The main finding of this study was that either 6 or 24 h of SI produced an increase in measures associated with sleepiness. Specifically, during the recovery period that followed SI, rats exhibited a reduction of the average latency to sleep onset (a direct measure of sleepiness), an elevation in the average duration of NREM sleep episodes, and an elevation in NREM EEG delta power (two indirect measures of sleepiness). In addition, during the course of SI exposure, a steady elevation of BF AD was observed (a putative neurochemical correlate of sleepiness). We thus conclude that, over time periods up to 30 h, hypersomnolence stemming from our model of sleep fragmentation was evident.
Although a common experience to all of us, sleepiness can be difficult to measure quantitatively. Variations in sleepiness are considered to reflect changes in the homeostatic sleep drive, which, in turn has two main environmental determinants: 1. the duration of prior wakefulness (manipulated in the present study), and 2. the circadian time of day. In the present study sleepiness was assessed directly by the use of a rodent multiple sleep latency test. This test revealed that SI made rats sleepy. The other indirect measures of the homeostatic sleep drive supported this finding.
The SI treatment successfully modeled characteristics of the sleep profile of the OSA patient, where sleep is very fragmented, particularly limiting entry into, and the amount of, deeper stages 3–4 of NREM sleep and REM sleep (Guilleminault et al., 1976
; Roehrs et al., 1985
; Stepanski, 2002
; Penzel et al., 2003
), most likely due to the continual microarousals that end apneic moments during the OSA patient’s sleep. The SI exposure effectively fragmented sleep, reducing the length of individual NREM sleep episodes from 2 min, 5 s in basal conditions to 58 s during the SI exposure. Within the first couple hours of SI exposure, rats began to sleep during the 90 s periods when the treadmill was off. Indeed, for the 24 h SI group, the total amount of NREM sleep obtained in the last 18 h of 24 h SI was equivalent to baseline values. Rats exposed to this 24 h SI model had fragmented sleep and greatly reduced amounts of REM sleep, whereas total NREM sleep time approached basal levels after the first 6 h of SI exposure. However, as noted in the results section, NREM sleep time in the last 3 hours of the 24 h SI exposure was slightly reduced compared to baseline (see ). In summary, compared to total sleep deprivation, SI exposure better models the sleep fragmentation characteristic of several clinical sleep pathologies. Indeed, the following characteristics of this SI model resemble the sleep pattern of patients with OSA: the frequency of arousals (~30 arousals/hr), the large reduction of REM sleep time, the restriction of average NREM sleep episode duration, and the feature that 24h total amounts of NREM sleep time approach baseline levels.
The increase in the average duration of NREM sleep episodes and NREM EEG delta power during the recovery period after SI exposure supports our hypothesis that the homeostatic sleep drive was elevated by the SI treatment. Various laboratories have proposed that delta power of NREM reflects the homeostatic sleep drive following such sleep manipulations as total sleep deprivation (Borbely and Neuhaus, 1979
; Borbely, 1982
; Franken et al., 1991
). Furthermore, an increase in the average NREM episode duration in the recovery period following sleep deprivation is also accepted as indicative of an increase in sleep pressure (Lancel and Kerhof, 1989; Franken et al., 1991
; Lancel et al., 1992
Although the measurement of sleep onset latencies using the rodent MSLT test provides a direct measure of sleepiness (i.e., sleep propensity), it may not be as sensitive a measure as are polysomnographic indicators of sleepiness. Thus, a decreased sleep onset latency was observed in the first two hours following 24 h SI whereas by the third hour of the recovery period the sleep latencies approached baseline values. In contrast, average NREM EEG delta power in the recovery period was elevated for more than three hours suggesting this may be a more sensitive, albeit indirect, measure of sleepiness.
Recently, Polotsky et al. (2006)
exposed C57BL/6J mice to experimental sleep fragmentation for 12 h/day in the light period and found no evidence of increased NREM delta power (analyzed in 12 h bins), or other changes in sleep that would suggest an increased sleep drive. However, in the present study, rats were on the SI schedule continuously for 24 h, a design that prevents the rats from napping during the dark period to make up for sleep lost during the light/inactive period. The present findings also indicate that increases in NREM delta power were evident in the first hours of the recovery period after SI.
Despite two days of habituation to the treadmill apparatus, during the first 6 h of SI exposure total sleep time was significantly reduced. Hence, one 6 h SI group was compared to rats exposed to 6h of total sleep deprivation produced by gentle handling (i.e., a positive control group). The elevations of BF AD levels produced by 6 h of SI and 6 h of total sleep deprivation were very similar.
Both short term (6 h) and longer term (30 h) SI exposures lead to significantly elevated levels of AD in BF. For example, 6 h of SI produced ~150% elevation of BF AD levels, very similar to the increase that is seen following 6 h of total sleep deprivation in the cat or rat (Porkka-Heiskanen et al., 1997
; Basheer et al., 1999
; Kalinchuk et al., 2003
; McKenna et al., 2003
; Murillo-Rodriguez et al., 2004
). Even though NREM sleep time approached baseline (control matched time of day) values in the last 18 h of the 24 h SI treatment () BF AD levels continued to rise, suggesting that sleep fragmentation alone (defined herein as the decrease in the average NREM episode duration), as opposed to cumulative prior wakefulness, appears to produce an increase in BF AD.
A growing body of evidence supports the role of AD as a mediator of the sleepiness following prolonged wakefulness (i.e., AD is an endogenous somnogen), including the findings of this study. The SI-induced elevation of BF AD levels appeared to be specifically due to the sleep manipulation and not to locomotor activity, since BF AD was not elevated in the exercise control rats. Also, SI did not elevate AD levels when microdialysis probes were placed just anterior to our target site, supporting the previous finding that the elevation of AD in response to sleep loss is brain site specific (Porkka-Heiskanen et al., 2000
The data indicate that either 6 or 24 h of SI can elevate behavioral and electrographic measures of sleepiness. The SI-induced increase in the homeostatic sleep drive is also correlated with an elevation of BF extracellular AD levels, a proposed neurochemical mediator of sleepiness. The combined findings are consistent with the following model: similar to total sleep deprivation, sleep fragmentation leads to an increase of AD in the BF which inhibits the activity of wake-promoting BF neurons, leading to decreased cortical activation and a subsequent increase in sleepiness.