Before the brain can interpret the world, it must first detect physical energy through sensory receptors. Sensation is the process of detecting this energy, while perception is the brain's interpretation and organization of that sensory information into meaningful experience. Psychologists have established important thresholds for measuring sensory ability. The absolute threshold is the minimum level of stimulation needed to detect a stimulus fifty percent of the time, such as the ability to see a candle flame from thirty miles away on a clear night. The difference threshold, or just noticeable difference, is the smallest difference between two stimuli that a person can detect half the time; Weber's Law states that this difference is a constant proportion of the original stimulus, not a fixed amount. Sensory systems also adapt over time, reducing sensitivity to constant, unchanging input. This is why you eventually stop noticing the feeling of clothes against your skin or the background hum of an air conditioner.
Consciousness, too, varies in predictable ways. Most adults cycle through stages of sleep roughly every ninety minutes, beginning with light NREM Stage 1 sleep, moving into Stage 2, which features sleep spindles, and deepening into Stage 3, also called slow-wave or deep sleep. The cycle then returns briefly through lighter stages before entering REM, or Rapid Eye Movement, sleep, which is characterized by fast brain waves, vivid dreaming, and temporary muscle paralysis. REM sleep is thought to play an important role in memory consolidation and emotional processing.
Underlying these cycles is the circadian rhythm, a roughly twenty-four-hour biological clock that regulates the sleep-wake cycle, body temperature, and the release of hormones. The circadian rhythm is influenced by environmental light and is controlled internally by the suprachiasmatic nucleus of the hypothalamus, helping to align the body's internal state with the external day-night cycle.