Cortical high-frequency oscillations (≈ 110 Hz) in cats are state-dependent and enhanced by a subanesthetic dose of ketamine.

Ketamine is an NMDA receptor antagonist that has antidepressant and anesthetic properties. At subanesthetic doses, ketamine induces transient psychosis in humans, and is used to model psychosis in experimental animals. In rodents, subanesthetic doses of ketamine increase the power of high-frequency oscillations (HFO, > 100 Hz) in the electroencephalogram (EEG), a frequency band linked to cognitive functions. However, to date, the effects of ketamine in carnivores and primates have been poorly investigated. Here, we examined in the cat, cortical HFO during wakefulness, sleep, and after administering a sub-anesthetic dose of ketamine. Four cats were prepared with cortical electrodes for chronic polysomnographic recordings in head-restrained conditions. The cortical HFO power, connectivity, direction of the information flow using Granger Causality (GC) analysis, their relationships with respiratory activity, and the effect of auditory stimulation were analyzed. During wakefulness, but not during sleep, we found that HFO were coupled with the inspiratory phase of the respiration. After ketamine administration, HFO power was enhanced and remained associated with the inspiratory phase. GC analysis suggests that ketamine-enhanced HFO originate from the olfactory bulb (OB) and stream towards the prefrontal cortex (Pf). Accordingly, occluding the nostrils significantly reduced the power of the ketamine-enhanced HFO in both the OB and Pf. Finally, auditory stimulation did not affect HFO. In conclusion, the HFO are associated with respiration during wakefulness, but not during sleep. The enhancement of this rhythm by ketamine may disrupt cortical information processing, which could contribute to some of the neuropsychiatric effects associated with ketamine.

Differential effects of haloperidol on neural oscillations during wakefulness and sleep.

The electrical activity of the brain, characterized by its frequency components, reflects a complex interplay between periodic (oscillatory) and aperiodic components. These components are associated with various neurophysiological processes, such as the excitation-inhibition balance (aperiodic activity) or interregional communication (oscillatory activity). However, we do not fully understand whether these components are truly independent or if different neuromodulators affect them in different ways. The dopaminergic system has a critical role for cognition and motivation, being a potential modulator of these power spectrum components. To improve our understanding of these questions, we investigated the differential effects of this system on these components using electrocorticogram recordings in cats, which show clear oscillations and aperiodic 1/f activity. Specifically, we focused on the effects of haloperidol (a D2 receptor antagonist) on oscillatory and aperiodic dynamics during wakefulness and sleep. By parameterizing the power spectrum into these two components, our findings reveal a robust modulation of oscillatory activity by the D2 receptor across the brain. Surprisingly, aperiodic activity was not significantly affected and exhibited inconsistent changes across the brain. This suggests a nuanced interplay between neuromodulation and the distinct components of brain oscillations, providing insights into the selective regulation of oscillatory dynamics in awake states.

Ibogaína: un psicodélico atípico con potencial antiadictivo / Ibogaine: an atypical psychedelic with anti-addictive potential

El trastorno por uso de sustancias es una enfermedad crónica de graves consecuencias. Actualmente, los tratamientos farmacológicos no apuntan a corregir los cambios neurobiológicos generados en el cerebro por el uso crónico de sustancias de abuso, sino que se enfocan principalmente en la atenuación de algunos de los síntomas que padece el consumidor. La ibogaína es un psicodélico atípico que, tanto en estudios observacionales como en ensayos clínicos abiertos, ha mostrado una propiedad antiadictiva que perdura en el tiempo. Sin embargo, su delicado perfil de toxicidad cardíaca, así como su uso en entornos sin adecuadas medidas de seguridad, han limitado su progresión en las investigaciones clínicas. Los efectos antiadictivos de ibogaína han disparado diversas líneas de investigación básica, preclínica y clínica, que buscan confirmar su efectividad, entender sus mecanismos de acción y delimitar su perfil de seguridad. Dada la poca información disponible para los profesionales de salud sobre esta sustancia, esta revisión busca aportar información acerca de su potencial terapéutico, posibles mecanismos de acción y riesgos asociados a su administración.

Substance use disorder is a chronic disease with severe consequences. Currently, pharmacological treatments do not aim to correct the neurobiological changes generated in the brain by the chronic use of substances of abuse, but rather focus mainly on attenuating some of the user's symptoms. Ibogaine is an atypical psychedelic that has shown long-lasting and interesting antiaddictive properties in both observational studies and open-label clinical trials. However, its delicate profile of cardiac toxicity, as well as its use in settings without adequate safety measures, have limited its progression in clinical research. The anti-addictive effects of ibogaine have triggered diverse scientific research in basic, preclinical, and clinical areas, which seek efficacy confirmation and to fully understand ibogaine´s underlying mechanisms of action and its safety profile. Given that there is little information available to health professionals about ibogaine and its antiaddictive properties, this review aims to provide published data about its therapeutic potential in drug addiction, its mechanisms of action, and risks associated with its administration.

Breathing modulates gamma synchronization across species.

Nasal respiration influences brain dynamics by phase-entraining neural oscillations at the same frequency as the breathing rate and by phase-modulating the activity of faster gamma rhythms. Despite being widely reported, we still do not understand the functional roles of respiration-entrained oscillations. A common hypothesis is that these rhythms aid long-range communication and provide a privileged window for synchronization. Here we tested this hypothesis by analyzing electrocorticographic (ECoG) recordings in mice, rats, and cats during the different sleep-wake states. We found that the respiration phase modulates the amplitude of cortical gamma oscillations in the three species, although the modulated gamma frequency bands differed with faster oscillations (90-130 Hz) in mice, intermediate frequencies (60-100 Hz) in rats, and slower activity (30-60 Hz) in cats. In addition, our results also show that respiration modulates olfactory bulb-frontal cortex synchronization in the gamma range, in which each breathing cycle evokes (following a delay) a transient time window of increased gamma synchrony. Long-range gamma synchrony modulation occurs during quiet and active wake states but decreases during sleep. Thus, our results suggest that respiration-entrained brain rhythms orchestrate communication in awake mammals.

Ketamine and sleep modulate neural complexity dynamics in cats.

There is increasing evidence that the level of consciousness can be captured by neural informational complexity: for instance, complexity, as measured by the Lempel Ziv (LZ) compression algorithm, decreases during anaesthesia and non-rapid eye movement (NREM) sleep in humans and rats, when compared with LZ in awake and REM sleep. In contrast, LZ is higher in humans under the effect of psychedelics, including subanaesthetic doses of ketamine. However, it is both unclear how this result would be modulated by varying ketamine doses, and whether it would extend to other species. Here, we studied LZ with and without auditory stimulation during wakefulness and different sleep stages in five cats implanted with intracranial electrodes, as well as under subanaesthetic doses of ketamine (5, 10, and 15 mg/kg i.m.). In line with previous results, LZ was lowest in NREM sleep, but similar in REM and wakefulness. Furthermore, we found an inverted U-shaped curve following different levels of ketamine doses in a subset of electrodes, primarily in prefrontal cortex. However, it is worth noting that the variability in the ketamine dose-response curve across cats and cortices was larger than that in the sleep-stage data, highlighting the differential local dynamics created by two different ways of modulating conscious state. These results replicate previous findings, both in humans and other species, demonstrating that neural complexity is highly sensitive to capture state changes between wake and sleep stages while adding a local cortical description. Finally, this study describes the differential effects of ketamine doses, replicating a rise in complexity for low doses, and further fall as doses approach anaesthetic levels in a differential manner depending on the cortex.

EEG Gamma Band Alterations and REM-like Traits Underpin the Acute Effect of the Atypical Psychedelic Ibogaine in the Rat.

Ibogaine is a psychedelic alkaloid that has attracted large scientific interest because of its antiaddictive properties in observational studies in humans as well as in animal models. Its subjective effect has been described as intense, vivid dream-like experiences occurring while awake; hence, ibogaine is often referred to as an oneirogenic psychedelic. While this unique dream-like profile has been hypothesized to aid the antiaddictive effects, the electrophysiological signatures of this psychedelic state remain unknown. We previously showed in rats that ibogaine promotes a waking state with abnormal motor behavior along with a decrease in NREM and REM sleep. Here, we performed an in-depth analysis of the intracranial electroencephalogram during "ibogaine wakefulness". We found that ibogaine induces gamma oscillations that, despite having larger power than control levels, are less coherent and less complex. Further analysis revealed that this profile of gamma activity compares to that of natural REM sleep. Thus, our results provide novel biological evidence for the association between the psychedelic state and REM sleep, contributing to the understanding of the brain mechanisms associated with the oneirogenic psychedelic effect of ibogaine.

Power and Coherence in the EEG of the Rat: Impact of Behavioral States, Cortical Area, Lateralization and Light/Dark Phases.

The sleep-wake cycle is constituted by three behavioral states: wakefulness (W), non-REM (NREM) and REM sleep. These states are associated with drastic changes in cognitive capacities, mostly determined by the function of the thalamo-cortical system, whose activity can be examined by means of intra-cranial electroencephalogram (iEEG). With the purpose to study in depth the basal activity of the iEEG in adult rats, we analyzed the spectral power and coherence of the iEEG during W and sleep in the paleocortex (olfactory bulb), and in neocortical areas. We also analyzed the laterality of the signals, as well as the influence of the light and dark phases. We found that the iEEG power and coherence of the whole spectrum were largely affected by behavioral states and highly dependent on the cortical areas recorded. We also determined that there are night/day differences in power and coherence during sleep, but not in W. Finally, we observed that, during REM sleep, intra-hemispheric coherence differs between right and left hemispheres. We conclude that the iEEG dynamics are highly dependent on the cortical area and behavioral states. Moreover, there are light/dark phases disparities in the iEEG during sleep, and intra-hemispheric connectivity differs between both hemispheres during REM sleep.

Nasal respiration entrains neocortical long-range gamma coherence during wakefulness.

Recent studies have shown that slow cortical potentials in archi-, paleo- and neocortex can phase-lock with nasal respiration. In some of these areas, gamma activity (γ: 30-100 Hz) is also coupled to the animal's respiration. It has been hypothesized that these functional relationships play a role in coordinating distributed neural activity. In a similar way, inter-cortical interactions at γ frequency have also been associated as a binding mechanism by which the brain generates temporary opportunities necessary for implementing cognitive functions. The aim of the present study is to explore whether nasal respiration entrains inter-cortical functional interactions at γ frequency during both wakefulness and sleep. Six adult cats chronically prepared for electrographic recordings were employed in this study. Our results show that during wakefulness, slow cortical respiratory potentials are present in the olfactory bulb and several areas of the neocortex. We also found that these areas exhibit cross-frequency coupling between respiratory phase and γ oscillation amplitude. We demonstrate that respiratory phase modulates the inter-cortical gamma coherence between neocortical electrode pairs. On the contrary, slow respiratory oscillation and γ cortical oscillatory entrainments disappear during non-rapid eye movement and rapid eye movement sleep. These results suggest that a single unified phenomenon involves cross-frequency coupling and long-range γ coherence across the neocortex. This fact could be related to the temporal binding process necessary for cognitive functions during wakefulness.

Decreased electrocortical temporal complexity distinguishes sleep from wakefulness.

In most mammals, the sleep-wake cycle is constituted by three behavioral states: wakefulness (W), non-REM (NREM) sleep, and REM sleep. These states are associated with drastic changes in cognitive capacities, mostly determined by the function of the thalamo-cortical system. The intra-cranial electroencephalogram or electocorticogram (ECoG), is an important tool for measuring the changes in the thalamo-cortical activity during W and sleep. In the present study we analyzed broad-band ECoG recordings of the rat by means of a time-series complexity measure that is easy to implement and robust to noise: the Permutation Entropy (PeEn). We found that PeEn is maximal during W and decreases during sleep. These results bring to light the different thalamo-cortical dynamics emerging during sleep-wake states, which are associated with the well-known spectral changes that occur when passing from W to sleep. Moreover, the PeEn analysis allows us to determine behavioral states independently of the electrodes' cortical location, which points to an underlying global pattern in the signal that differs among the cycle states that is missed by classical methods. Consequently, our data suggest that PeEn analysis of a single EEG channel could allow for cheap, easy, and efficient sleep monitoring.

Electrocortical high frequency activity and respiratory entrainment in 6-hydroxydopamine model of Parkinson's disease.

Parkinson's disease is characterized by motor symptoms (akinesia, rigidity, etc.), which are associated with the degeneration of the dopaminergic neurons of the midbrain. In addition, olfactory impairment that usually develops before the detection of motor deficits, is detected in 90% of Parkinsonian patients. Recent studies in mammals, have shown that slow cortical potentials phase-lock with nasal respiration. In several cortical areas, gamma synchronization of the electrographic activity is also coupled to respiration, suggesting than nasal respiratory entrainment could have a role in the processing of olfactory information. In the present study, we evaluate the role of midbrain dopaminergic neurons, in the modulation of the electrocorticogram activity and its respiratory entrainment during wakefulness and sleep. For this purpose, we performed a unilateral lesion of dopaminergic neurons of the substantia nigra pars compacta of the rat, with 6-hydroxydopamine. An increase in beta (20-35 Hz) together with a decrease in gamma power (60-95 Hz) in the motor cortex ipsilateral to the lesion was observed during wakefulness. These results correlated with the degree of motor alterations and dopamine measured at the striatum. Moreover, we found a decline in gamma coherence between the ipsilateral olfactory bulb and motor cortex. Also, at the olfactory bulb we noticed an increase in respiratory-gamma cross-frequency coupling after the lesion, while at the motor cortex, a decrease in respiratory potential entrainment of gamma activity was observed. Interestingly, we did not observe any significant modification either during Non-REM or REM sleep. These waking dysrhythmias may play a role both in the anosmia and motor deficits present in Parkinson disease.

Acute effect of vaporized Cannabis on sleep and electrocortical activity.

The use of Cannabis for medical purposes is rapidly expanding and is usually employed as a self-medication for the treatment of insomnia disorder. However, the effect on sleep seems to depend on multiple factors such as composition of the Cannabis, dosage and route of administration. Vaporization is the recommended route for the administration of Cannabis for medical purposes; however, there is no published research about the effects of vaporized Cannabis on sleep, neither in laboratory animals, nor in humans. Because previous reports suggested that low doses of THC have sedating effects, the aim of the present study was to characterize in rats, the acute effects on sleep induced by the administration of low doses of THC by means of vaporization of a specific type of Cannabis (THC 11.5% and negligible amounts of other cannabinoids). For this purpose, polysomnographic recordings in chronically prepared rats were performed during 6 h in the light and dark phases. Animals were treated with 0 (control), 40, 80 and 200 mg of Cannabis immediately before the beginning of recordings; the THC plasma concentrations with these doses were low (up to 6.7 ng/mL with 200 mg). A quantitative EEG analyses by means of the spectral power and coherence estimations was also performed for the highest Cannabis dose. Compared to control, 200 mg of Cannabis increased NREM sleep time during the light phase, but only during the first hour of recording. Interestingly, no changes on sleep were observed during the dark (active) phase or with lower doses of Cannabis. Cannabis 200 mg also produced EEG power reductions in different cortices, mainly for high frequency bands during W and REM sleep, but only during the light phase. On the contrary, a reduction in the sleep spindles intra-hemispheric coherence was observed during NREM sleep, but only during the dark phase. In conclusion, administration of low doses of THC by vaporization of a specific type of Cannabis produced a small increment of NREM sleep, but only during the light (resting) phase. This was accompanied by subtle modifications of high frequency bands power (during the light phase) and spindle coherence (during the dark phase), which are associated with cognitive processing. Our results reassure the importance of exploring the sleep-promoting properties of Cannabis.

EEG dissociation induced by muscarinic receptor antagonists: Coherent 40 Hz oscillations in a background of slow waves and spindles.

Mesopontine and basal forebrain cholinergic neurons are involved in the control of behavioral states and cognitive functions. Animals treated with cholinergic muscarinic receptor antagonists display a dissociated state characterized by behavioral wakefulness (W) associated with high amplitude slow oscillations and spindles in the electroencephalogram (EEG), similar to those that occur during non-REM (NREM) sleep. Oscillations in the gamma frequency band (≈ 40 Hz) of the EEG also play a critical role during W and cognition. Hence, the present study was conducted to determine the effect of muscarinic antagonists on the EEG gamma band power and coherence. Five cats were implanted with electrodes in different cortices to monitor the EEG. The effects of atropine and scopolamine on power and coherence within the low gamma frequency band (30-45 Hz) from pairs of EEG recordings were analyzed and compared to gamma activity during sleep and W. Muscarinic antagonists induced a NREM sleep-like EEG profile that was accompanied by a large increase in gamma power and coherence. The values of gamma coherence were similar to that occurring during alert W (AW), and greater than in quiet W, NREM and REM sleep. We conclude that under atropine or scopolamine, functional interactions between cortical areas in the gamma frequency band remain high, as they are during AW. This significant functional connectivity at high frequency may explain why the animals remain awake in spite of the presence of slow waves and spindles.

Distribution of MCH-containing fibers in the feline brainstem: Relevance for REM sleep regulation.

Neurons that utilize melanin-concentrating hormone (MCH) as a neuromodulator are localized in the postero-lateral hypothalamus and incerto-hypothalamic area. These neurons project diffusely throughout the central nervous system and have been implicated in critical physiological processes, such as sleep. Unlike rodents, in the order carnivora as well as in humans, MCH exerts its biological functions through two receptors: MCHR-1 and MCHR-2. Hence, the cat is an optimal animal to model MCHergic functions in humans. In the present study, we examined the distribution of MCH-positive fibers in the brainstem of the cat. MCHergic axons with distinctive varicosities and boutons were heterogeneously distributed, exhibiting different densities in distinct regions of the brainstem. High density of MCHergic fibers was found in the dorsal raphe nucleus, the laterodorsal tegmental nucleus, the periaqueductal gray, the pendunculopontine tegmental nucleus, the locus coeruleus and the prepositus hypoglossi. Because these areas are involved in the control of REM sleep, the present anatomical data support the role of this neuropeptidergic system in the control of this behavioral state.

EEG 40 Hz Coherence Decreases in REM Sleep and Ketamine Model of Psychosis.

Cognitive processes are carried out during wakefulness by means of extensive interactions between cortical and subcortical areas. In psychiatric conditions, such as psychosis, these processes are altered. Interestingly, REM sleep where most dreams occurs, shares electrophysiological, pharmacological, and neurochemical features with psychosis. Because of this fact, REM sleep is considered a natural model of psychosis. Ketamine is a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist that at sub-anesthetic dose induces psychotomimetic-like effects in humans and animals, and is employed as a pharmacological model of psychosis. Oscillations in the gamma frequency band of the electroencephalogram (EEG), mainly at about 40 Hz, have been involved in cognitive functions. Hence, the present study was conducted to analyze the EEG low gamma (30-45 Hz) band power and coherence of the cat, in natural (REM sleep) and pharmacological (sub-anesthetic doses of ketamine) models of psychosis. These results were compared with the gamma activity during alert (AW) and quiet wakefulness (QW), as well as during non-REM (NREM) sleep. Five cats were chronically prepared for polysomnographic recordings, with electrodes in different cortical areas. Basal recordings were obtained and ketamine (5, 10, and 15 mg/kg, i.m.) was administrated. Gamma activity (power and coherence) was analyzed in the abovementioned conditions. Compared to wakefulness and NREM sleep, following ketamine administration gamma coherence decreased among all cortical regions studied; the same coherence profile was observed during REM sleep. On the contrary, gamma power was relatively high under ketamine, and similar to QW and REM sleep. We conclude that functional interactions between cortical areas in the gamma frequency band decrease in both experimental models of psychosis. This uncoupling of gamma frequency activity may be involved in the cognitive features shared by dreaming and psychosis.

Power and coherence of cortical high-frequency oscillations during wakefulness and sleep.

Recently, a novel type of fast cortical oscillatory activity that occurs between 110 and 160 Hz (high-frequency oscillations (HFO)) was described. HFO are modulated by the theta rhythm in hippocampus and neocortex during active wakefulness and REM sleep. As theta-HFO coupling increases during REM, a role for HFO in memory consolidation has been proposed. However, global properties such as the cortex-wide topographic distribution and the cortico-cortical coherence remain unknown. In this study, we recorded the electroencephalogram during sleep and wakefulness in the rat and analyzed the spatial extent of the HFO band power and coherence. We confirmed that the HFO amplitude is phase-locked to theta oscillations and is modified by behavioral states. During active wakefulness, HFO power was relatively higher in the neocortex and olfactory bulb compared to sleep. HFO power decreased during non-REM and had an intermediate level during REM sleep. Furthermore, coherence was larger during active wakefulness than non-REM, while REM showed a complex pattern in which coherence increased only in intra and decreased in inter-hemispheric combination of electrodes. This coherence pattern is different from gamma (30-100 Hz) coherence, which is reduced during REM sleep. This data show an important HFO cortico-cortical dialog during active wakefulness even when the level of theta comodulation is lower than in REM. In contrast, during REM, this dialog is highly modulated by theta and restricted to intra-hemispheric medial-posterior cortical regions. Further studies combining behavior, electrophysiology and new analytical tools are needed to plunge deeper into the functional significance of the HFO.

Neocortical 40 Hz oscillations during carbachol-induced rapid eye movement sleep and cataplexy.

Higher cognitive functions require the integration and coordination of large populations of neurons in cortical and subcortical regions. Oscillations in the gamma band (30-45 Hz) of the electroencephalogram (EEG) have been involved in these cognitive functions. In previous studies, we analysed the extent of functional connectivity between cortical areas employing the 'mean squared coherence' analysis of the EEG gamma band. We demonstrated that gamma coherence is maximal during alert wakefulness and is almost absent during rapid eye movement (REM) sleep. The nucleus pontis oralis (NPO) is critical for REM sleep generation. The NPO is considered to exert executive control over the initiation and maintenance of REM sleep. In the cat, depending on the previous state of the animal, a single microinjection of carbachol (a cholinergic agonist) into the NPO can produce either REM sleep [REM sleep induced by carbachol (REMc)] or a waking state with muscle atonia, i.e. cataplexy [cataplexy induced by carbachol (CA)]. In the present study, in cats that were implanted with electrodes in different cortical areas to record polysomnographic activity, we compared the degree of gamma (30-45 Hz) coherence during REMc, CA and naturally-occurring behavioural states. Gamma coherence was maximal during CA and alert wakefulness. In contrast, gamma coherence was almost absent during REMc as in naturally-occurring REM sleep. We conclude that, in spite of the presence of somatic muscle paralysis, there are remarkable differences in cortical activity between REMc and CA, which confirm that EEG gamma (≈40 Hz) coherence is a trait that differentiates wakefulness from REM sleep.

Heart rate variability during carbachol-induced REM sleep and cataplexy.

The nucleus pontis oralis (NPO) exerts an executive control over REM sleep. Cholinergic input to the NPO is critical for REM sleep generation. In the cat, a single microinjection of carbachol (a cholinergic agonist) into the NPO produces either REM sleep (REMc) or wakefulness with muscle atonia (cataplexy, CA). In order to study the central control of the heart rate variability (HRV) during sleep, we conducted polysomnographic and electrocardiogram recordings from chronically prepared cats during REMc, CA as well as during sleep and wakefulness. Subsequently, we performed statistical and spectral analyses of the HRV. The heart rate was greater during CA compared to REMc, NREM or REM sleep. Spectral analysis revealed that the low frequency band (LF) power was significantly higher during REM sleep in comparison to REMc and CA. Furthermore, we found that during CA there was a decrease in coupling between the RR intervals plot (tachogram) and respiratory activity. In contrast, compared to natural behavioral states, during REMc and CA there were no significant differences in the HRV based upon the standard deviation of normal RR intervals (SDNN) and the mean squared difference of successive intervals (rMSSD). In conclusion, there were differences in the HRV during naturally-occurring REM sleep compared to REMc. In addition, in spite of the same muscle atonia, the HRV was different during REMc and CA. Therefore, the neuronal network that controls the HRV during REM sleep can be dissociated from the one that generates the muscle atonia during this state.

Statistical, spectral and non-linear analysis of the heart rate variability during wakefulness and sleep.

As a first step in a program designed to study the central control of the heart rate variability (HRV) during sleep, we conducted polysomnographic and electrocardiogram recordings on chronically-prepared cats during semi- restricted conditions. We found that the tachogram, i.e. the pattern of heart beat intervals (RR intervals) was deeply modified on passing from alert wakefulness through quiet wakefulness (QW) to sleep. While the tachogram showed a rhythmical pattern coupled with respiratory activity during non-REM sleep (NREM), it turned chaotic during REM sleep. Statistical analyses of the RR intervals showed that the mean duration increased during sleep. HRV measured by the standard deviation of normal RR intervals (SDNN) and by the square root of the mean squared difference of successive intervals (rMSSD) were larger during REM and NREM sleep than during QW. SD-1 (a marker of short- term variability) and SD-2 (a marker of long-term variability) measured by means of Poincaré plots increased during both REM and NREM sleep compared to QW. Furthermore, in the spectral analysis of RR intervals, the band of high frequency (HF) was larger in NREM and REM sleep in comparison to QW, whereas the band of low frequency (LF) was larger only during REM sleep in comparison to QW. The LF/HF ratio was larger during QW compared either with REM or NREM sleep. Finally, sample entropy analysis used as a measure of complexity, was higher during NREM in comparison to REM sleep. In conclusion, HRV parameters, including complexity, are deeply modified across behavioral states.