Evoked potentials are utilized in clinical settings to evaluate the integrity of a sensory pathway, at least up to the level of the primary sensory receiving area. These potentials can be recorded in comatose individuals, as well as in infants too young to undergo a sensory examination. The initial parts of the auditory evoked potential actually reflect activity in the brainstem, thus this evoked potential can be used to assess the function of brainstem structures.

The sleep-wake cycle, among other bodily functions, exhibits circadian periodicity of approximately 1 day. Changes in the EEG can be correlated with changes in the behavioral state during this cycle. An awake, aroused individual predominantly displays beta wave activity. The EEG is considered desynchronized during this state, exhibiting low-voltage, high-frequency activity. When relaxed individuals close their eyes, the dominant EEG waves are alpha waves. As a person falls asleep, they sequentially pass through four stages of slow-wave sleep (referred to as stages 1 through 4) over a period of 30 to 45 minutes. In stage 1, alpha waves are interspersed with lower frequency theta waves. In stage 2, the waves slow down further, but the slow-wave activity is interrupted by sleep spindles (bursts of activity at 12 to 14 Hz) and large K complexes (large, slow potentials). Stage 3 sleep is associated with delta waves and occasional sleep spindles. Stage 4 is characterized by delta waves without spindles.

During slow-wave sleep, muscle relaxation occurs with intermittent posture adjustments. The heart rate and blood pressure decrease, while gastrointestinal motility increases. The ease of awakening progressively decreases as individuals pass through these sleep stages. When individuals awaken, they pass through the sleep stages in reverse order.

Approximately every 90 minutes, slow-wave sleep transitions into a different form of sleep called rapid eye movement (REM) sleep. In REM sleep, the EEG once again becomes desynchronized. The low-voltage, fast activity seen in REM sleep resembles that observed in the EEG of an aroused individual. Due to the similarity of the EEG to that of an awake state and the difficulty in awakening a person, this type of sleep is characterized as paradoxical sleep. During REM sleep, muscle tone is completely lost, but phasic contractions occur in various muscles, particularly the eye muscles. These rapid eye movements give this type of sleep its name. Numerous autonomic changes also take place during REM sleep. Temperature regulation is lost, and pupils constrict. Penile erection may occur during this sleep stage. Heart rate, blood pressure, and respiration intermittently change. Several episodes of REM sleep occur each night, and although it is challenging to awaken a person from REM sleep, internal arousal is common. Most dreaming occurs during REM sleep.

The sleep-wake cycle has an endogenous periodicity of approximately 25 hours, but it typically becomes entrained to the day-night cycle. The suprachiasmatic nucleus of the hypothalamus appears to be the source of this circadian periodicity. This nucleus receives projections from the retina, and its neurons form a biological clock that adapts to the light-dark cycle. However, entrainment can be disrupted when a person is isolated from the environment or experiences changes in time zones (jet lag). Destruction of the suprachiasmatic nucleus disrupts several biological rhythms, including the sleep-wake cycle.

The proportion of slow-wave (non-REM) sleep to REM sleep varies with age. Newborns spend approximately half of their sleep time in REM sleep, whereas elderly individuals have minimal REM sleep. Around 20% to 25% of the sleep of young adults is REM sleep.
Evokedpotentialsareusedclinicallytoassesstheintegrityofasensorypathway,atleasttotheleveloftheprimarysensoryreceivingarea.Thesepotentialscanberecordedincomatoseindividuals,aswellasininfantstooyoungtoundergoasensoryexamination.Theinitialpartsoftheauditoryevokedpotentialactuallyreflectactivityinthebrainstem;therefore,thisevokedpotentialcanbeusedtoassessthefunctionofbrainstemstructures.
Sleep and wakefulness are among the many functions of the body that show circadian (about 1-day) periodicity. Characteristic changes in the EEG can be correlated with changes in the behavioral state during the sleep-wake cycle. Beta wave activity dominates in an awake, aroused individual. The EEG is said to be desynchronized; it displays low-voltage, high-frequency activity. In relaxed individuals with their eyes closed, the EEG is dominated by alpha waves (see
Fig. 10.5 ). A person falling asleep passes sequentially through four stages of slow-wave sleep (called stages 1 through 4) over a period of 30 to 45 minutes (see
Fig. 10.5 ). In stage 1, alpha waves are interspersed with lower frequency waves called theta waves. In stage 2, the waves slow further, but the slow-wave activity is interrupted by sleep spindles, which are bursts of activity at 12 to 14 Hz, and by large K complexes (large, slow potentials). Stage 3 sleep is associated with delta waves and with occasional sleep spindles. Stage 4 is characterized by delta waves without spindles.
During slow-wave sleep, the muscles of the body relax, but the posture is adjusted intermittently. The heart rate and blood pressure decrease, and gastrointestinal motility increases. The ease with which individuals can be awakened decreases progressively as they pass through these sleep stages. As individuals awaken, they pass through the sleep stages in reverse order.
About every 90 minutes, slow-wave sleep changes to a different form of sleep, called rapid eye movement (REM) sleep. In REM sleep, the EEG again becomes desynchronized. The low-voltage, fast activity of REM sleep resembles that seen in the EEG from an aroused subject (see
Fig.
10.5 , bottom trace). Because of the similarity of the EEG to that of an awake individual and the difficulty awaking the person, the term paradoxical sleep characterizes this type of sleep. Muscle tone is completely lost, but phasic contractions occur in a number of muscles, most notably the eye muscles. The resulting rapid eye movements are basis of the name for this type of sleep. Many autonomic changes also take place. Temperature regulation is lost, and meiosis occurs. Penile erection may occur during this type of sleep. Heart rate, blood pressure, and respiration change intermittently. Several episodes of REM sleep occur each night. Although it is difficult to arouse a person from REM sleep, internal arousal is common. Most dreaming occurs during REM sleep.
Thesleep-wakecyclehasanendogenousperiodicityofabout25hours,butitnormallybecomesentrainedtotheday-nightcycle.Thesourceofcircadianperiodicityappearstobethesuprachiasmaticnucleusofthehypothalamus.Thisnucleusreceivesprojectionsfromtheretina,anditsneuronsseemtoformabiologicalclockthatadaptstothelight-darkcycle.However,theentrainmentcanbedisruptedwhenthesubjectisisolatedfromtheenvironmentorchangestimezones(jetlag).Destructionofthesuprachiasmaticnucleusdisruptsanumberofbiologicalrhythms,includingthesleep-wakecycle.
The proportion of slow-wave (non-REM) sleep to REM sleep varies with age. Newborns spend about half of their sleep time in REM sleep, whereas elderly people have little REM sleep. About 20% to 25% of the sleep of young adults is REM sleep.