Introduction

It is believed that good sleep is crucial to the maintenance and restoration of health. Middle-aged adults and elderly people often complain about their sleep quality [1, 2]. Such self-reports were corroborated by results of the vast majority of polysomnographic studies revealing significant worsening of objectively determined characteristics of sleep quality with advancing of age [3,4,5]. Amazingly, such worsening was also detected in healthy, non-complaining and carefully screened elderly people [6]. Therefore, complaints about poor sleep quality were suggested to be secondary to health problems, whereas the sleep–aging process seems not to be the major reason for such complaints [7, 8]. The values of the polysomnographic sleep characteristics for good sleep in older people correspond to the values for poor sleep in younger people because older people appear to weaken their criteria for good sleep [9]. Earlier, Buysse et al. [10] hypothesized the ability of healthy people with objectively deteriorated sleep adjust their expectations about their night sleep to what they accept as being its age-related changes, and Zilli et al. [11] showed that healthy people older than 65 years clearly perceive worsening of their night sleep characteristics, but do not differ in the level of sleep satisfaction from young people, because they heed the perceived freshness after awakening not paying much attention to the frequent nighttime awakenings [11].

The cited studies focused on the association between sleep satisfaction and night sleep, whereas a potential link between sleep satisfaction and age-associated changes in sleep–wake timing has not been tested yet. Therefore, the purpose of the present analysis is to examine whether a higher level of sleep satisfaction can be associated with the typical for a given age’s characteristics of human sleep–wake pattern.

Materials and methods

Sleep satisfaction and a set of characteristics of the sleep–wake pattern were self-reported by 160 healthy participants of the sleep deprivation experiments. More detailed analyses of their objective (electroencephalographic) and subjective measures of sleepiness were provided elsewhere [12,13,14,15]. The ages of the 67 male and 93 female participants ranged from 15 to 67 years (mean ± standard deviation = 31.0 ± 13.4).

Each morning during a week prior to the experiment, the participants reported the duration of their nap, sleep latency and times for going to bed and awakening. Sleep Satisfaction Score (SSS) was defined as a response to a single question: “Did you have a good sleep? (No = 1… [I slept] Very well = 5)”. Everyday self-reports were averaged over the pre-experimental week with exception of the duration of nap that was transformed into a nap frequency score (1 = no, 2 = once, and 3 = more than once).

The 72-item Sleep–Wake Pattern Assessment Questionnaire [16] was administered for self-assessment of anytime and daytime wakeability, anytime and nighttime sleepability, and evening and morning lateness on six 12-item scales. Each score varies from − 12 to + 12. Positive score signifies either the ability, namely, anytime wakeability, daytime wakeability, anytime sleepability, nighttime sleepability (W, V, F, S > 0, respectively), or lateness, namely, morning lateness, evening lateness (M, E > 0, respectively), and negative score signifies either the inability (W, V, F, S ≤ 0) or earliness (M, E ≤ 0). The scales were earlier validated against the objective and subjective measures of the phase position of circadian rhythms of physiological and hormonal variables, wave-forms of diurnal variations in alertness–sleepiness, self-reported bed and rising times, etc. [12,13,14,15, 17,18,19,20]. In particular, nighttime sleepability scale (S) was validated against such objective and subjective measures as percentage of slow wave sleep, nap and all-night sleep latency, subjectively and objectively assessed sleepiness, and self-reported total sleep time [18]. Anytime sleepability scale (F) and anytime and daytime wakeability scales (W and V) were validated in sleep deprivation experiments against subjective and electroencephalographic indexes of sleepiness and homeostatic sleep pressure [12,13,14, 19, 20]. The last of the analyzed self-reports was subjective sleepiness scored with the 9-step Karolinska Sleepiness Scale (KSS) [21] self-reported at 9:00 after the first night of sleep deprivation. The whole list of these self-reports is given in Tables 1 and 2.

Table 1 Correlations of age and Sleep Satisfaction Score (SSS) with their suggested predictors
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Table 2 Stepwise linear regression analysis aimed on the prediction of age and SSS
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The sample was subdivided into three age groups and three SSS groups (Tables 1, 2). To test significance of the relationship between sleep satisfaction and sleep–wake pattern in the whole sample and in each of the groups, correlation analysis (Table 1), stepwise linear regression analysis (Table 2), and three- and two-way MANOVAs (Fig. 1) were used.

Fig. 1
figure 1

Self-assessment of the sleep–wake times and traits in groups with different ages and Sleep Satisfaction Score (SSS). a–e Estimated marginal means ± confidence interval (CI, vertical lines) for participants subdivided into nine groups (three ranges of age × three rages of SSS) were calculated by means of the three-way MANOVA

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Results

The results presented in Tables 1 and 2 and Fig. 1 suggest that the self-reported characteristics of sleep–wake pattern exhibited significant changes across the ages and that any of these changes was in the expected direction. Particularly, the age of the study participants correlated with the advance shift of times for going to bed and awakening, morning earliness, shortening of sleep duration, reduction of anytime and nighttime sleepability, and reduced sleepiness in the morning after sleepless night (Table 1). SSS did not correlate with age. However, the stepwise linear regression analysis yielded three significant predictors of older age that included higher SSS in addition to early awakening and reduced nighttime sleepability (Table 2). When this analysis was performed separately for the three SSS groups, the explained variance was found to be maximal (57%) in the group of those participants who reported that they slept very well. In contrast, none of the characteristics of sleep–wake pattern significantly contributed to difference in age of participants from the group with the lowest SSS (Table 2).

A set of characteristics of the sleep–wake pattern that correlated or predicted SSS differed from the set of correlates and predictors of age. Some of correlations and predictors revealed in analyses of the whole sample were intuitively expected, e.g., the correlations suggesting association of higher SSS with longer nighttime sleep and shorter sleep latency (Table 1) and such a predictor of higher SSS as higher nighttime sleepability (Table 2). However, higher SSS was additionally associated with both morning earliness and late awakening (Table 1) and significant predictors of higher SSS score were both morning earliness and later time for going to bed (Table 2).

Results of analyses performed separately on three age sub-samples provided an explanation for these contradictive relationships. In the group of youngest participants (≤ 25 years), the significant correlates were, as expected, evening lateness, later time for going to bed and awakening, and longer sleep duration, whereas in the group of oldest participants (46–67 years), higher SSS was, as expected, associated with morning earliness and earlier time for going to bed (Table 1). If the strongest predictor of this score in the group of youngest participants was late awakening, such a predictor in the groups of older participants (≥ 26 years) was morning earliness (Table 2).

These results were further confirmed by results of the three-way MANOVA of the whole dataset yielding significant interaction between the factors “Age” and “SSS” for earliness and times for going to bed and awakening. The interaction between the factors “Age” and “SSS” are illustrated in Fig. 1. It shows that sleep satisfaction was linked to age-typical characteristics of sleep–wake pattern, such as lateness in young adults and earliness in older adults. The results of the MANOVAs additionally revealed a significant relationship between sleep satisfaction and wakeability characteristics of the sleep–wake pattern (Fig. 1). Higher SSS was always associated with higher wakeability (Fig. 1d). In contrast, either earlier or later phase of wake–sleep cycle were important contributors to sleep satisfaction only in separate age groups (Fig. 1a–c).

Discussion

It is believed that a good sleep is absolutely crucial for health, but it is not excluded that a good night’s sleep is the exception rather than the norm for elderly people. Approximately, half of them in the United States population experiences sleep disturbance that includes difficulty of falling asleep and nighttime awakenings, unsatisfactory nocturnal sleep quality and disturbed or “light” sleep, insufficient sleep duration and unwanted early morning awakenings, reduced level of daytime alertness and undesired daytime drowsiness [2]. Moreover, the worsening of objective characteristics of sleep quality was documented for healthy non-complaining elderly people [6]. Do such findings indicate that elderly people mostly fail to get the good sleep they really need? The answer might be no, because a perceived level of sleep satisfaction is not necessarily linked to age-associated deterioration of objective and subjective indicators of night sleep quality. It was suggested that, when these people rate their sleep satisfaction, they can rely more on the perceived freshness after awakening rather than on high frequency of nighttime awakenings that they also can clearly perceive [11]. The present results supported and extended such findings. They indicate that the self-assessed characteristics of the sleep–wake pattern exhibited notable shifts already on the age interval from early to late adulthood. Moreover, those of characteristics of this pattern that can be linked to quality of night sleep (e.g., score on S scale) also exhibited significant change. However, despite these changes, sleep satisfaction did not decline in the older study participants as compared to younger participants. It remained adjusted to what is considered to be the age-specific pattern of the sleep–wake cycle.

Elderly people were proposed to be able to adjust their expectations about sleep to the changes they accept as age-related [10]. The present results indicate that such an adjustment is also notable in groups of people of younger ages (till 67 years), and that it persists across the interval of ages from early to late adulthood. Can the sleep aging process underlie this ability in spite of biological rather than the psychological nature of this process? In the following discussion, I am arguing for possibility that age-associated changes in sleep–wake regulating processes can determine people’s ability to adjust their perception of “sleep goodness” to the typical for their age’s sleep–wake pattern. Namely, I propose that the effect of aging on the relative strength of the antagonistic wake and sleep drives can determine the ability of older people to adjust expectations about their night sleep quality to age-specific sleep–wake pattern.

According to the two-process conceptualization of the sleep–wake regulating mechanisms, the sleep drive (i.e., the sleep-promoting process) arises from combination of two major processes, homeostatic and circadian, and the strength of this drive is indicated by amplitude of slow-wave activity and percentage of slow wave sleep [22]. Factor analysis of data of longitudinal intra-individual studies reported by Åkerstedt et al. [23, 24] suggested that the items designed for subjective evaluation of “sleep goodness” were sorted into two major factors. The first factor was represented by subjective sleep quality, calmness of sleep, ease of falling asleep, number of awakenings, sleep latency, etc., and they predicted “better” sleep with longer sleep duration, lower number of awakenings, higher amount and percentage of slow wave sleep, etc.,. The second factor was represented by ease of awakening and feeling refreshed after sleep, and its relationship with objective measures of night sleep quality and circadian phase was found to oppose the relationship shown by the first factor [23, 24]. One can suggest that contribution of the items loading at the second factor to general sleep satisfaction can increase due to age-associated weakening of the sleep drive in combination with advance of phase of the sleep–wake cycle and other circadian rhythms. Indeed, the idea that older ages are characterized by reduction of the homeostatic sleep drive was corroborated by numerous experimental findings suggesting that the reduction of slow-wave activity and slow wave sleep is the most obvious age-related modification of the sleep electroencephalographic spectrum [25, 26]. Since such reduction appears to be already present in middle-aged adults [27], the age-associated dumping of amplitude of slow-wave activity and decrease of percentage of slow wave sleep signifies the earliest phase of the process of sleep aging [4].

However, neither reduction of the sleep drive nor advance of the sleep–wake cycle can explain why those older healthy people who report good night sleep fully ignore the clearly recognized signs of worsening of their night sleep quality. It seems that the answer can be found in the theoretic framework that is slightly different from the two-process model conceptualization of sleep–wake regulation known as the opponent process model [28, 29]. It conceptualizes the sleep–wake regulation as an interaction between the competing drives for sleep and wake (i.e., the sleep- and wake-promoting processes). We earlier showed that a weakening of the sleep drive in older participants of nap and sleep deprivation studies was associated with the electroencephalographic changes pointing at possibility of disinhibition of their wake drive [18, 30, 31]. Such relative strengthening of this drive can bring some advantages to healthy older people living in our post-industrial 24-hour societies. For instance, they may better tolerate sleep deprivation compared to younger people [32, 33]. Another example is the age-associated changes in maximal sleep capacity [34]. The present results indicate that sleep satisfaction remained positively linked to the wakeability characteristics of the sleep–wake cycle on the whole interval of ages from early to late adulthood. Therefore, the expected age-associated decline of sleepability characteristics of the sleep–wake cycle shown by participants with high SSS was compensated by an increase of all their wakeability characteristics. It seems that ability of adjustment of the perception of a good night’s sleep to the typical for this age’s sleep–wake pattern persists in middle-aged adults due to the age-associated strengthening of their wake drive relative to their drive for sleep. Unimportance of the perceived signs of deterioration of night sleep quality (i.e., due to the weakening of the drive for sleep) can be explained by the appearance of feeling of full refreshment after night sleep and easiness of awakening in people of this and older ages (i.e., due to the strengthening of the opposing drive for wake).

To conclude, it seems that the feeling of a good night’s sleep is not declining with advancing of age. There exists a significant link between sleep satisfaction and the typical for each age’s features of sleep–wake pattern. Sleep satisfaction can remain adjusted to the age-typical sleep–wake pattern due to changes in relative strength of the opposing drives for sleep and wake across the lifespan.