dreams.
Rapid eye movements occur beneath closed eyelids, and breathing and heart rate become more irregular. Dreaming is most vivid and frequently recalled during REM sleep.
The Purpose of Sleep Stages
The cyclical progression through sleep stages serves various biological functions, contributing to overall health and cognitive performance. Each stage plays a distinct role in the body’s nightly restorative and organizational processes. The deepest NREM sleep, NREM Stage 3, is particularly important for physical restoration.
During NREM Stage 3, the body undergoes significant repair and growth. Growth hormone release peaks, supporting tissue repair and cellular regeneration. This deep sleep also plays a role in immune system support.
REM sleep is primarily associated with cognitive and emotional processing. This stage aids memory consolidation, where newly acquired information is processed and integrated into long-term memory. REM sleep also contributes to emotional regulation and processing daily experiences.
Beyond these roles, sleep stages collectively facilitate brain waste removal, clearing metabolic byproducts accumulated during wakefulness, and energy conservation.