Newborn full-term infants spend approximately 50 percent of their sleep in the REM stage (although their EEG and eye movements differ from those of adults). The newborn sleep cycle lasts about 60 min (50 percent REM, 50 percent NREM, generally alternating through a 3to 4-h interfeeding period); with age, the sleep cycle lengthens to 90 to 100 min. Approximately 20 to 25 percent of total sleep time in young adults is spent in REM sleep, 3 to 5 percent in stage N1, 50 to 60 percent in stage N2, and 10 to 20 percent in stage N3 combined. The amount of sleep in N3 decreases with age, and persons older than 70 years of age have virtually no very deep slow-wave sleep (Fig. 18-3). The 90to 100-min cycle is fairly stable in any one person and is believed to continue to operate to a less-perceptible degree during wakefulness in relation to a number of other cyclic phenomena, such as core body temperature, gastric motility, hunger, urinary output, alertness, and capacity for cognitive activity.
A comparison of the physiologic changes in NREM and REM sleep is of interest. The change in the EEG pattern has already been indicated. Cortical neurons tend to discharge in synchronized bursts during NREM sleep and in nonsynchronized bursts during wakefulness. In REM sleep, the EEG pattern is generally asynchronous as well. Much of the night’s complex visual dreaming has been found to occur in the REM period, with the qualifications noted below, and dreams are recalled most consistently if the subject is awakened during this time. It is important to point out, however, that dreaming activity is reported by subjects awakened from NREM sleep, although less consistently. Because the overall time spent in NREM is so much greater than that in REM, approximately 20 percent of dreaming occurs outside of REM periods but REM sleep nonetheless maintains a special relationship to dreaming.
Subjects are easily aroused from REM sleep, but arousing a person during stage N3 is more difficult; full arousal may take minutes or more, during which time the subject may be slightly disoriented and confused (for which reason, physicians called at night should, if possible, avoid making complex medical decisions during this brief period).
As mentioned previously, tonic muscle activity is minimal during REM sleep, although small twitches in facial and digital muscles (hand and foot) can still be detected. Eye movements of REM sleep are conjugate and occur in all directions (horizontal more than vertical). They can be appreciated through the closed eyelids. Gross body movements occur every 15 min or so in all stages of sleep but are maximal in the transition between REM and NREM sleep, at which time the sleeping person changes position, usually from side to side (most people sleep on their sides).
On closer study, REM sleep has been found to have phasic and tonic components. In addition to the REMs, phasic phenomena include activation of the sympathetic nervous system with attendant alternate dilatation and constriction of the pupils and fluctuation of the blood pressure, heart rate, and respiration. The phasic activities are related to bursts of neuronal activity in the pontine, vestibular, and median raphe nuclei and are conducted through the corticobulbar and corticospinal tracts. In the nonphasic periods of REM sleep, alpha and gamma spinal neurons are inhibited, the H responses diminish (see Chap. 43), and the tendon and postural and flexor reflexes diminish or are abolished. This flaccidity and atonia, which are prominent in the abdominal, upper airway, and intercostal muscles, may compromise breathing during REM sleep and pose a threat to infants with respiratory difficulty and to adults who are obese or have respiratory difficulty as a result of kyphoscoliosis, muscular dystrophy, hypoplastic or otherwise compromised airways, and neuromuscular paralyses.