Variations in period-analysed EEG asymmetry in REM and NREM sleep

Interhemispheric asymmetry during NREM sleep in the dog

Functional hemispheric asymmetry was evidenced in many species during sleep. Dogs seem to show hemispheric asymmetry during wakefulness; however, their asymmetric neural activity during sleep was not yet explored. The present study investigated interhemispheric asymmetry in family dogs using non-invasive polysomnography. EEG recordings during 3-h-long afternoon naps were carried out (N = 19) on two occasions at the same location. Hemispheric asymmetry was assessed during NREM sleep, using bilateral EEG channels. To include periods with high homeostatic sleep pressure and to reduce the variance of the time spent in NREM sleep between dogs, the first two sleep cycles were analysed. Left hemispheric predominance of slow frequency range was detected in the first sleep cycle of sleep recording 1, compared to the baseline level of zero asymmetry as well as to the first sleep cycle of sleep recording 2. Regarding the strength of hemispheric asymmetry, we found greater absolute hemispheric asymmetry in the second sleep cycle of sleep recording 1 and 2 in the frequency ranges of alpha, sigma and beta, compared to the first sleep cycle. Differences between sleep recordings and consecutive sleep cycles might be indicative of adaptation-like processes, but do not closely resemble the results described in humans.

Modulation of the ultradian human nasal cycle by sleep stage and body position

Objective:

The nasal cycle, which is present in a significant number of people, is an ultradian side-to-side rhythm of nasal engorgement associated with cyclic autonomic activity. We studied the nasal cycle during REM/non-REM sleep stages and examined the potentially confounding influence of body position on lateralized nasal airflow.

Methods:

Left- and right-side nasal airflow was measured in six subjects during an eight-hour sleep period using nasal thermistors. Polysomnography was performed. Simultaneously, body positions were monitored using a video camera in conjunction with infrared lighting.

Results:

Significantly greater airflow occurred through the right nasal chamber (relative to the left) during periods of REM sleep than during periods of non-REM sleep (p<0.001). Both body position (p < 0.001) and sleep stage (p < 0.001) influenced nasal airflow lateralization.

Conclusions:

This study demonstrates that the lateralization of nasal airflow and sleep stage are related. Some types of asymmetrical somatosensory stimulation can alter this relationship.

Unihemispheric sleep and asymmetrical sleep: behavioral, neurophysiological, and functional perspectives

Sleep is a behavior characterized by a typical body posture, both eyes' closure, raised sensory threshold, distinctive electrographic signs, and a marked decrease of motor activity. In addition, sleep is a periodically necessary behavior and therefore, in the majority of animals, it involves the whole brain and body. However, certain marine mammals and species of birds show a different sleep behavior, in which one cerebral hemisphere sleeps while the other is awake. In dolphins, eared seals, and manatees, unihemispheric sleep allows them to have the benefits of sleep, breathing, thermoregulation, and vigilance. In birds, antipredation vigilance is the main function of unihemispheric sleep, but in domestic chicks, it is also associated with brain lateralization or dominance in the control of behavior. Compared to bihemispheric sleep, unihemispheric sleep would mean a reduction of the time spent sleeping and of the associated recovery processes. However, the behavior and health of aquatic mammals and birds does not seem at all impaired by the reduction of sleep. The neural mechanisms of unihemispheric sleep are unknown, but assuming that the neural structures involved in sleep in cetaceans, seals, and birds are similar to those of terrestrial mammals, it is suggested that they involve the interaction of structures of the hypothalamus, basal forebrain, and brain stem. The neural mechanisms promoting wakefulness dominate one side of the brain, while those promoting sleep predominates the other side. For cetaceans, unihemispheric sleep is the only way to sleep, while in seals and birds, unihemispheric sleep events are intermingled with bihemispheric and rapid eye movement sleep events. Electroencephalogram hemispheric asymmetries are also reported during bihemispheric sleep, at awakening, and at sleep onset, as well as being associated with a use-dependent process (local sleep).

Ultradian and circadian modulation of dream recall: EEG correlates and age effects

Dreaming occurs during non-rapid eye movement (NREM) and rapid eye movement (REM) sleep, which both are regulated by homeostatic, ultradian, and circadian processes. However, the magnitude of how ultradian REM and NREM sleep and its EEG correlates impact onto dream recall remains fairly unknown. In this review, we address three questions: 1. Is there an ultradian NREM-REM sleep modulation in successful dream recall, which is gated by the circadian clock? 2. What are the key electrophysiological correlates that account for dream recall during NREM and REM sleep and 3. Are there age-related changes in the ultradian and circadian regulation in dream recall and its electrophysiological correlates? Knowledge on the specific frequency and topography NREM and REM sleep differences prior to dream recall may pinpoint to the cerebral correlates that account for this cognitive process, and hint to their possible physiological meaning.

Inter-hemispheric oscillations in human sleep

Sleep is generally categorized into discrete stages based on characteristic electroencephalogram (EEG) patterns. This traditional approach represents sleep architecture in a static way, but it cannot reflect variations in sleep across time and across the cortex. To investigate these dynamic aspects of sleep, we analyzed sleep recordings in 14 healthy volunteers with a novel, frequency-based EEG analysis. This approach enabled comparison of sleep patterns with low inter-individual variability. We then implemented a new probability dependent, automatic classification of sleep states that agreed closely with conventional manual scoring during consolidated sleep. Furthermore, this analysis revealed a previously unrecognized, interhemispheric oscillation during rapid eye movement (REM) sleep. This quantitative approach provides a new way of examining the dynamic aspects of sleep, shedding new light on the physiology of human sleep.

Interhemispheric EEG asymmetries during unilateral bright-light exposure and subsequent sleep in humans

We tested whether evening exposure to unilateral photic stimulation has repercussions on interhemispheric EEG asymmetries during wakefulness and later sleep. Because light exerts an alerting response in humans, which correlates with a decrease in waking EEG theta/alpha-activity and a reduction in sleep EEG delta activity, we hypothesized that EEG activity in these frequency bands show interhemispheric asymmetries after unilateral bright light (1,500 lux) exposure. A 2-h hemi-field light exposure acutely suppressed occipital EEG alpha activity in the ipsilateral hemisphere activated by light. Subjects felt more alert during bright light than dim light, an effect that was significantly more pronounced during activation of the right than the left visual cortex. During subsequent sleep, occipital EEG activity in the delta and theta range was significantly reduced after activation of the right visual cortex but not after stimulation of the left visual cortex. Furthermore, hemivisual field light exposure was able to shift the left predominance in occipital spindle EEG activity toward the stimulated hemisphere. Time course analysis revealed that this spindle shift remained significant during the first two sleep cycles. Our results reflect rather a hemispheric asymmetry in the alerting action of light than a use-dependent recovery function of sleep in response to the visual stimulation during prior waking. However, the observed shift in the spindle hemispheric dominance in the occipital cortex may still represent subtle local use-dependent recovery functions during sleep in a frequency range different from the delta range.

EEG correlates of emotions in dream narratives from typical young adults and individuals with autistic spectrum disorders

The relationship between emotional dream content and Alpha and Beta REM sleep EEG activity was investigated in typical individuals and in Autistic Spectrum Disorders (ASD). Dream narratives of persons with ASD contained fewer emotional elements. In both groups, emotions correlated positively with slow Alpha (8.0-10.0 Hz) spectral power over parieto-occipital and left central regions, as well as with a right occipital EEG asymmetry. Slow Alpha activity in ASD individuals was lower over midline and parasagittal areas and higher over lateral areas compared to controls. Both groups displayed a right-biased slow Alpha activity for midparietal and occipital (significantly higher in control) sites. Results indicate that Alpha EEG activity may represent a neurophysiological substrate associated with emotional dream content. Distinctive Alpha EEG patterns and asymmetries suggest that dream generation implies different brain connectivity in ASD.

High frequency EEG activity during sleep: characteristics in schizophrenia and depression

Previous studies indicate that high frequency power (>20Hz) in the electroencephalogram (EEG) are associated with feature binding and attention. It has been hypothesized that hallucinations and perceptual abnormalities might be linked to irregularities in fast frequency activity. This study examines the power and distribution of high frequency activity (HFA) during sleep in healthy control subjects and unmedicated patients with schizophrenia and depression. This is a post-hoc analysis of an archival database collected under identical conditions. Groups were compared using multivariate analyses of covariance (MANCOVA) using group frequency by stage analysis. A multiple regression analyzed the association between HFA power and clinical symptoms. Schizophrenic (SZ) and major depressive disorder (MDD) patients showed significantly greater high frequency (HF) power than healthy controls (HC) in all sleep stages (p<0.0001). SZs also exhibited significantly greater HF power than MDD patients in all sleep stages except wakefulness (W) (p<0.05). In all groups, gamma (35-45Hz) power was greater in W, decreased during slow wave sleep (SWS) and decreased further during rapid eye movement (REM). Beta 2 (20-35 Hz) power was greater in W and REM than in SWS. Only positive symptoms exhibited an association with HF power. Elevated HFA during sleep in unmedicated patients with SZ and MDD is associated with positive symptoms of illness. It is not clear how HFA would change in relation to clinical improvement, and further study is needed to clarify the association of HFA to the state/trait characteristics of SZ and MDD.

The effects of vagus nerve stimulation on sleep EEG in depression: a preliminary report

OBJECTIVE

The present study evaluated the effects of vagus nerve stimulation (VNS) on sleep in seven treatment-resistant depressed outpatients.

METHODS

Sleep studies were conducted in the laboratory at baseline and 10-12 weeks after VNS implantation while the concomitant psychotropic medication regimen was unchanged. Standard sleep macroarchitecture based on visual stage and assessment of ultradian sleep electroencephalographic (EEG) rhythms were measured on all nights.

RESULTS

An overall significant treatment effect on sleep macroarchitecture was obtained by MANOVA. Decreased awake time, decreased Stage 1 sleep and increased Stage 2 sleep were evident post-VNS, although univariate analyses did not reach significance. In addition, the strength or amplitude of ultradian sleep EEG rhythms more than doubled on VNS and was restored to within normal range.

CONCLUSION

VNS improved the clinical symptoms of depression and sleep architecture. Results suggest that treatment-resistant depressed patients have dampened sleep EEG rhythms that are restored to near-normal amplitudes with VNS treatment.

Visual dream content, graphical representation and EEG alpha activity in congenitally blind subjects

It is currently claimed that congenitally blind do not have visual imagery and are therefore unable to present visual contents in their dreams. The aim of our study was to quantitatively evaluate the existence of visual imagery in born-blind dreams and to correlate it with objective measures, such as sleep EEG frequency components, namely with alpha attenuation (regarded as an indicator of visual activity), and graphical analysis of dream pictorial representations. The investigation was carried out via simultaneous recordings of dream reports and polysomnography, during nocturnal sleep at volunteers' homes; scheduled regular awakenings during the night provided the data for dream and EEG analysis. In the morning, subjects were asked to make a drawing of their dream images. Congenitally blind (n=10) were comparable to normal sighted subjects (n=9): the two groups presented equivalent visual activity indices, and no differences in the analysis of graphical representation of dreaming imagery. However, blind subjects presented a lower rate of dream recall than sighted (27% versus 42%). Both groups had significant negative correlation between Visual Activity Index (VAI) and alpha power in the central and occipital O2 derivations (blind: C4: r=-0.615, P<0.005; O2: r=-0.608, P<0.006; sighted: C4: r=-0.633, P<0.01; O2: r=-0.506, P<0.05). This correlation was weaker for the blind in O1 (r=-0.573, P<0.05) and non-existent for the sighted. Blind individuals have significantly lower alpha activity in the central derivation. In conclusion, the congenitally blind have visual content in their dreams and are able to draw it and, as expected, their VAI is negatively correlated with EEG alpha power.

Interhemispheric asymmetry of human sleep EEG in response to selective slow-wave sleep deprivation

Recent evidence suggests that the human sleep electroencephalogram (EEG) shows regional differences over both the sagittal and coronal planes. In the present study, in a group of 10 right-handers, the authors investigated the presence of hemispheric asymmetries in the homeostatic regulation of human sleep EEG power during and after selective slow-wave sleep (SWS) deprivation. The SWS deprivation was slightly more effective over the right hemisphere, but the left hemisphere showed a markedly larger increase of EEG power in the 1.00-24.75 Hz range during recovery-night non-REM sleep, and a larger increase of EEG power during both deprivation-night and recovery-night REM sleep. These results support the greater need for sleep recuperative processes of the left hemisphere, suggesting that local sleep regulation processes may also act during REM sleep.

EEG source identification: frequency analysis during sleep

This article deals with a new approach in sleep characterization that combines EEG source localisation methods with standard frequency analysis of multielectrode EEGs. First, we describe the theoretical methodology and the benefits that we get from a three-dimensional image (LORETA) of the cerebral activity related to a frequency band. Then, this new application is used as signal-processing technique on sleep EEG recordings obtained from young male adults using four frequency bands (delta 0.5-3.5 Hz, theta 4.0-7.5 Hz, alpha 8.0-12.5 Hz and beta 13.0-32.0 Hz) in different sleep stages. Finally, we show that the obtained results are highly consistent with other physiological assessments (standard EEG mapping, functional magnetic resonance imaging, etc.), but give us more realistic additional information on the generators of electromagnetic cerebral activity.

Behavioral, neurophysiological and evolutionary perspectives on unihemispheric sleep

Several animals mitigate the fundamental conflict between sleep and wakefulness by engaging in unihemispheric sleep, a unique state during which one cerebral hemisphere sleeps while the other remains awake. Among mammals, unihemispheric sleep is restricted to aquatic species (Cetaceans, eared seals and manatees). In contrast to mammals, unihemispheric sleep is widespread in birds, and may even occur in reptiles. Unihemispheric sleep allows surfacing to breathe in aquatic mammals and predator detection in birds. Despite the apparent utility in being able to sleep unihemispherically, very few mammals sleep in this manner. This is particularly interesting since the reptilian ancestors to mammals may have slept unihemispherically. The relative absence of unihemispheric sleep in mammals suggests that a trade off exists between unihemispheric sleep and other adaptive brain functions occurring during sleep or wakefulness. Presumably, the benefits of sleeping unihemispherically only outweigh the costs under extreme circumstances such as sleeping at sea. Ultimately, a greater understanding of the reasons for little unihemispheric sleep in mammals promises to provide insight into the functions of sleep, in general.

Ultradian rhythms and temporal coherence in sleep EEG in depressed children and adolescents

BACKGROUND

It has been suggested that a primary ultradian (80-120 minute) rhythm disturbance in EEG underlies sleep abnormalities in adults with depression. The present study evaluated ultradian rhythm disturbances in childhood and adolescent depression.

METHODS

Sleep macroarchitecture and temporal coherence in quantitative EEG rhythms were investigated in 50 medication-free outpatients with major depression (25 children and 25 adolescents) and 15 healthy normal controls (5 children and 10 adolescents).

RESULTS

Few of the macroarchitectural measures showed significant group effects. In fact, age and sex effects were stronger than disease-dependent components. Temporal coherence of EEG rhythms during sleep did differentiate those with MDD from controls. Both depressed children and adolescents had lower intrahemispheric coherence, whereas interhemispheric was only lower in depressed adolescents in comparison with controls. Gender differences were evident in adolescents, but not children, with MDD with lowest interhemispheric coherence in adolescent girls.

CONCLUSIONS

These findings are in keeping with increased risk for depression in females beginning at adolescence and extending throughout adulthood. It was suggested that low temporal coherence in depression reflects a disruption in the fundamental basic rest-activity cycle of arousal and organization in the brain that is strongly influenced by gender.

Interhemispheric differences in awake and sleep human EEG: a comparison between non-linear and spectral measures

Interhemispheric differences in the EEG of nine healthy right-handed human subjects (C3 vs. C4 derivations) were investigated during resting wake with closed eyes (CE) and sleep stages I, II, III, IV and REM. The harmonic power spectral density within the EEG main spectral bands, the fractal (Dr) and the correlation (D2) dimension as well as the largest Lyapunov exponent (lambda1) of both hemispheres were compared. In addition, the relationships between non-linear and spectral measures were analyzed. Dr, D2, lambda1 and the power in alpha band exhibited interhemispheric differences during waking, the values from the right hemisphere (RH) being higher than those of the left (LH) except for lambda1. During slow wave sleep (SWS), non-linear parameters detected opposite EEG asymmetries (D2 in stage III and lambda1 in stage IV) to those found in the other behavioural stages. In addition, both D2 and lambda1 were correlated (negatively) with the power in the delta band, but lambda1 was also correlated (positively) with the power in the alpha and beta bands. In conclusion, RH appears to be more complex though more predictable than the LH during CE and sleep stages I and II, these characteristics changing to the LH during SWS.

REM sleep dream mentation in right hemispherectomized patients

Investigations of dream mentation in brain damaged patients have shed some light on the controversial issue of cerebral lateralization of dreaming. To examine further the relationships between brain function and dreaming, we studied REM sleep dream recall and content in four patients having undergone right functional or anatomical hemispherectomy and eight matched control subjects. Patients were found to have the capacity to report dreams to much the same extent as control subjects. Further, the patients' dream content was overall similar to that of the control subjects. The results provide strong evidence that dreaming is not a right-hemisphere function, and that the left hemisphere may be more critical for the generation of dreams. In addition, some characteristics of hemispherectomized patients' dream content (characters, smells) are consistent with the possibility that a history of epilepsy may influence REM sleep imagery over the long term.

A comparison of period amplitude and power spectral analysis of sleep EEG in normal adults and depressed outpatients

Three experiments were carried out to evaluate the relationship between two techniques for quantifying electroencephalographic (EEG) data during sleep: period amplitude analysis (PAA) and power spectral analysis (PSA). In Experiment 1, canonical correlations and regression analyses were computed on PSA and PAA data from 40 undergraduate volunteers. The results yielded an average canonical correlation of 0.98. Further, multiple regression analyses demonstrated that the PSA variables accounted for approximately 66% of the variance in the PAA data, whereas PAA variables captured 88% of the variance in the PSA data. Epoch-to-epoch correlations were higher for PAA measures than for PSA data, perhaps indicating greater stability of PSA measures across epochs of sleep. In Experiment 2, PSA and PAA data were compared in 17 unmedicated outpatients with unipolar depression. Canonical correlations and regression analyses indicated that the overlap in variance between PSA and PAA did not exceed 50%, regardless of whether PSA or PAA variables were used as predictors. Epoch-to-epoch correlations between PAA measures were significantly higher than correlations among PSA variables, again suggesting greater stability of PAA data across epochs of sleep. The range of correlations for either data set was, however, substantially lower in the depressed than in the normal group. Experiment 3 evaluated the possibility that filter settings and artifact-rejection procedures had contributed to reduced overlap in PSA-PAA variance and reduced stability in depressed patients. An additional group of eight healthy volunteers served as subjects. Findings in Experiment 3 indicated that methodological differences between Experiments 1 and 2 did not account for the reduced correlations in the depressed group. It was concluded that PSA and PAA data should be comparable in normal subjects but are relatively independent in depressed patients. Epoch-to-epoch correlations were higher for PAA data than those found between PSA measures in both normal subjects and depressed patients, suggesting that PAA may be more stable across sleep epochs. Reduced stability may be a reflection of nonstationarity in the EEG of depressed patients.

Microarchitectural findings in sleep EEG in depression: diagnostic implications

A 10-year review of sleep electroencephalogram (EEG)-frequency analysis in depression reveals several consistent microarchitectural abnormalities. Decreased delta amplitude or incidence, particularly in the first 100 min of sleep, has been reported. Elevated fast-frequency EEG has been shown in both remitted and symptomatic depressed patients, especially in the right hemisphere. Further, interhemispheric coherence is reduced in both depressed groups. These microarchitectural features may not be present in narcolepsy, obsessive-compulsive disorders, or schizophrenia, despite similarities in sleep-stage characteristics. Collectively, these findings suggest that computer analysis of the sleep EEG may differentiate depressed patients from normal controls and from other clinical populations.

Dynamical properties of the sleep EEG in different frequency bands

The information concerning the dynamic behavior of the sleep process gained by the usual evaluation of sleep EEGs according to the criteria of Rechtschaffen and Kales is limited. Therefore a new methodical approach is presented, which is a special case of spectral analyzed data processing. After digital band-pass filtering of the sleep EEG the root-mean-square (RMS) value of successive 20 s EEG epochs is calculated in defined frequency ranges. This procedure ensures to take into account the influence of the phase relation between different frequency components. The temporal course of these RMS values during the night reveals smooth curves with continuous transitions between different sleep states. In all frequency bands slow oscillations according to the sleep cycles are observable. Whereas the slow frequency bands have a temporal course with local maxima during non-REM and local minima during REM sleep, the fast frequency bands beta and gamma show the opposite behavior revealing higher RMS values during REM sleep. The relationship between the activities in different frequency bands is evaluated calculating the cross correlation coefficient. Taken together the procedure allows an objective and automated quantitative analysis of the sleep EEG. The main advantage of this approach is the characterization of the sleep cycle as a dynamic and continuous process. Compared to the classical analysis it provides a more detailed analysis of the sleep process, especially concerning the dynamics and microstructure of sleep.

Period-amplitude analysis and power spectral analysis: a comparison based on all-night sleep EEG recordings

Both period-amplitude analysis (PAA) and power spectral analysis (PSA) were performed on all-night human sleep EEG recordings obtained from 11 subjects. The comparison of the two methods was based on the PAA variables time in band (a wave incidence measure) and rectified amplitude, and on the PSA variables spectral power density and spectral amplitude (the square root of power). The mean time course of these variables was determined for the first 4 nonREM-REM sleep cycles. Spectral power density and spectral amplitude in the delta range were high in nonREM sleep and low in REM sleep, and showed a declining trend over consecutive nonREM sleep episodes. In the frequency range below 2 Hz, rectified amplitude was highly correlated with both time in band and spectral amplitude, and there was no evidence for a dissociation between wave amplitude and wave incidence measures. However, in frequencies above 2 Hz, the modulation of time in band was a mirror image of that below 2 Hz. This result does not reflect a property of the data, but is inherent to the methodology applied. The reversal point of modulation was merely shifted when the high-pass filter settings were changed. It is concluded that band-pass filtering is necessary prior to PAA even for the analysis of the lowest frequency range, and that the indiscriminate use of PAA may give rise to spurious results.

The ultradian rhythm of alternating cerebral hemispheric activity

This review covers cognitive and electroencephalographic studies of the ultradian rhythm of alternating cerebral hemispheric activity found in humans and animals. This endogenous alternation of right and left dominance ranges in periodicity from about 25 to 300 min with peaks between 90-200 min during waking and around 100 min during sleep. Studies of lateralized EEG activity during sleep are reported as correlates of the REM-NREM sleep cycle. Studies of lateralized ultradian rhythms of EEG during wakefulness reveal a correlation between hemispheric dominance and the nasal cycle. The rhythm of cerebral dominance has also been identified with tests of lateralized cognitive performance using left and right hemisphere dependent tasks. Awakening from REM or NREM sleep is associated with different effects on left or right hemispheric dominance. This rhythm plays an important role in cognitive performance, memory processes, visual perception, levels of arousal and performance, mood, and individual and social behavior.

Digital period analysis of EEG in depression: periodicity, coherence, and interhemispheric relationships during sleep

1. Interhemispheric EEG differences were compared between 12 symptomatic depressed outpatients, 12 asymptomatic patients and 12 normal controls during two consecutive nights in the Sleep Study Unit. 2. EEG was quantified using digital period analysis (DPA), a time-domain analysis of successive polarity changes (zero-cross) and instances of zero slope (first derivative), yielding percent-time in each frequency band. 3. The degree of hemispheric asymmetry (L-R) was computed for delta, beta and theta percentages from REM, Stage 2 and Slow-Wave (SW) sleep. 4. Normals showed small asymmetries throughout sleep with largest differences in SW, with no consistent relationship between right and left activity and sleep stage. 5. Both depressed groups showed largest asymmetries in REM sleep, with significantly more beta, theta and delta in the right hemisphere consistently. None of the 24 depressed patients showed greater left hemisphere activity throughout sleep.

Digital period analysis of sleep EEG in depression

The period-analyzed sleep electroencephalogram (EEG) was compared in a group of 9 depressed outpatients and 9 age-matched normal controls. Both groups showed rhythms in beta, delta, and theta activity with an approximately 90-min period. The phase and coherence between fast and slow frequency EEG measures, however, differed significantly in the two groups. Beta and delta rhythms were less coherent in the depressed outpatient sample. The control group showed higher coherence and a strong coupling of beta and delta activity. These preliminary data suggest that depression may be associated with some degree of ultradian rhythm disturbances though periodicity is unaffected.

Comparison of the delta EEG in the first and second non-REM periods in depressed adults and normal controls

The distribution of period-analyzed delta activity in the first and second non-rapid eye movement (NREM) periods was compared in nine symptomatic depressed outpatients and nine normal controls. The groups did not differ in ratios of delta zero-cross or delta power in the first to the second NREM periods. Further, neither group showed a systematic change in delta count or delta power across the first two NREM periods. Our findings suggest that ratios of delta activity in the first two NREM periods may not systematically differentiate depressed adults from normal subjects.