Physiological correlates of prolonged sleep deprivation in rats.

The issue of whether sleep is physiologically necessary has been unresolved because experiments that reported deleterious effects of sleep deprivation did not control for the stimuli used to prevent sleep. In this experiment, however, experimental and control rats received the same relatively mild physical stimuli, but stimulus presentations were timed to reduce sleep severely in experimental rats but not in controls. Experimental rats suffered severe pathology and death; control rats did not.

The circadian clock mutation alters sleep homeostasis in the mouse.

The onset and duration of sleep are thought to be primarily under the control of a homeostatic mechanism affected by previous periods of wake and sleep and a circadian timing mechanism that partitions wake and sleep into different portions of the day and night. The mouse Clock mutation induces pronounced changes in overall circadian organization. We sought to determine whether this genetic disruption of circadian timing would affect sleep homeostasis. The Clock mutation affected a number of sleep parameters during entrainment to a 12 hr light/dark (LD 12:12) cycle, when animals were free-running in constant darkness (DD), and during recovery from 6 hr of sleep deprivation in LD 12:12. In particular, in LD 12:12, heterozygous and homozygous Clock mutants slept, respectively, approximately 1 and approximately 2 hr less than wild-type mice, and they had 25 and 51% smaller increases in rapid eye movement (REM) sleep during 24 hr recovery, respectively, than wild-type mice. The effects of the mutation on sleep are not readily attributable to differential entrainment to LD 12:12 because the baseline sleep differences between genotypes were also present when animals were free-running in DD. These results indicate that genetic alterations of the circadian clock system and/or its regulatory genes are likely to have widespread effects on a variety of sleep and wake parameters, including the homeostatic regulation of sleep.

Effect of total sleep deprivation on 5'-deiodinase activity of rat brown adipose tissue.

Prolonged sleep deprivation of the rat produces a progressive increase in energy expenditure and an eventual decrease in body temperature, which suggests a profound derangement in thermoregulation. Because increased thermogenic activity in brown adipose tissue (BAT) is a likely mechanism mediating the observed increase in energy expenditure, we focused our attention on the effect of total sleep deprivation on BAT type II 5'-deiodinase (5'D-II), since its activation indicates BAT stimulation and is essential for full BAT thermogenic response. Five euthyroid rats were subjected to total (92%) sleep deprivation (euD-rats). Sharing the sleep deprivation apparatus, yoked control rats (euC-rats) received the same degree of physical stimulation as the D-rats, but were only partially (25%) sleep deprived. Additional cage controls (euCC-rats) were housed in the same room. Since during sleep deprivation the animals undergo a reduction in plasma T4 concentration and inability to maintain body temperature heralds death, an identical study was performed in five trios of hyperthyroid rats (hyperD-, hyperC-, and hyper CC-rats) given daily ip injections of 15 micrograms T4/100 g BW, 10 days before and throughout the deprivation period. Experiments were carried out at an ambient temperature of 29 C, close to thermoneutrality for rats. Sleep deprivation in hyperD-rats was maintained until death seemed imminent (9-14 days), and in euD-rats for 12-15 days. Sleep deprivation induced a significant increase in BAT 5'D-II activity in both hyperD- and euD-rats compared with that in euCC-rats (P less than 0.01). BAT 5'D-II in euC-rats was also significantly higher than that in euCC-rats (P less than 0.05), probably because they were partially sleep deprived. BAT 5'D-II activity in hyperD-rats was increased compared to that in both hyperC- and hyperCC-rats (P less than 0.05), in which the activity was slightly but not significantly lower than that in euCC-rats. No significant differences were observed in liver and kidney type I 5'-D (5'D-I) and in pituitary 5'D-II among euD-rats, euC-rats, and euCC-rats. As expected, the hyperthyroid groups (hyperD-rats, hyperC-rats, and hyperCC-rats) had significantly higher kidney 5'D-I and lower pituitary 5'D-II than the euCC-rats. Liver 5'D-I was also significantly increased in the hyperC-rats and hyperCC-rats, but not in the hyperD-rats. These observations indicate that total sleep deprivation is associated with a marked increase in BAT 5'D-II activity in both euthyroid and hyperthyroid rats.(ABSTRACT TRUNCATED AT 400 WORDS)

EEG delta power during sleep in young and old rats.

Delta EEG power density, which has been viewed as a measure of intensity of NREM sleep, declines across the lifetime in humans, cats, and hamsters, but data in rats have been unclear. It is also uncertain whether older rats differ from younger animals in the degree of change in delta power during recovery sleep following short-term sleep deprivation. We have examined delta power density in NREM sleep under baseline conditions and following 48 h of sleep deprivation in young (3 months), middle-aged (12 months), and older (24 months) rats. The presence or absence of age effects was highly dependent on the method of normalizing the data. When expressed as a fraction of total NREM EEG power, there was no age effect on baseline delta power density, or on the change from baseline to recovery conditions. When expressed as a multiple of delta power in REM under the same condition, the younger rats had higher delta power density than the middle-aged and older rats. For all the ages combined, there was an increase in delta power density in the recovery condition. When examined by age, the younger rats (which started from a higher level of delta power density than the other groups) did not have an increase in delta during recovery; the middle-aged rats tended to, and the older rats (which started from lower baseline levels) significantly increased delta power density in the recovery condition. This suggests that the lower delta power seen during baseline in older rats is not due to decreased ability to generate delta activity.

Age-dependent changes in recovery sleep after 48 hours of sleep deprivation in rats.

To characterize possible changes in homeostatic regulation of sleep with aging, we have examined sleep stages during recovery sleep after 48 h of sleep deprivation in young (3 months), middle aged (12 months), and old (24 months) rats. It was found that young and middle aged, in contrast to old rats, had large (21-24%) increases in total sleep time during recovery sleep; the old rats experienced a quantitatively small (8%) but significant rise in total sleep. NREM sleep increased significantly during the recovery period in young and middle aged, but not older rats. High voltage NREM sleep (HS2) declined by 30% during recovery in the young animals, but remained unchanged compared to baseline in the middle aged and old animals. The young and middle aged rats had increases in REM sleep during recovery compared to their baseline by 96% and 93%, respectively, which was significantly greater than a 65% increase during recovery in the old rats. Increases in total sleep and REM sleep during recovery were largely confined to the first 6 h in young and middle aged rats, but maxima for the old rats occurred in the second 6 h.

Increased basal REM sleep but no difference in dark induction or light suppression of REM sleep in flinders rats with cholinergic supersensitivity.

Increased cholinergic sensitivity in the central nervous system has been postulated to account for some of the neuroendocrine abnormalities and sleep disturbances seen in human depressives. The Flinders Sensitive Line (FSL) rats, which exhibit increased sensitivity to cholinergic agents, have been shown to have REM sleep patterns similar to those seen in depressives, including shorter REM sleep latency and increased daily percentage of REM sleep. We studied the response of FSL and control rats to brief dark pulses administered during the normal light period (which are known to stimulate REM sleep in albino rats) and to brief light pulses during the normal dark period (which suppress REM sleep in albino rats) to determine whether these responses are affected by central cholinergic hypersensitivity. FSL rats showed REM sleep patterns indistinguishable from controls during light or dark pulses, which does not support the primary involvement of cholinergic systems in this mechanism of REM sleep regulation. We also examined REM and non-REM (NREM) sleep patterns in FSL rats and their controls to determine whether they show sleep continuity disturbances or decreased sleep intensity as seen in depression. In agreement with an earlier study, we found that FSL rats had more daily REM sleep and accumulated less NREM sleep between REM bouts than controls. Duration of NREM sleep bouts, total daily NREM sleep time, and EEG amplitude of NREM sleep did not differ between FSL and control rats, suggesting that the cholinergic abnormalities in FSL rats do not produce substantial NREM sleep changes.

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.

Effects of pinealectomy on baseline sleep and response to sleep deprivation.

STUDY OBJECTIVES: We have previously reported that older (24 mo.) Fischer rats manifest a diminished post-sleep deprivation increase in NREM and REM sleep. In order to examine whether this decline reflects an age-related change in pineal function, we are now reporting on baseline and recovery sleep parameters in pinealectomized 3-, 12-, and 24-month old rats following 24 hours of sleep deprivation using the disk-over-water method. DESIGN: Three independent age groups; within each group there were sequential measures of sleep under baseline conditions and during recovery from sleep deprivation. SETTING: The Sleep Research Laboratory at the University of Chicago PARTICIPANTS: 56 male Fisher (F344) rats INTERVENTIONS: 24 hours of total sleep deprivation using the disk-over-water method MEASUREMENTS: Sleep staging of EEG and EMG, and power spectral analysis of the EEG RESULTS: Pinealectomized (pinex) rats did not differ from sham-operated (sham) rats in total sleep, REM sleep, super-modal high-amplitude NREM sleep (HS2), a measure of NREM EEG delta power, or circadian rhythm amplitude. In the pinex rats, there was a modest (2.5%) age-independent increase in NREM sleep (p<0.02). The pinex rats of all ages failed to manifest the increase in NREM sleep during recovery seen in the sham-operated animals (p<0.04). CONCLUSIONS: We found no evidence that altered pineal function is responsible for age-related changes in baseline sleep in the rat. These data also suggest that, independent of age, normal pineal function may be relevant to the ability to generate increased NREM sleep in response to prior sleep deprivation.

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.

Relationships among wake episode lengths, contiguous sleep episode lengths, and electroencephalographic delta waves in rats with suprachiasmatic nuclei lesions.

The lengths of sleep and wake episodes during 2 consecutive days of recording were measured in five rats lacking circadian rhythms owing to lesions of the suprachiasmatic nuclei. Total sleep (TS) episode lengths and the amount of NREM sleep and paradoxical sleep (PS) within each episode were examined in relationship to the lengths of the immediately preceding and the immediately following wake episodes. As putative measures of sleep intensity, average and maximum delta wave (1-4 Hz) incidence and amplitude within NREM were also examined in relation to adjacent wake episode lengths. For sleep episodes longer than 50 min (78% of daily sleep), TS episode lengths and amount of NREM within these episodes showed significant positive correlations with both prior and subsequent wake episode lengths. PS durations within sleep episodes also showed significant positive correlations with subsequent wake episode lengths, but little correlation with prior wake episode lengths. The results suggest that in the absence of sleep-wake circadian rhythms, sleep time is subject to short-term homeostatic regulation. Amounts of PS within sleep episodes were highly correlated (r = 0.84) with amounts of NREM. NREM delta wave incidence and amplitude showed no significant relationships with the lengths of prior or subsequent wake episodes, suggesting that variations in sleep intensity may not play a prominent role in the short-term homeostatic regulation of ad lib sleep. Delta wave incidence and amplitude were also not correlated with the duration of NREM episodes, but incidence during wake was positively correlated with wake episode duration, suggesting that delta density during wake may be an electrophysiological indicator of the propensity to sleep.

Sleep deprivation in the rat: III. Total sleep deprivation.

Ten rats were subjected to total sleep deprivation (TSD) by the disk apparatus. All TSD rats died or were sacrificed when death seemed imminent within 11-32 days. No anatomical cause of death was identified. All TSD rats showed a debilitated appearance, lesions on their tails and paws, and weight loss in spite of increased food intake. Their yoked control (TSC) rats remained healthy. Since dehydration was ruled out and several measures indicated accelerated use rather than failure to absorb nutrients, the food-weight changes in TSD rats were attributed to increased energy expenditure (EE). The measurement of EE, based upon caloric value of food, weight, and wastes, indicated that all TSD rats increased EE, with mean levels reaching more than twice baseline values.

Sleep deprivation in the rat: IV. Paradoxical sleep deprivation.

Twelve rats were subjected to paradoxical sleep deprivation (PSD) by the disk apparatus. All PSD rats died or were sacrificed when death seemed imminent within 16-54 days. No anatomical cause of death was identified. All PSD rats showed a debilitated appearance, lesions on their tails and paws, and weight loss in spite of increased food intake. Their yoked control (PSC) rats remained healthy. Since dehydration was ruled out and several measures indicated normal or accelerated use of nutrients, the food-weight changes in PSD rats were attributed to increased energy expenditure (EE). The measurement of EE, based upon caloric value of food, weight, and wastes, indicated that all PSD rats increased EE, with mean levels reaching more than twice baseline values. All of these changes had been observed in rats deprived totally of sleep; the major difference was that they developed more slowly in PSD rats.

Sleep deprivation in the rat: VIII. High EEG amplitude sleep deprivation.

The disk apparatus was used to deprive six rats of the portion of non-rapid eye movement (NREM) sleep with high electroencephalogram (EEG) amplitude (HS2). All HS2 deprived (HS2D) rats died or were sacrificed when death seemed imminent within 23 to 66 days. No anatomical cause of death was identified. All deprived rats showed a debilitated appearance, lesions on their tails and paws, and weight loss in spite of increased food intake. Energy expenditure (calculated from the caloric value of food, weight change, and wastes) increased to more than twice baseline values. With one exception, yoked control rats remained generally healthy. It was not clear whether the changes in HS2D rats resulted from the loss of HS2 or the general disruption of NREM sleep that accompanied this loss. Also, it was not possible to produce major HS2 loss without incurring some loss of paradoxical sleep (PS). Control studies indicated that the partial PS loss in HS2D rats could not, in and of itself, account for all the pathological effects. However, an interaction of HS2D and partial PS loss in producing pathological effects cannot be ruled out.

Effects of sleep deprivation on sleepiness, sleep intensity, and subsequent sleep in the rat.

The effects of 24 hr of sleep deprivation on cortical EEG and ventral hippocampus EEG recordings, ventral hippocampus spike rates, sleep stages percentages, and bout length measures were studied in rats. Two groups, differing only in the rate and distance they were forced to walk during deprivation by the water wheel method, were recorded continuously (23 hr per day) for one baseline, one deprivation, and two recovery days. During deprivation, microsleeps, increased hippocampal spike rates, and increased amplitude of the EEG recordings all suggested the intrusion of sleep processes. Nonetheless, there was no evidence to support the idea that these animals were not substantially deprived of sleep. No important differences were found in the recovery data of the two groups, even though one group walked three times as far as the other during deprivation. This supports the idea that, in conjunction with large amounts of sleep deprivation, changes in exercise and energy depletion may have little effect on sleep measures. During recovery, increased hippocampal spike rates and bout lengths, as well as increases in EEG amplitude, were interpreted in terms of increased sleep "intensity." High amplitude NREM sleep rebounded first, followed by rebounds in both paradoxical sleep and low amplitude NREM sleep. This pattern was compared to patterns previously reported for humans, cats, and rats. Finally, the tendency for some measures to fall below their baseline levels after an initial rebound was discussed in terms of "sleep inhibition" and servomechanism theory.

Sleep deprivation in the rat: XIV. Comparison of waking hypothalamic and peritoneal temperatures.

Earlier studies of rats subjected to total sleep deprivation (TSD) by the disk-over-water method had shown an initial increase in waking peritoneal temperature (T(ip)) followed by an even greater decrease as deprivation proceeded. In the present study, hypothalamic temperature (T(hy)), as well as T(ip), were recorded continuously. As in the earlier studies, TSD rats showed an increase in energy expenditure and an initial increase followed by a decrease in T(ip). Waking T(hy) showed a more prolonged initial rise and a smaller late decline than waking T(ip) Assuming that, as the literature suggests, T(hy) is held closer to temperature setpoint (TSET) than is T(ip), the present results suggest an elevated waking TSET during deprivation. T(ip) became progressively lower than T(hy) over the course of deprivation, indicating a decreased ability to maintain the whole body near TSET. This decreased ability could result from insufficient thermogenesis or excessive heat loss. Because thermogenesis rose progressively throughout deprivation, heat loss must have increased even more than heat production. Thus, the results are consistent with other data which indicate that TSD in the rat produces two opposing effects on waking temperature, an elevation of setpoint and excessive heat loss, which together increase the demand for energy expenditure.

Sleep deprivation in the rat: XVI. Effects in a light-dark cycle.

To avoid a possible confound between the effects of sleep loss and disturbed circadian rhythms in previous studies of total sleep deprivation (TSD) by the disk-over-water method, TSD rats and their yoked control (TSC) rats had been maintained in constant light both before and during the experiment. With circadian rhythms of both groups flattened by constant light, group differences in outcome measures could be attributed to sleep loss. However, the constant light control entailed the possibility that the sleep loss effects might obtain only in constant light. To evaluate this possibility, three TSD-TSC rat pairs maintained on a 12 hour light: 12 hour dark (LD) schedule were studied. TSC rats showed only minor changes during the deprivation period. As in previous studies, TSD rats showed increased food intake; decreased weight; increased energy expenditure; debilitated appearance; lesions on the tail and paws; an initial increase followed by a large decrease in body temperature; impending death; and recovery sleep, which featured large, selective, sustained rebounds of paradoxical sleep and a reversal of all observed TSD-induced changes. Thus, TSD produced the same changes during an LD schedule as during constant light. The amplitude of the diurnal body temperature rhythm declined over the course of TSD and then almost completely recovered during the first day of recovery sleep. The decline was interpreted as the result of deprivation-induced thermoregulatory changes.

Vascular resistance in the rat during baseline, chronic total sleep deprivation, and recovery from total sleep deprivation.

Rats subjected to total sleep deprivation (TSD) by the disk-over-water method exhibit an elevated temperature set point, increased energy expenditure (EE), and increased circulating norepinephrine--all of which should militate for an increase in body temperature. Instead, after a small rise early in TSD, intraperitoneal temperature (T(ip)) fell progressively, indicating a reduced ability to retain body heat. To evaluate whether vasoconstrictor defenses against heat loss in the regions of major heat dissipation in the rat (hindpaws and tail) were impaired, peripheral vascular resistance (PVR) was calculated from aortic blood pressure (BP) and blood flow (BF) (BP and BF were continuously recorded at the aortic-iliac junction). TSD rats and their yoked control (TSC) rats were subjected to TSD for 10 to 22 days. As in earlier studies, TSD rats showed excessive heat loss indicated by a falling T(ip) (after an initial rise) while EE was elevated. Temperature set point was presumably raised throughout deprivation as shown previously. Although a small decline in PVR early in deprivation could have increased heat loss, there was no evidence of a massive vasodilation in the region examined which could, in itself, account for the progressive inability to retain heat over the course of TSD. In fact, PVR was near baseline levels during the latter half of TSD. Nevertheless, there was evidence of impaired vasoconstrictive defenses in TSD rats inasmuch as they showed significantly lower PVR than TSC rats during most of the deprivation period in spite of indications that they were farther below set point. It is not yet clear whether this impairment was a major determinant of the heat loss in TSD rats. A rapid PVR rebound during recovery suggested a release from a TSD-linked blockage of vasomotor compensation for excessive heat loss.

Effects of paradoxical sleep deprivation on thermoregulation in the rat.

Rats were subjected to chronic paradoxical sleep deprivation (PSD) by the disk-over-water method to determine if they would develop the sustained increase in core (hypothalamic) temperature (T(hy)); elevated temperature setpoint (Tset); and the attenuation of the normal decline in core temperature during the transition from wake to sleep observed in rats subjected to total sleep deprivation (TSD). PSD rats did not show a significant elevation in T(hy). PSD rats and their yoked controls (PSC) were provided with a continuously available operant by which they could increase ambient temperature (Tamb). Change in Tset was assessed by evaluating operant behavior as a function of hypothalamic and intraperitoneal temperature (T(ip)). Unlike TSD rats, PSD and PSC rats maintained near-baseline Tamb at all T(hy) and T(ip) values throughout the deprivation, indicating no change in Tset. As deprivation progressed, PSD rats displayed an attenuation of the normal fall of T(hy) and T(ip) during the transition from wake to sleep. PSC rats did not. During the final quarter of survival time, T(ip) in PSD rats actually rose above waking values during the transition to NREM. These results indicate that PS loss may alter thermoregulation during sleep. It would appear that selective PSD is sufficient to attenuate the normal decline in T(hy) and T(ip) during NREM sleep, whereas NREM loss is required for elevations in T(hy) and Tset.

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.

Recovery sleep following sleep deprivation in intact and suprachiasmatic nuclei-lesioned rats.

Recovery sleep was studied for 3-5 days following 24 h of sleep deprivation (TSD) in normal rats and in rats lacking circadian rhythms (CRs) of sleep because of prior lesioning of the suprachiasmatic nuclei (SCN). One group of lesioned rats was run in constant dim light. Another lesioned group and an intact group were run on a 12:12 dark-light schedule with TSD and recovery beginning at lights-off. All groups showed immediate rebounds of high-amplitude NREM sleep and paradoxical sleep, confined mostly to the first 12-18 h of recovery, and decreases in moderate and low-amplitude NREM sleep during the first 6-12 h of recovery. Thus, sleep stage rebound priorities were little affected by CRs. Total sleep rebound was initially greatest in intact rats, but limited mostly to the first 12 h of recovery. Total sleep rebound was distributed over a longer period in SCN rats, but total accumulated rebound was similar in all groups. Thus, CRs appear to modulate the timing but not the amount of accumulated total sleep rebound. Results were interpreted in terms of ceiling effects on total sleep, delayed rebounds, and competition between CRs and homeostatic recovery processes. Recovery sleep of lesioned rats on the dark-light schedule was marked by a transient diurnal rhythm.