Spontaneous arousal activity in infants during NREM and REM sleep

The infant arousal response involves subcortical and cortical responses occurring as a sequence of stereotyped behaviour regardless of the eliciting stimulus. The spontaneous activity of these responses during sleep, however, is uncertain. We examined the spontaneous arousal pattern in normal infants to determine the sequence of responses, and to examine their periodicity and the effects of sleep state. We performed a nap polysomnographic study on 10 normal infants between 2 and 10 weeks of age. Electroencephalographic and electro-oculographic activity, and respiratory airflow and movements were measured, and video recordings were made throughout each study. Different levels of arousal behaviour were examined. We found that spontaneous arousal activity occurred frequently and the majority of responses occurred as a sequence involving an augmented breath followed by a startle and then cortical arousal. Subcortical arousals as reflected by augmented breaths and startles were more common than cortical arousals. Additionally, augmented breaths followed by apnoea were recorded and were not usually associated with other arousal responses. All of the responses occurred periodically either as bursts of activity or as isolated responses. Each of the responses occurred more frequently during rapid eye movement (REM) sleep than during non-rapid eye movement (NREM) sleep. We conclude that there is an endogenous rhythm of spontaneous activity in infants involving excitatory processes from the brainstem, which may or may not be closely followed by cortical excitation. The spontaneous arousal responses occur periodically but with a high level of irregularity and the level of activity is affected by sleep state.     

Survival analysis indicates that age-related decline in sleep continuity occurs exclusively during NREM sleep

A common complaint of older persons is disturbed sleep, typically characterized as an inability to return to sleep after waking. As every sleep episode (i.e., time in bed) includes multiple transitions between wakefulness and sleep (which can be subdivided into rapid eye movement [REM] sleep and non-REM [NREM] sleep), we applied survival analysis to sleep data to determine whether changes in the “hazard” (duration-dependent probability) of awakening from sleep and/or returning to sleep underlie age-related sleep disturbances. The hazard of awakening from sleep—specifically NREM sleep—was much greater in older than in young adults. We found, however, that when an individual had spontaneously awakened, the probability of falling back asleep was not greater in young persons. Independent of bout length, the number of transitions between NREM and REM sleep stages relative to number of transitions to wake was approximately 6 times higher in young than older persons, highlighting the difficulty in maintaining sleep in older persons. Interventions to improve age-related sleep complaints should thus target this change in awakenings.     

Activity of respiratory neurons during NREM sleep

The purpose of this study was to analyze differences in the activity of medullary respiratory neurons in the unanesthetized, intact cat during wakefulness and non-rapid-eye-movement (NREM) sleep. We studied single respiratory neurons located within a 1-2 mm deep, 8-10 mm long zone that followed, and included in its dorsal aspect, the retrofacial and ambiguus nuclei. The analysis of variance was used to detect respiratory activity, and cycle-triggered histograms were plotted. The respiratory signal strength and consistency of the respiratory activity were quantified with the eta 2 statistic. We determined for each breath in wakefulness and NREM sleep the average discharge rate during the active phase of the cell, the number of action potentials during the active phase of the cell, and durations of both the cycle and inspiration. Differences in discharge rates and in the number of discharges between wakefulness and NREM sleep were tested with the t test. A bimodal distribution of eta 2 values for the population of neurons indicated there were two groups of respiratory cells: those with eta 2 values less than 0.3 and those with values greater than 0.3. The former we call weak respiratory cells; the latter, strong respiratory cells. Strong and weak cells were classified further as inspiratory or noninspiratory on the basis of the shape of their cycle-triggered histograms. Within the class of strong inspiratory cells, those with the highest eta 2 values 1) reached their peak discharge rate early, 2) discharged at high rates throughout inspiration, and 3) were inactive during expiration. The values of these variables diminished progressively in inspiratory cell groups with lower eta 2 values. Most cells were less active in NREM sleep than in wakefulness. Similar proportions of weak and strong cells and inspiratory and noninspiratory cells were affected by sleep. The reduction in sleep of the activity of strong inspiratory cells was consistent with a general relationship between this activity and the duration of inspiration. Lower discharge rates were associated with longer breaths; higher rates with shorter breaths. This relationship existed within both NREM sleep and wakefulness, and the plot of the relationship across these states formed a continuous function. The reduction in discharge rate in sleep was greater for weak than for strong inspiratory cells: the correlation coefficient between percent change in rate and eta 2 values was -0.636 for inspiratory cells, but it was not significant (-0.265) for noninspiratory cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Long-term facilitation of ventilation in humans during NREM sleep

The purpose of this study was to determine whether episodic hypoxic exposure would elicit long term facilitation (LTF) of ventilation (V(I)) in sleeping humans. Twenty subjects gave written informed consent. Of these, six subjects were unable to maintain stable stage 2 sleep or deeper for a majority of the experiment and their data were excluded from the analysis. On night 1 after subjects had reached stable sleep (stage 2 or deeper), the subjects breathed room air for 5 minutes, followed by 3 minutes of hypoxia (F(I)O2 = 8%). This sequence was repeated 10 times, and the breathing pattern was observed for a further 60 minutes. Subjects returned to the laboratory for a second visit, which served as a sham night. Instrumentation and study time were the same as on night 1, but subjects breathed room air only. Airflow, tidal volume (V(T)), end tidal O2 and CO2, and estimation of arterial O2 saturation (%) were measured. Seven of the subjects had long-term facilitation (LTF), which was manifested as a significant increase in V(I) that persisted for up to 40 minutes following the last hypoxic exposure. In the other seven subjects, no substantial increase in V(I) was found. We could not explain this difference based on body size (BMI), gender, level of hypoxemia, or magnitude of the hyperpnea during hypoxia. The difference between the two groups was that the LTF group consisted of habitual snorers, and that the NLTF were not inspiratory-flow-limited during the experiment.     

Ventilatory long-term facilitation in non-snoring subjects during NREM sleep

We hypothesized that very brief episodes of hypoxia (<1 min) would evoke long-term facilitation (LTF) in individuals free of inspiratory flow limitation (IFL). We studied 12 healthy participants who were self-reported non-snorers and confirmed the absence of IFL. We induced 15 brief episodes of hypoxia during non-REM sleep, reducing arterial oxygen saturation to 84-85%, followed by 1 min of room air. Ventilatory variables and resistance were measured during the control period, hypoxic trials, room air controls, and for 20 min following the last hypoxic episode. There was a significant increase in minute ventilation (108+/-1.3% of control, P < 0.05) and tidal volume (105+/-1.7% of control, P < 0.05) and a significant decrease in upper airway resistance (88+/-9.8% control, P < 0.05) during the recovery period. However, there were no significant changes in any variable during sham studies. We have shown for the first time that LTF can be elicited in sleeping humans free of IFL.     

Auditory event-related potentials during stage 2 NREM sleep in humans

SUMMARY  Event-related potential (ERP) recordings were used to investigate the nature of auditory stimulus evaluation during stage 2 sleep. Frequent and rare stimuli, differing in intensity and frequency, were presented to six adult subjects while awake and asleep. The latency and voltage distribution of one of the long-latency components evoked during sleep resembled the P3 component evoked while awake. However, it was attenuated in voltage and superimposed on N3, a large late negative component, most probably the slow potential of the K complex. The identification of a P3-like potential during sleep suggests that the P3 potential is not solely a marker of active cognitive processes, but contains a small component which reflects automatic, pre-attentive evaluation of deviant stimuli.     

Asymmetrical auditory probe evoked potentials during REM and NREM sleep

Two experiments were conducted to evaluate interhemispheric differences in late auditory evoked potentials (AEPs) during rapid-eye-movement (REM), Stage 2, and Stage 4 sleep. In the first study, 1,000-Hz stimuli [80 db (SPL)] were presented binaurally at a rate of 1/1.1 s. Analyses of variance were computed on the absolute difference in the amplitude of right and left evoked responses. N1 AEPs were significantly more asymmetrical in Stage 4 sleep compared to either REM or Stage 2 sleep. A second study was conducted with a wider topographical electrode distribution. This study used both a long and a short interstimulus interval with 500-Hz 80-db SPL tone pips. The absolute difference in the amplitude of right and left AEPs was compared across sleep stage. Asymmetries were larger in Stage 4 sleep than in either REM or Stage 2. Examination of these data indicated relative hemispheric balance in REM and Stage 2 sleep with largest asymmetries in Stage 4. The results do not support the view that REM and non-REM sleep stages are associated with differential activation of the two cerebral hemispheres. Rather, they suggest that the sleep cycle is characterized by variations in the degree of asymmetry. Asymmetries in Stage 4 sleep were not consistently in favor of the left hemisphere.     

Genioglossal muscle response to CO2 stimulation during NREM sleep

STUDY OBJECTIVES: The objective was to evaluate the responsiveness of upper airway muscles to hypercapnia with and without intrapharyngeal negative pressure during non-rapid eye movement (NREM) sleep and wakefulness. DESIGN: We assessed the genioglossal muscle response to CO2 off and on continuous positive airway pressure (CPAP) (to attenuate negative pressure) during stable NREM sleep and wakefulness in the supine position. SETTING: Laboratory of the Sleep Medicine Division, Brigham and Women's Hospital. PATIENTS OR PARTICIPANTS: Eleven normal healthy subjects. INTERVENTIONS: During wakefulness and NREM sleep, we measured genioglossal electromyography (EMG) on and off CPAP at the normal eupneic level and at levels 5 and 10 mm Hg above the awake eupneic level. MEASUREMENTS AND RESULTS: We observed that CO2 could increase upper-airway muscle activity during NREM sleep and wakefulness in the supine position with and without intrapharyngeal negative pressure. The application of nasal CPAP significantly decreased genioglossal EMG at all 3 levels of PETCO2 during NREM sleep (13.0 +/- 4.9% vs. 4.6 +/- 1.6% of maximal EMG, 14.6 +/- 5.6% vs. 7.1 +/- 2.3% of maximal EMG, and 17.3 +/- 6.3% vs. 10.2 +/- 3.1% of maximal EMG, respectively). However, the absence of negative pressure in the upper airway did not significantly affect the slope of the pharyngeal airway dilator muscle response to hypercapnia during NREM sleep (0.72 +/- 0.30% vs. 0.79 +/- 0.27% of maximal EMG per mm Hg PCO2, respectively, off and on CPAP). CONCLUSIONS: We conclude that both chemoreceptive and negative pressure reflex inputs to this upper airway dilator muscle are still active during stable NREM sleep.     

Response properties of long-latency event-related potentials evoked during NREM sleep

The experiments reported here sought to investigate whether the K-complex evoked during sleep is comprised of activity from two separate physiological systems with different response properties. To that end, the parameters of stimulation in a two tone auditory 'odd-ball' task were varied systematically as stimuli were presented to subjects during NREM sleep. During experiment 1, the frequency (pitch) of the odd-ball stimulus varied systematically while intensity (loudness) was matched between tones. During experiment 2, pitch was matched between tones while the loudness of the odd-ball stimulus was varied. The long-latency event-related potentials (ERPs) N2 and P3 could be dissociated from the K-complex (N3 and P4) in response to these parametric manipulations. Information processing occurs during sleep, and is reflected in ERPs with a morphology largely analogous to those observed under similar conditions while subjects are awake. The second (K-complex) system is sleep specific. A model was constructed to explain the activity of these two hypothesized systems. As predicted by the model, K-complex latency was longer in Stage 2 when N2 and P3 were also active, than in Stage 4 where N2-P3 activity was lessened. These results support the two-system hypothesis; electrical brain activity evoked during sleep should not be considered a unitary sleep-specific response. Furthermore, the data indicate that the K-complex is sensitive to the physical characteristics of stimuli, that the sleeping brain processes information to a high degree, and that the 'endogenous' components of the ERP observed in awake humans reflect more automatic processes than previously suspected.     

Clonidine has a paradoxical effect on cyclic arousal and sleep bruxism during NREM sleep

Study Objective:

Clonidine disrupts the NREM/REM sleep cycle and reduces the incidence of rhythmic masticatory muscle activity (RMMA) characteristic of sleep bruxism (SB). RMMA/SB is associated with brief and transient sleep arousals. This study investigates the effect of clonidine on the cyclic alternating pattern (CAP) in order to explore the role of cyclic arousal fluctuation in RMMA/SB.

Design:

Polysomnographic recordings from a pharmacological study.

Setting:

University sleep research laboratory.

Participants and Interventions:

Sixteen SB subjects received a single dose of clonidine or placebo at bedtime in a crossover design.

Measurements and Results:

Sleep variables and RMMA/SB index were evaluated. CAP was scored to assess arousal instability between sleep-maintaining processes (phase A1) and stronger arousal processes (phases A2 and A3). Paired t-tests, ANOVAs, and cross-correlations were performed. Under clonidine, CAP time, and particularly the number of A3 phases, increased (P ≤ 0.01). RMMA/SB onset was time correlated with phases A2 and A3 for both placebo and clonidine nights (P ≤ 0.004). However, under clonidine, this positive correlation began up to 40 min before the RMMA/SB episode.

Conclusions:

CAP phase A3 frequency increased under clonidine, but paradoxically, RMMA/SB decreased. RMMA/SB was associated with and facilitated in CAP phase A2 and A3 rhythms. However, SB generation could be influenced by other factors besides sleep arousal pressure. NREM/REM ultradian cyclic arousal fluctuations may be required for RMMA/SB onset.

Citation:

Carra MC; Macaluso GM; Rompré PH; Huynh N; Parrino L; Terzano MG; Lavigne GJ. Clonidine has a paradoxical effect on cyclic arousal and sleep bruxism during NREM sleep. SLEEP 2010;33(12):1711-1716.     

Resetting the baroreflex during snoring: Role of resistive loading and intra-thoracic pressure

Baroreflex sensitivity (BRS) is reduced during snoring in humans and animal models. We utilised our rabbit model to examine the contribution of increased upper airway resistance to baroreflex resetting during snoring, by comparing BRS and baroreflex operating point (OP) values during IS to those obtained during tracheostomised breathing through an external resistive load (RL) titrated to match IS levels of peak inspiratory pleural pressure (Ppl). During both IS and RL, BRS decreased by 45% and 49%. There was a linear relationship between the change in Ppl and the decrease in BRS, which was similar for IS and RL. During both RL and IS, there was a shift in OP driven by ～16% increase in HR and no change in arterial pressure. Snoring related depression of BRS is likely mediated via a HR driven change in OP, which itself may be the outcome of negative intra-thoracic pressure mediated effects on right atrial wall stretch reflex control of heart rate.     

Increased EEG sigma and beta power during NREM sleep in primary insomnia

The hyperarousal model of primary insomnia suggests that a deficit of attenuating arousal during sleep might cause the experience of non-restorative sleep. In the current study, we examined EEG spectral power values for standard frequency bands as indices of cortical arousal and sleep protecting mechanisms during sleep in 25 patients with primary insomnia and 29 good sleeper controls. Patients with primary insomnia demonstrated significantly elevated spectral power values in the EEG beta and sigma frequency band during NREM stage 2 sleep. No differences were observed in other frequency bands or during REM sleep. Based on prior studies suggesting that EEG beta activity represents a marker of cortical arousal and EEG sleep spindle (sigma) activity is an index of sleep protective mechanisms, our findings may provide further evidence for the concept that a simultaneous activation of wake-promoting and sleep-protecting neural activity patterns contributes to the experience of non-restorative sleep in primary insomnia.     

The determinants of the apnea threshold during NREM sleep in normal subjects

STUDY OBJECTIVE: To determine whether (1) postmenopausal women have a higher apnea threshold than premenopausal women and men and (2) hormone replacement therapy would decrease the apnea threshold in postmenopausal women. DESIGN: Protocol #1: Analysis of a prospectively collected database of 55 subjects who had undergone an apnea-threshold protocol. Protocol #2: Intervention study: apnea threshold compared in 6 postmenopausal women before and after 30 days of replacement therapy with progestin and estrogen. SETTING: Research sleep laboratory. PARTICIPANTS: Healthy volunteers aged 18 to 65 years without evidence of sleep-disordered breathing. INTERVENTIONS: Hypocapnia was induced via nasal mechanical ventilation for 3 minutes during stable non-rapid eye movement sleep. Cessation of mechanical ventilation resulted in hypocapnic central apnea or hypopnea, depending upon the magnitude of the hypocapnia. The change in endtidal CO2 at the apnea threshold was defined as the change in end-tidal CO2 associated with the apnea closest to the last hypopnea. MEASUREMENTS AND RESULTS: The change in the end-tidal CO2 at the apnea threshold was highest in the premenopausal women (4.6+/-0.6 mm Hg), with no difference between the postmenopausal women (3.1+/-0.5 mm Hg) and men (3.4+/-0.7 mm Hg). Determinants of the change in endtidal CO2 at the apnea threshold included sex and menopause status. Hormone replacement therapy increased the change in end-tidal CO2 at the apnea threshold from 2.9+/-0.4 mm Hg to 4.8+/-0.4 mm Hg (P<.001). CONCLUSIONS: These data support the hypothesis that estrogens and progestins positively influence the apnea threshold and control of breathing during non-rapid eye movement sleep.     

Increased EEG sigma and beta power during NREM sleep in primary insomnia

The hyperarousal model of primary insomnia suggests that a deficit of attenuating arousal during sleep might cause the experience of non-restorative sleep. In the current study, we examined EEG spectral power values for standard frequency bands as indices of cortical arousal and sleep protecting mechanisms during sleep in 25 patients with primary insomnia and 29 good sleeper controls. Patients with primary insomnia demonstrated significantly elevated spectral power values in the EEG beta and sigma frequency band during NREM stage 2 sleep. No differences were observed in other frequency bands or during REM sleep. Based on prior studies suggesting that EEG beta activity represents a marker of cortical arousal and EEG sleep spindle (sigma) activity is an index of sleep protective mechanisms, our findings may provide further evidence for the concept that a simultaneous activation of wake-promoting and sleep-protecting neural activity patterns contributes to the experience of non-restorative sleep in primary insomnia.     

Accessing previous mental sleep experience in REM and NREM sleep

This study investigated the processes by which contents previously stored in memory are retrieved and inserted into mental sleep experience (MSE). MSE reports were collected from six subjects awakened three times on each of eight nights in two alternate sequences of awakenings (NREM-REM-NREM; REM-REM-REM). The occurrences of interrelations between contents of report pairs were scored using Clark's (1970) feature matching model. These were greater for same night pairs than for different night pairs, and did not differ with respect to sequence of awakenings or order of report pairs (first-second, second-third, first-third). Contents of previous MSEs, therefore, seem to be accessible in both sleep types for insertion into current MSE. The interrelated units were more frequently lexical than propositional, with more paradigmatic than syntagmatic relationships in report pairs from both sequences of awakenings. Thus, the re-elaboration of contents of previous MSEs seems to occur mainly at the level of single contents in both types of sleep, with similar modalities of processing.     

Reduced NREM sleep instability in children with sleep disordered breathing

STUDY OBJECTIVES: To evaluate NREM sleep instability, as measured by the cyclic alternating pattern (CAP), in a cohort of children with mild sleep disordered breathing (SDB) or frank obstructive sleep apnea (OSA) and normal controls. DESIGN: Prospective study. SETTINGS: Sleep laboratory in academic center. PARTICIPANTS: Twenty-two patients (13 boys; mean age 6.5 +/- 2.4 years; 10 with mild SDB and 12 with OSA) and 15 normal children matched for age underwent overnight polysomnographic recordings in a standard laboratory setting. Sleep was visually scored for sleep macrostructure and CAP in a blinded fashion using standard criteria. Markovian analysis was also performed. MEASUREMENTS AND RESULTS: Participants with OSA had reduced total CAP rates than normal controls and mild SDB patients. Children with mild SDB or OSA had a lower number of A1, lower A1 percentage, and lower A1 index than controls. Children with OSA also showed longer intervals between consecutive A phases and a decrease in entropy in the Markovian analysis. CONCLUSIONS: The lower CAP rate and its reduced entropy in children with mild SDB or OSA seem to indicate the presence of subtle sleep alterations that are not clearly identifiable with other approaches and might provide more robust correlates of neurocognitive and behavioral dysfunction in snoring children.     

Effect of sleep deprivation on NREM sleep ERPs and related activity at sleep onset.

This research assessed the impact of one night of sleep deprivation on the amplitudes of NREM-sleep event-related potentials (NREM ERPs) and on the frequency of occurrence of related electroencephalogram activity including sleep spindles, arousals, K-complexes, and vertex sharp waves (VSWs). The NREM ERPs identified included P220, N350, P450, N550 and P900. During a pre-deprivation night, ten subjects took two 20-min naps separated by a 20-min break at their normal bedtime. Brief tones were presented at three intensity levels (60, 75 and 90 dB) with a 5-s interstimulus interval. Following these naps, subjects were kept awake until their normal bedtime the following day. At that time, they repeated the two-nap procedure. The ERPs obtained for each tone and wake/sleep state for pre- and post-deprivation conditions were analyzed using repeated measures statistical procedures. As anticipated, NREM ERP amplitudes recorded both pre- and post-deprivation increased with tone intensity and with approaching sleep. Also, sleep deprivation was associated with more rapid sleep onset, reduced arousability, and greater spindle production. While sleep deprivation had no effect on the amplitude of P220. Post-deprivation amplitudes of N350, N550 and P900 were greater, especially following the 90-dB tone. There was a corresponding increase in VSWs and K-complexes. These findings are inconsistent with the view that NREM ERPs reflect arousal. The underlying mechanism(s) may facilitate initiation and maintenance of sleep.     

Long-term facilitation of genioglossus activity is present in normal humans during NREM sleep

Episodic hypoxia (EH) is followed by increased ventilatory motor output in the recovery period indicative of long-term facilitation (LTF). We hypothesized that episodic hypoxia evokes LTF of genioglossus (GG) muscle activity in humans during non-rapid eye movement sleep (NREM) sleep. We studied 12 normal non-flow limited humans during stable NREM sleep. We induced 10 brief (3 min) episodes of isocapnic hypoxia followed by 5 min of room air. Measurements were obtained during control, hypoxia, and at 5, 10, 20, 30 and 40 min of recovery, respectively, for minute ventilation (V(I)), supraglottic pressure (P(SG)), upper airway resistance (R(UA)) and phasic GG electromyogram (EMG(GG)). In addition, sham studies were conducted on room air. During hypoxia there was a significant increase in phasic EMG(GG) (202.7+/-24.1% of control, p<0.01) and in V (I) (123.0+/-3.3% of control, p<0.05); however, only phasic EMG(GG) demonstrated a significant persistent increase throughout the recovery. (198.9+/-30.9%, 203.6+/-29.9% and 205.4+/-26.4% of control, at 5, 10, and 20 min of recovery, respectively, p<0.01). In multivariate regression analysis, age and phasic EMG(GG) activity during hypoxia were significant predictors of EMG(GG) at recovery 20 min. No significant changes in any of the measured parameters were noted during sham studies. CONCLUSION: (1) EH elicits LTF of GG in normal non-flow limited humans during NREM sleep, without concomitant ventilatory or mechanical LTF. (2) GG activity during the recovery period correlates with the magnitude of GG activation during hypoxia, and inversely with age.     

Medullary respiratory neural activity during hypoxia in NREM and REM sleep in the cat

Intact unanesthetized cats hyperventilate in response to hypocapnic hypoxia in both wakefulness and sleep. This hyperventilation is caused by increases in diaphragmatic activity during inspiration and expiration. In this study, we recorded 120 medullary respiratory neurons during sleep in hypoxia. Our goal was to understand how these neurons change their activity to increase breathing efforts and frequency in response to hypoxia. We found that the response of medullary respiratory neurons to hypoxia was variable. While the activity of a small majority of inspiratory (58%) and expiratory (56%) neurons was increased in response to hypoxia, the activity of a small majority of preinspiratory (57%) neurons was decreased. Cells that were more active in hypoxia had discharge rates that averaged 183% (inspiratory decrementing), 154% (inspiratory augmenting), 155% (inspiratory), 230% (expiratory decrementing), 191% (expiratory augmenting), and 136% (expiratory) of the rates in normoxia. The response to hypoxia was similar in non-rapid-eye-movement (NREM) and REM sleep. Additionally, changes in the profile of activity were observed in all cell types examined. These changes included advanced, prolonged, and abbreviated patterns of activity in response to hypoxia; for example, some inspiratory neurons prolonged their discharge into expiration during the postinspiratory period in hypoxia but not in normoxia. Although changes in activity of the inspiratory neurons could account for the increased breathing efforts and activity of the diaphragm observed during hypoxia, the mechanisms responsible for the change in respiratory rate were not revealed by our data.     

NREM sleep with low-voltage EEG in the rat

NREM sleep in the rat has traditionally been defined by electroencephalographic (EEG) amplitudes above those of wakefulness (W) and paradoxical sleep (PS); we refer to this high-amplitude NREM sleep as "HS." We have found that approximately 5% of total time is occupied by episodes in which EEG amplitude is low, distinguishing it from HS; theta amplitude is low, distinguishing it from PS; and electromyographic (EMG) amplitude is low, distinguishing it from W. We have called these low-EEG, low-theta, low-EMG episodes "low-amplitude sleep" (LS). Three studies are done to elucidate additional characteristics of LS. Polygraphically scored 30-s epochs were matched with independent classifications of rat behavior as W, NREM, or PS; 87% of polygraphically scored LS epochs were matched with NREM sleep behavior. Response thresholds to noxious stimuli were lowest in W, intermediate and similar in LS and HS, and highest in PS. The incidence of PGO-type (ponto-geniculo-occipital) waves in W, HS, and LS were all very low in comparison with rates in PS. Thus, LS and HS exhibited similarly quiescent spontaneous behavior, similar intermediate response thresholds, and similar low rates of PGO-type activity. Accordingly, we have proposed that LS, along with HS, is an NREM sleep stage.