Consciousness and sleep
Consciousness
Consciousness is your awareness of both internal sensations and the external world. This includes:
- Internal awareness: Recognizing bodily signals like pain, hunger, thirst, and sleepiness, as well as your own thoughts and emotions.
- External awareness: Perceiving environmental stimuli such as sunlight, warmth, and the sound of a friend’s voice.
Consciousness exists on a continuum - from full alertness during wakefulness to the deep inactivity of sleep. Wakefulness involves high sensory awareness, active thought, and purposeful behavior. Sleep, in contrast, involves reduced physical activity and decreased sensory perception. Other states along this continuum include daydreaming, intoxication, and medically induced unconsciousness.
States of consciousness
Alertness
Alertness is the ability to stay attentive and focused on the surrounding environment.
- Influences: Regulated by the reticular activating system (RAS) in the brainstem and coordinated with the body’s natural 24-hour circadian rhythm.
- Disruptions: Can be affected by drugs, toxins, head injuries, or conditions like ADHD, depression, and chronic fatigue.
Sleep
Sleep is a distinct state characterized by diminished sensory awareness and low physical activity. It’s different from simply resting while awake.
- Measurement: Polysomnography monitors sleep by recording eye movements (EOG), brain electrical activity (EEG), and muscle activity (EMG).
Brain waves and sleep stages
- Awake: Dominated by beta waves - high frequency () and low amplitude.
- NREM sleep: Divided into three stages:
- Stage 1: A brief transition into sleep. Brain activity slows (alpha waves transitioning to theta waves), and heart rate, respiration, muscle tension, and body temperature decrease. This stage is easily disrupted.
- Stage 2: Characterized by continued theta wave activity interspersed with sleep spindles - brief bursts of faster brain waves that may support learning and memory.
- Stage 3 (deep sleep): Marked by low-frequency, high-amplitude delta waves (less than ). Physiological processes slow significantly, and sleep becomes deeper and harder to disrupt.
- REM sleep: Distinguished by rapid eye movements, brain wave patterns similar to wakefulness, and temporary paralysis of most voluntary muscles. This stage is associated with vivid dreaming.
- REM rebound: When someone is deprived of REM sleep, the body tends to compensate with longer REM periods once sleep is allowed.
Circadian rhythms and sleep regulation
- Circadian rhythm: A roughly 24-hour cycle that governs sleep-wake patterns and is influenced by natural light-dark cycles.
- Suprachiasmatic nucleus (SCN): Located in the hypothalamus, this internal clock receives light input from the retina to synchronize circadian rhythm.
- Melatonin: A hormone from the pineal gland that promotes sleep. It’s released in response to darkness and suppressed by light.
- Chronotypes: Individual differences in sleep patterns, such as being a morning person or a night owl.
Sleep cycles and changes throughout the night
A complete sleep cycle lasts about 90 minutes. It typically moves through NREM stages (1 to 3) and then transitions into REM sleep. As the night goes on, deep sleep (stage 3) becomes shorter, while REM periods become longer.
Theories on the functions of sleep
Dreaming
Occurrence: Dreams mainly occur during REM sleep.
Sleep-wake disorders
Parasomnias: Abnormal behaviors during sleep, such as sleepwalking, restless leg syndrome, and night terrors. These typically occur during slow-wave (deep) sleep.
Dyssomnias: Disruptions in sleep quantity, quality, or timing, including:
- Sleep apnea: Episodes where breathing stops temporarily, leading to fragmented sleep and daytime fatigue.
- Narcolepsy: Sudden, uncontrollable sleep episodes, sometimes accompanied by muscle weakness (cataplexy).
- Insomnia: Persistent difficulty falling or staying asleep, often linked to stress and sometimes treated with medication.