Period-amplitude analysis of rat electroencephalogram: stage and diurnal variations and effects of suprachiasmatic nuclei lesions

Period-amplitude analysis was used to measure the number of waves per unit time (wave incidence) and wave amplitude for 19 wavelength categories in the lateral cortical electroencephalogram (EEG) of five intact and four suprachiasmatic nuclei-lesioned rats during NREM sleep, waking, and paradoxical sleep (PS) over a period of 24 h. The analysis confirmed several parallels between rat electroencephalogram (EEG) and human EEG: The wave incidence and amplitude at all wavelengths are both practically indistinguishable between wake, PS, and NREM sleep onset. As NREM sleep EEG amplitude increases, slow wave incidence and amplitude increase. The incidence and amplitude of slow waves are greatest at the start of the diurnal NREM sleep period and lowest at its end. The pattern of diurnal variation of the NREM EEG may be modeled using two wave generators (sources of variation), one between 1 and 4 Hz, and the other between 5 and 16 Hz. The diurnal patterns for wake and PS are less clear, but both appear to require three generators, one below 3 Hz, one between 3.5 and 6 Hz, and one above 9 Hz. The EEG of suprachiasmatic nuclei-lesioned rats does not show any shift to longer wavelengths in NREM sleep. Wake, PS, and NREM EEG in these rats have a lower incidence and amplitude of slow waves than the corresponding stages in intact rats. One explanation is an inhibition of the slow wave generator as a result of the lesions.     

Period-amplitude analysis of rat electroencephalogram: effects of sleep deprivation and exercise

Electroencephalogram (EEG) wavelength and amplitude within NREM sleep, paradoxical sleep (PS), and wake were measured by computer in five intact rats and four rats with suprachiasmatic nucleus (SCN) lesions for the first recovery day following 24-h total sleep deprivation (TSD) achieved by keeping them on a rotating cylinder over water. To assess exercise effects, EEG within NREM was also analyzed in four intact rats for 8 h after separate 4-h TSD sessions at low and high rates of cylinder rotation (high rate = 12 times low rate). During recovery from 24-h TSD, EEG changed most dramatically in NREM. The number of slow waves per unit time (1-4 Hz wave incidence) and the amplitude at all wavelengths from 1 to 16 Hz were increased for up to 12 h and then fell below baseline levels for most of the next 12 h. Fast (5-16 Hz) wave incidence changed inversely with slow wave incidence. Wake and PS also showed initially increased amplitude, but shifts in incidence were from slow to fast waves. Relative to baseline, intact and SCN-lesioned rats showed similarly shaped recovery functions, indicating that EEG responses to sleep loss are largely independent of diurnal rhythms. Four-hour TSD at a low rotation rate affected NREM EEG similarly to 24-h TSD, but more mildly. The high rotation rate further increased slow wave incidence during recovery without further increasing slow wave amplitude. The results suggest that both EEG wave incidence and amplitude are responsive to prior wakefulness, but only incidence is responsive to prior exercise.     

All night spectral analysis of EEG sleep in young adult and middle-aged male subjects

The sleep EEGs of 9 young adult males (age 20–28 years) and 8 middle-aged males (42–56 years) were analyzed by visual scoring and spectral analysis. In the middle-aged subjects power density in the delta, theta and sigma frequencies were attenuated as compared to the young subjects. In both age groups power density in the delta and theta frequencies declined from NREM period 1 to 3. In the sigma frequencies, however, no systematic changes in power density were observed over the sleep episode. In both age groups the decay of EEG power (0.75–7.0 Hz) over successive NREM-REM cycles and the time course of EEG power during NREM sleep was analyzed. The decay rate of both EEG power density over successive NREM-REM cycles and EEG power density during NREM sleep was smaller in the middle-aged subjects than in the young subjects. It is concluded that the age-related differences in human sleep EEG power spectra are not identical to the changes in EEG power spectra observed in the course of the sleep episode. Therefore age-related differences in EEG power spectra cannot be completely explained by assuming a reduced need for sleep in older subjects. The smaller decay rate of EEG power during NREM sleep in the middle-aged subjects is interpreted as a reduced sleep efficiency. The results are discussed in the frame work of the two-process model of sleep regulation.     

Age-related changes in sleep in the rat

Human sleep in old age is characterized by a number of changes, including reductions in sleep efficiency, amounts of visually scored slow-wave and REM sleep, and amplitude of the diurnal sleep/wake rhythm. In older rats, some, but not all, of these traits have been reported, including a decrease in the mean duration of sleep bouts, an increase in the number of sleep bouts, and a modest reduction of REM sleep. Studies of the diurnal rhythm of total sleep have had varied results. There are, however, virtually no data indicating at what point across the rat's lifetime the changes seen in old age begin to occur. In order to more fully characterize sleep in older rats, and to develop data on when they first appear, we have examined sleep in young adult (3 months), middle-aged (12 months), and older (24 months) rats during 24 hours under constant dim light. Analyses of variance revealed no age-related changes in total sleep, NREM or REM sleep, wake time after sleep onset, or three different measures of the amplitude of the sleep/wake circadian rhythm. There were, however, significant age-related reductions in high-voltage NREM sleep ("HS2"), the mean length of sleep bouts, and REM-onset duration. These were seen in the 1-year-old rats, indicating that the changes seen in the older animals were evident by midlife.     

Brain temperature variations during natural sleep and arousal in white rats

In 15 free moving white rats brain temperature (in sensorimotor cortical area), dorsal hippocampal and neocortical EEG and EMG of the neck muscles were recorded. It was found that total (neocortical and hippocampal) EEG desynchronization per se (during slow wave sleep, in EEG arousal without behavioral activiation, in passive avoidance responses) was not capable of eliciting a brain temperature rise and frequently (especially during slow wave sleep) led to its decrease. Brain temperature was found to increase upon the appearance of distinct synchronization of the regular hippocampal rhythm (during fast sleep phase, in behavioral arousal with orienting and exploratory responses). Cyclic increases in synchronization of slow waves and spindles divided by short-term desynchronization periods (slow sleep microcycles) were accompanied by small but distinct temperature elevations on the background of slow temperature fall or plateau.     

Diurnal changes of blood pressure,heart rate and body temperature during sleep in the rat

We have studied diurnal changes in mean arterial pressure (MAP), heart rate (HR) and body temperature (Tb) during wake (W), non-rapid eye movement sleep (NREMS) and REM sleep (REMS) in the rat. Although HR and Tb show a similar sinusoidal diurnal variation during all vigilance states, the diurnal profile for the MAP is vigilance-state dependent. During W, MAP values are higher during the dark phase, during NREMS, no significant diurnal change is seen, and during REMS, the MAP exhibits a reversed diurnal change, being higher during the light phase. The low frequency component (0.25–0.725 Hz) in the power spectral density of the blood pressure, an index of sympathetic activity, is also higher during the light phase than the dark phase in REMS. The present findings suggest that diurnal changes in MAP in the rat result from the wake rhythm, and that the mechanism for the diurnal control of MAP may be different from that for HR or Tb.     

Separation of circadian and wake duration-dependent modulation of EEG activation during wakefulness

The separate contribution of circadian rhythmicity and elapsed time awake on electroencephalographic (EEG) activity during wakefulness was assessed. Seven men lived in an environmental scheduling facility for 4 weeks and completed fourteen 42.85-h 'days', each consisting of an extended (28.57-h) wake episode and a 14.28-h sleep opportunity. The circadian rhythm of plasma melatonin desynchronized from the 42.85-h day. This allowed quantification of the separate contribution of circadian phase and elapsed time awake to variation in EEG power spectra (1-32 Hz). EEG activity during standardized behavioral conditions was markedly affected by both circadian phase and elapsed time awake in an EEG frequency- and derivation-specific manner. The nadir of the circadian rhythm in alpha (8-12 Hz) activity in both fronto-central and occipito-parietal derivations occurred during the biological night, close to the crest of the melatonin rhythm. The nadir of the circadian rhythm of theta (4.5-8 Hz) and beta (20-32 Hz) activity in the fronto-central derivation was located close to the onset of melatonin secretion, i.e. during the wake maintenance zone. As time awake progressed, delta frequency (1-4.5 Hz) and beta (20-32 Hz) activity rose monotonically in frontal derivations. The interaction between the circadian and wake-dependent increase in frontal delta was such that the intrusion of delta was minimal when sustained wakefulness coincided with the biological day, but pronounced during the biological night. Our data imply that the circadian pacemaker facilitates frontal EEG activation during the wake maintenance zone, by generating an arousal signal that prevents the intrusion of low-frequency EEG components, the propensity for which increases progressively during wakefulness.     

Variations in slow wave activity during sleep in the rat

Scoring of human electroencephalogram (EEG) recordings usually includes subdivisions of non-rapid eye movement (NREM) sleep based on amount of slow wave activity. This procedure has revealed relationships between slow wave activity and many other variables. In animals, however, few experimenters have described variations in slow wave activity within NREM sleep. The present study quantifies, by filtering and integration techniques, variations in amount of slow wave activity during NREM sleep in the rat. Slow wave activity was found to be greatest at the start of the light period; the diurnal variation of slow wave activity within NREM sleep was correlated with variations in amount of NREM sleep. An amplitude criterion was used to define NREM sleep, but overall EEG amplitude during NREM sleep did not show the same diurnal variation as slow wave activity. The results indicate the value of measuring variations in slow wave amplitude during NREM sleep in animals in addition to overall EEG amplitude.     

Diurnal preference, sleep habits, circadian sleep propensity and melatonin rhythm in healthy human subjects

After 24-h sleep deprivation, 33 healthy young subjects entered the 10/20 min ultra-short sleep–wake schedule for 26 h. Melatonin rhythm was hourly assessed simultaneously. Results indicated that morning preference was significantly correlated with habitual sleep onset (r=−0.41, P=0.04), habitual sleep offset (r=−0.52, P=0.002), melatonin peak time (r=−0.36, P=0.04), and sleep propensity onset time (r=−0.36, P=0.04). The intervals between habitual sleep mid-point and melatonin peak time and between habitual sleep mid-point and sleep propensity onset time were significantly longer in morning-preference subjects than in evening-preference subjects (P<0.05). These findings suggest that the variance of diurnal preference may be related to differences in phase relations between habitual sleep timing and the circadian pacemaker.     

Chronobiology of aging: Temperature, sleep-wake rhythms and entrainment

—Studies were carried out on a group of six young (ages 23–30) and six older (ages 53–70) normal men who lived under conditions of temporal, but not social isolation, from three to eight weeks. During entrained and non-entrained (free-running) conditions, comparative measurements were made of sleep-wake cycles, sleep stages and rectal temperature rhythms for these two age groups. Results demonstrated a reduction in the period and amplitude of the body temperature rhythms during free-running in the older group. Sleep efficiency, total sleep time, REM sleep latency, REM episode length, percent REM in the last 2 hours of sleep, the length and frequency of arousals during sleep, and the terminal wake latency were all age related and dependent on entrainment. The period of the sleep-wake cycle, terminal awakenings from REM and percent REM in the first 3 hours of sleep were not age related but were dependent on entrainment. Sleep stages as percents of total sleep time were found to be age related but independent of entrainment, while sleep latency, mid-REM to mid-REM cycle length and the ratio of sleep to total time were neither age related nor dependent on entrainment. In addition, individual chronobiological differences were prominent in the older group. Changes of period and of the phase relationship of sleep-wake and temperature rhythms occurred in several subjects during the non-entrained condition.     

EEG predictors of dreaming outside of REM sleep

The stream of human consciousness persists during sleep, albeit in altered form. Disconnected from external input, the mind and brain remain active, at times creating the bizarre sequences of thought and imagery that comprise “dreaming.” Yet despite substantial effort toward understanding this unique state of consciousness, no reliable neurophysiological indicator of dreaming has been discovered. Here, we identified electroencephalographic (EEG) correlates of dreaming using a within‐subjects design to characterize the EEG preceding awakenings from sleep onset, REM (rapid eye movement) sleep, and N2 (NREM Stage 2) sleep from which participants were asked to report their mental experience. During the transition into sleep, compared to periods during which participants reported thinking, emergence of dream imagery was associated with increased absolute power below 7 Hz. During later N2, dreaming conversely occurred during periods of decreased relative power below 1 Hz, accompanied by an increase in relative power above 4 Hz. No EEG predictors of dreaming were identified during REM. These observations suggest an inverted‐U relationship between dreaming and the prevalence of low‐frequency EEG rhythms, such that dreaming first emerges in concert with EEG slowing during the sleep‐wake transition, but then disappears as high‐amplitude slow oscillations come to dominate the recording during later N2 sleep.     

The diurnal rhythm of adenosine levels in the basal forebrain of young and old rats

There are significant decrements in sleep with age. These include fragmentation of sleep, increased wake time, decrease in the length of sleep bouts, decrease in the amplitude of the diurnal rhythm of sleep, decrease in rapid eye movement sleep and a profound decrease in electroencephalogram Delta power (0.3-4 Hz). Old rats also have less sleep in response to 12 h-prolonged wakefulness (W) indicating a reduction in sleep drive with age. The mechanism contributing to the decline in sleep with aging is not known but cannot be attributed to loss of neurons implicated in sleep since the numbers of neurons in the ventral lateral preoptic area, a region implicated in generating sleep, is similar between young (3.5 months) and old (21.5 months) rats. One possibility for the reduced sleep drive with age is that sleep-wake active neurons may be stimulated less as a result of a decline in endogenous sleep factors. Here, we test this hypothesis by focusing on the purine, adenosine (AD), one such sleep factor that increases after prolonged W. In experiment 1, microdialysis measurements of AD in the basal forebrain at 1 h intervals reveal that old (21.5 months) rats have more extracellular levels of AD compared with young rats across the 24 h diurnal cycle. In experiment 2, old rats kept awake for 6 h (first half of lights-on period) accumulated more AD compared with young rats. If old rats have more AD then why do they sleep less? To investigate whether changes in sensitivity of the AD receptor contribute to the decline in sleep, experiments 3 and 4 determined that for the same concentration of AD or the AD receptor 1 agonist, cyclohexyladenosine, old rats have less sleep compared with young rats. We conclude that even though old rats have more AD, a reduction in the sensitivity of the AD receptor to the ligand does not transduce the AD signal at the same strength as in young rats and may be a contributing factor to the decline in sleep drive in the elderly.     

Automatic analysis of electro-encephalogram sleep spindle frequency throughout the night

A fully automatic method to analyse electro-encephalogram (EEG) sleep spindle frequency evolution during the night was developed and tested. Twenty allnight recordings were studied from ten healthy control subjects and ten sleep apnoea patients. A total of 22 868 spindles were detected. The overall mean spindle frequency was significantly higher in the control subjects than in the apnoea patients (12.5Hz against 11.7Hz, respectively; p<0.004). The proposed method further identified the sleep depth cycles, and the mean spindle frequency was automatically determined inside each sleep depth cycle. In control subjects, the mean spindle frequency increased from 12.0Hz in the first sleep depth cycle to 12.6Hz in the fifth cycle. No such increase was observed in the sleep apnoea patients. This difference in the spindle frequency evolution was statistically significant (p<0.004). The advantage of the method is that no EEG amplitude thresholds are needed.     

Sleep homeostasis in the rat: Simulation of the time course of EEG slow-wave activity

According to the two-process model of sleep regulation, a homeostatic Process S increases during waking and declines during sleep. For humans, the time course of S has been derived from the changes in EEG slow-wave activity (SWA; spectral power density in the 0.75–4.0 Hz range) during sleep. We tested the applicability of the model to sleep in the rat. The simulation was based on the vigilance states for consecutive 8-s epochs of a 96-h experiment in 9 animals. The level of S was made to decrease in epochs of non-REM sleep (NREMS), and to increase in epochs of waking or REM sleep according to exponential functions. By optimizing the initial value and the time constants of S, a close fit between the hourly values of SWA in NREMS and of S was obtained. The biphasic time course of SWA during baseline, its enhancement in the initial recovery period after 24-h sleep deprivation, and its subsequent prolonged undershoot were present in the simulation. We conclude that sleep homeostasis as conceptualized in the two-process model may be a general property of mammalian sleep.     

Reduced Alpha power associated with the recall of mentation from Stage 2 and Stage REM sleep

Relationships between Alpha (8-12 Hz) activity and cognitive processes during wakefulness raise the possibility of similar relationships between Alpha and cognitive activity during sleep. We hypothesized that Alpha power decreases during both Stage 2 and REM sleep would index the presence of sleep mentation in these stages. Absolute power for six classical EEG bands and three Alpha subbands was calculated for Stage 2 and REM sleep awakenings both with and without mentation recall. In both stages, recall was associated with lower Alpha power, especially with middle Alpha power (9.5-11.5 Hz). Unexpectedly, a similar effect for Delta power (0.5-4.0 Hz) was also observed. The Alpha effect may reflect cognitive elaboration active in the minutes preceding awakening; however, attention and memory processes cannot be excluded. The Delta effect is consistent with prior observations of regular linkages between Alpha and Delta power during sleep.     

Homeostatic behavior of fast Fourier transform power in very low frequency non-rapid eye movement human electroencephalogram

Basic research shows that the physiological and molecular mechanisms of very low frequency (<1 Hz) electroencephalogram (EEG) waves of non-rapid eye movement (NREM) sleep differ from those of the higher (1–4 Hz) delta frequencies. Human studies show that the across-NREM period dynamics of very low frequency and 1–4 Hz EEG also differ. These differences and the reported failure of very low frequency EEG power to increase after a night of total sleep deprivation raise the question of whether very low frequency EEG shows the other homeostatic properties established for higher delta frequencies. Here we tested the relation of very low frequency EEG power density to prior waking duration across a normal day and whether these low frequencies meet another criterion for homeostatic sleep EEG: conservation of power across a late nap and post-nap sleep. Data from 19 young adults recorded in four separate sessions of baseline, daytime nap and post-nap sleep were analyzed. Power density in very low frequency NREM EEG increased linearly when naps were taken later in the day (i.e. were preceded by longer waking durations). In the night following an 18:00 h nap, very low frequency power was reduced by roughly the amount of power in the nap. Thus, very low frequency EEG meets two major homeostatic criteria. We hypothesize that these low frequencies reflect the executive rather than the functional processes by which NREM sleep reverses the effects of waking brain activity.     

Effects of aging on food-entrained circadian rhythms in the rat

Twenty-four hour schedules of restricted food availability entrain a component of the circadian activity rhythm in rats via a food-entrainable pacemaker separate from the light-entrainable pacemaker. The effect of aging on food-entrained circadian rhythms was examined in 6 rats maintained on a restricted diurnal feeding schedule from age 3–21 months and again from 24–25 months. Food-entrainment, measured as behavioral anticipation of a 1-hr daily mealtime during the middle of the light period and persistence of this anticipation rhythm during food deprivation, was apparent in the aged rats when recorded in wheel-running cages from 20–21 months of age. Despite the long duration of restricted diurnal food access, the aged rats, like young rats, rapidly reverted to nocturnal activity when transferred to ad lib feeding. When restricted diurnal feeding was reinstated at 24 months age, these rats, now recorded in food-bin monitoring cages, required more time for a food anticipation pattern to emerge and showed a lower amplitude food anticipation rhythm compared to a group of young adult rats. These age-related changes are similar to those that characterize photically entrained circadian rhythms and suggest that both components of the rat's multioscillatory circadian timekeeping system deteriorate in parallel over the life span.     

Diurnal variation in continuous measures of the rat EEG power spectra.

EEG measures that vary on a continuous scale, without separating behavior into discrete states, may complement sleep staging as a means of characterizing diurnal variation in level of arousal. The object of the present study was to evaluate diurnal variation in the EEG power spectrum averaged independently of sleep state, and to determine which parameters best reflect this variation. The EEG from rats maintained with chronic cortical electrodes was continuously digitized at 256 Hz, and power spectra computed by fast Fourier transformation every four seconds. Artifact-free spectra occurring over one-hour periods were averaged. Spectral edge, calculated from 66 percent of the area of spectra, and relative power in delta and theta band-widths derived from averaged spectra vary in a consistent and highly significant diurnal pattern. The trend of relative delta power over the daytime, inactive period (when sleep occurs in nocturnal rodents) resembles that seen in human subjects during sleep, with peak levels occurring at the onset, followed by a steady decline during remaining hours of the daytime rest period.     

High-amplitude theta wave bursts during REM sleep and cataplexy in hypocretin-deficient narcoleptic mice

Neurons that release hypocretin (HCRT; orexin) peptides control wake–sleep states and autonomic functions, and are lost in patients with narcolepsy with cataplexy. Bursts of high‐amplitude electroencephalographic (EEG) activity have been reported during behavioural arrests and rapid eye movement sleep (REMS) episodes at sleep onset in HCRT‐deficient narcoleptic mice. Quantitative information on these EEG phenomena is lacking. We aimed to quantify EEG frequency, occurrence rate, daily rhythm and cardiovascular correlates of high‐amplitude EEG bursts during REMS and cataplexy. Twenty HCRT‐deficient mice and 15 congenic wild‐type controls were instrumented with electrodes for sleep recordings and a telemetric blood pressure transducer. Short (1–2 s) high‐amplitude bursts of pointed theta waves (7 Hz) occurred during either REMS or cataplexy in 80% of HCRT‐deficient mice without any significant accompanying modification in systolic blood pressure or heart period. Theta bursts were significantly more likely to occur during the dark period and in the last third of REMS episodes. Similar EEG events were detected in a significantly lower fraction (27%) of wild‐type mice and with a significantly lower occurrence rate (0.8 versus 5 per hour of REMS). These data demonstrate that occurrence of high‐amplitude theta bursts is facilitated during REMS and cataplexy in narcoleptic mice. Analysis of EEG frequency and daily and intra‐episode patterns of event occurrence do not support interpretation of theta bursts as temporally displaced pre‐REMS spindles. Facilitation of high‐amplitude theta bursts may thus represent a novel neurophysiological abnormality associated with chronic HCRT deficiency.     

Exercise in a cold environment after sleep deprivation

Summary Seven subjects exercised to thermal comfort in a cold environment (0 C, 2.5 m·s^–1) after normal sleep (control) and following a 50-h period of sleep deprivation. Resting core temperature (rectal) taken before the subject entered the cold environment was significantly lower (–0.5 C,P<0.05) following the 50-h period of wakefulness. However, rectal temperature was not different after 15 min of exercise during the two exposures, suggesting that the subjects stored heat more rapidly during the first 15 min of exercise after sleep deprivation. No significant differences in self-chosen exercise intensity, heart rate, metabolic rate, or exercise time were evident between the control and sleep deprived exposures. Fifty hours of sleep deprivation failed to alter the core temperature response during exercise in severe cold stress, and subjects chose identical work rates to minimize fatigue and cold sensation. The results suggest that the 50-h sleep deprivation period was not a true physiological stress during exercise in a cold environment. (Supported by Contract # DAMD 17-81-C1023.)