In our analysis of how responses on the PSQ questions were related, we found a high correlation between responses to the ‘soundness of sleep’ and ‘evaluation of sleep’ questions (r = 0.88, P < 0.0001). From this, we concluded that these two questions were not independent and for our subsequent analyses we only included responses to the ‘evaluation of sleep’ question.
There were 72 nights of baseline sleep from the 24 subjects. Data from all subjects for night 1 were omitted from our analysis due to the so-called ‘first night effect’ (Agnew et al., 1966). Data from the PSQs on nights 2 and 3 were not available for one subject and PSG data from night 2 for two subjects were omitted from analysis due to more than 5% of the epochs being unscorable because of equipment or sensor problems. We therefore included a total of 44 baseline sleep episodes from 23 of the subjects in our analysis.
The objective baseline sleep was similar in composition to what has been reported for healthy older adults and was comprised of 60.6 ± 6.3% NREM, 18.1 ± 5.2% REM and 20.9 ± 8% wakefulness (see Table 1). Subjects evaluated their overall baseline sleep as between ‘average’ and ‘good’ (4.73 ± 1.25) and stated that they felt between ‘somewhat refreshed’ and ‘refreshed’ (3.64 ± 1.53) and only ‘slightly sleepy’ (2.02 ± 0.96) at wake time.
Table 1. Mean, standard deviation and range for objective and subjective sleep parameters during baseline and forced desynchrony
| ||Baseline||Forced desynchrony|
|N||Mean ± SD||Range||N||Mean ± SD||Range|
| TST (min)||44||377.5 ± 37.6||292.5–442.5||702||290.6 ± 65.9||59.5–393|
| Stage 1 (min)||44||52.9 ± 28.4||6–128||702||37.9 ± 22.7||3.0–143.5|
| Stage 2 (min)||44||186.4 ± 40.2||94.5–265||702||138.2 ± 43.7||17.5–268.0|
| SE (%)||44||79 ± 8||60–95||702||72.8 ± 16.5||14.9–98.4|
| SWS (min)||44||51.5 ± 34.5||0.5–121||702||46.0 ± 26.7||0–157.5|
| Time awake before WT (min)||44||8.1 ± 19.8||0–89.5||702||23.5 ± 47.4||0–312.5|
| WASO (min)||44||92.5 ± 37.1||21.5–181||702||102.3 ± 65.6||5.5–338.5|
| SOLo (min)||44||7.8 ± 8.8||0.5–53||702||6.4 ± 6.8||0–74.5|
| NREM (min)||44||290.8 ± 30.0||214–348||702||222.2 ± 49.5||50.0–324.0|
| REM (min)||44||86.7 ± 24.8||42.5–162||702||68.5 ± 26.7||0–143.0|
| TST (min)||44||404.1 ± 63.4||240–600||702||389.7 ± 87.5||5–600|
| Time awake before scheduled WT (min)||44||19.4 ± 29.6||0–120||700||34.5 ± 50.8||0–300|
| SOLs (min)||43||19.2 ± 20.7||2–120||697||17.4 ± 26.0||0.05–480|
| Evaluation of sleep||44||4.7 ± 1.3||1–7||702||4.9 ± 1.2||1–7|
| Sleepiness||43||2.0 ± 1||1–5||702||2.5 ± 1.4||1–7|
| Refreshment||44||3.6 ± 1.5||1–7||701||3.7 ± 1.6||1–7|
When the objective and subjective assessments of baseline sleep were assessed, there was a significant correlation between subjective sleep onset latency (SOLs) and objective sleep onset latency (SOLo) (see Table 2). Subjective evaluation of sleep negatively correlated with the duration of stage 1 sleep and positively correlated with the duration of stage 2 sleep (see Fig. 2 and Table 2). Total perceived sleep time correlated with the duration of SWS and negatively correlated with stage 1 and WASO (see Table 2). Decreased subjective sleepiness and increased refreshment after wake time were associated with the amount of wakefulness prior to scheduled wake time (see Table 2).
Table 2. Correlations between objective and subjective sleep parameters during baseline and forced desynchrony
|Objective measures||Subjective measures|
|TST (min)||Sleep latency (min)||Time awake before WT (min)||Number of awakenings||Evaluation of sleep||Sleepiness at wake time||Refreshment at wake time|
| Sleep latency (min)|| ||0.32|| || || ||0.46|| |
| TST (min)|| || || || || || || |
| Stage 1 (min)||−0.32|| || ||0.32||−0.50|| || |
| Stage 2 (min)|| || || || ||0.33|| || |
| SWS (min)||0.38|| || || || || || |
| NREM (min)|| || || || || || || |
| REM (min)|| || || ||−0.33|| || || |
| WASO (min)||−0.30|| || || || || || |
| Time awake before WT (min)|| || || || || ||−0.32||0.32|
| Wake (min)|| || || || || || || |
| Number of awakenings|| || || || || || || |
| Average duration of awakenings|| || || || || || || |
| SE (%)|| || || || || || || |
| Sleep latency (min)||−0.11||0.26|| || || ||0.09|| |
| TST (min)||0.32||−0.22||−0.50||−0.15||0.25||0.23||−0.11|
| Stage 1 (min)||0.16|| ||−0.14||0.30||−0.17|| || |
| Stage 2 (min)||0.18||−0.08||−0.33||−0.15||0.27||0.15|| |
| SWS (min)||0.22||−0.09||−0.17||−0.13|| ||0.19||−0.26|
| NREM (min)||0.32||−0.16||−0.45||−0.09||0.15||0.21||−0.11|
| REM (min)||0.24||−0.18||−0.37||−0.10||0.29||0.16|| |
| WASO (min)||−0.31||0.19||0.51||0.15||−0.25||−0.26||0.12|
| Time awake before WT (min)||−0.08|| ||0.51|| || ||−0.30||0.22|
| Wake (min)||−0.32||0.22||0.50||0.15||−0.25||−0.24||0.11|
| Number of awakenings|| ||0.24||−0.08||0.25|| || || |
| Average duration of awakenings||−0.21|| ||0.42|| ||−0.20||−0.17||0.10|
| SE (%)||0.32||−0.22||−0.49||−0.15||0.25||0.23||−0.11|
Figure 2. Subjective evaluation of sleep with respect to amount of stage 1 and stage 2 sleep during baseline condition. Minutes of stage 1 (left panel) and stage 2 (right panel) sleep during the 480-min baseline sleep episodes were obtained from scored PSG records. Subjective evaluations of sleep quality were ranked on a seven-point scale obtained from the postsleep questionnaire. Each point represents the average (±SEM) evaluation of sleep and amount of stage 1 or 2 sleep for an individual subject.
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There were 30 scheduled sleep episodes during the FD condition per subject. One subject did not complete the final FD sleep episode because she became ill and her study was ended early, and the PSG file from one sleep episode from another subject was lost. A total of 13 sleep episodes from eight of the subjects were not included in our FD analysis due to having more than 5% of the epochs unscorable and three sleep episodes from two other subjects were not included because of problems with the PSQ. This resulted in a total of 702 FD sleep episodes available to be included in our analysis, with each subject contributing between 28 and 30 FD sleep episodes to the analysis.
Objective sleep quality varied widely throughout the FD condition, with SE averaging more than 6% lower (P < 0.0001) than in the baseline condition (see Table 1). Objective sleep latency was not significantly different between the baseline and FD conditions nor was the duration or percentage of SWS. There were fewer minutes of stage 2 and NREM sleep in the FD than in the baseline condition (see Table 1) and the percentage of these sleep stages was also less in the FD condition than in the baseline (stage 2: 34.6 ± 10.9% versus 38.8 ± 8.4%, P < 0.01; NREM: 55.5 ± 12.4% versus 60.6 ± 6.3%, P < 0.0001). There were also fewer minutes of stage 1 and REM sleep in FD than in baseline (see Table 1), but the overall percentage of these sleep stages was not significantly different between FD and baseline (stage 1: 9.5 ± 5.7% versus 11.0 ± 5.9%, P = 0.08; REM: 17.1 ± 6.7% versus 18.1 ± 5.2%, P = 0.26). Overall, during the FD condition the subjects spent significantly longer lying awake just prior to scheduled wake time than they did in the baseline condition (P < 0.0001; see Table 1). While the overall duration of wakefulness during the scheduled sleep episode was not significantly different in the FD and baseline conditions, it was a significantly greater percentage of the scheduled sleep time during the FD condition (27.2 ± 16.5% versus 20.9 ± 8.0%, P < 0.0001).
Subjects rated their subjective sleep latency, TST, number of awakenings, overall evaluation of sleep and feeling of refreshment at wake time similarly in both FD and baseline conditions (see Table 1). Subjects estimated their early awakenings as being longer during FD than during baseline (P < 0.01) and rated themselves as sleepier at wake time in the FD condition (P < 0.01), although these latter two subjective measures did not reach our stringent criteria for significance.
The correlation between objective sleep and the subjective assessment of sleep under FD conditions was performed in the same manner as for the baseline nights. Those significant associations between objective and subjective sleep in the baseline condition were also significant in the FD condition, although in most cases there was a weaker association between the factors (see Table 2). These included the correlation between SOLs and SOLo; total perceived sleep time and stage 1, SWS and WASO; subjective evaluation of sleep and duration of stage 1 and stage 2; sleepiness at wake time and SOLo; and refreshment at wake time and amount of wakefulness prior to scheduled wake time (see Table 2).
In addition, there were associations between subjective and objective sleep that were not significantly correlated at baseline, but which became significantly correlated during the FD condition. Subjective TST significantly correlated with objective TST, with duration of NREM sleep and with SE (see Fig. 3) and negatively correlated with scored wakefulness (see Table 2). Subjective estimation of the duration of wakefulness prior to scheduled wake time positively correlated with objectively scored wakefulness prior to scheduled wake time (see Fig. 4) and WASO. Subjective wake prior to scheduled wake time was also associated with objective TST, SE and duration of stage 2 (see Table 2).
Figure 3. Subjective total sleep time as a function of sleep efficiency (SE) during forced desynchrony condition. SE (% of 400-min scheduled sleep episode where any stage of sleep occurred) was obtained from scored PSG records. Self-assessed total sleep time (in min) was reported on the postsleep questionnaire. Each point represents the average (±SEM) for an individual subject.
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Figure 4. Objective versus subjective estimation of waking prior to scheduled wake time during forced desynchrony condition. Objective duration between the final epoch of any sleep stage and scheduled wake time was obtained from scored PSG records, while subjective estimation was obtained from the postsleep questionnaire. Each point represents the average (±SEM) for an individual subject.
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When we examined the influence of circadian phase on all-night objective sleep parameters, we found a significant influence of circadian phase on sleep latency (F7,671 = 8.68, P < 0.0001), SE (F7,671 = 24.73, P < 0.0001), NREM (F7,671 = 15.69, P < 0.0001) and REM sleep (F7,671 =24.25, P < 0.0001), as well as in the amount of wakefulness at the end of the night (F7,671 = 11.41, P < 0.0001), the total amount of wakefulness during the scheduled sleep episode (F7,671 = 26.33, P < 0.0001) and the average duration of awakenings (F7,671 = 8.06, P < 0.0001). With the exception of subjective number of awakenings (F7,671 = 0.66, P = 0.7), all subjective sleep estimates (latency: F7,666 = 4.15, P < 0.001; TST: F7,671 = 3.90, P < 0.001; duration of waking prior to scheduled time: F7,669 = 13.40, P < 0.0001; overall evaluation of sleep: F7,671 = 5.35, P < 0.0001; feeling of sleepiness at wake time: F7,671 = 10.85, P < 0.0001; feeling refreshed at wake time: F7,670 = 3.73, P < 0.001) also showed significant circadian variations.
Because of these significant circadian variations in objective and subjective sleep during the FD, we conducted a second comparison of FD versus baseline sleep and restricted FD sleep episodes to those which occurred at similar circadian phases to the baseline nights. The phase at wake time during baseline ranged from 10.75° to 92.25° between the subjects and there were 152 FD sleep episodes in this range. When we re-did our comparison of baseline versus FD sleep on this restricted data set, we found that objective sleep latency and the percentage of stage 1, stage 2, SWS and NREM sleep were similar in the two conditions. The percentage of REM sleep was significantly higher in the FD condition (22.08 ± 5.0% versus 18.06 ± 5.2%, P < 0.0001) as was SE (84.15 ± 8.22% versus 79.05 ± 8.03%, P < 0.001). While the number (P = 0.043) and average duration (P = 0.011) of awakenings did not reach the stringent significance level we set of <0.0025, they were lower in the FD condition, resulting in the overall percentage of wakefulness being significantly lower in the FD condition than in baseline (15.81 ± 8.2% versus 20.89 ± 8.02%, P < 0.001). Subjective ratings of TST and number of awakenings did not differ between the two conditions, nor did the subjective feeling of refreshment at wake time. Subjective sleep latency (11.22 ± 7.91 versus 19.22 ± 20.71 min, P = 0.017) and subjective duration of premature awakening (10.39 ± 21.67 versus 19.43 ± 29.57 min, P = 0.064) tended to be shorter in the FD condition, and subjects rated themselves as feeling slightly more sleepy at wake time (P < 0.001) and evaluated their sleep as significantly better in the FD condition (P < 0.01).