From both anecdotal observation and empirical evidence, it is clear that the sleep of older persons is quite different from that of younger adults. Compared to younger adults, even the most healthy of seniors spend about twice as much time during the night in unwanted wakefulness, and are more likely to wake up earlier in the morning than they would like to (Ohayon et al., 2004; Prinz, 2004). There are many possible reasons why this should be so, not least of which are the many troubles of ill health that advancing age is often heir to. Not only pain and discomfort, but also neurological disorders of late life such as Parkinson's disease and Alzheimer's disease are known to significantly impair sleep (Bliwise, 1993). Additionally, though it is plausible that at least some of this sleep dysfunction, especially in the healthy elderly, is a product of age-related changes in the phase (timing) of the human circadian system (reviewed by Monk, 2005; Prinz, 2004). In particular, the elderly often choose to go to bed and wake up several hours earlier than young adults (Buysse et al., 1992; Czeisler et al., 1992), have higher ‘morningness’ scores (more like a ‘morning lark’) on the Horne and Ostberg (1976) instrument (Monk et al., 1991), and have circadian phases (as indicated by temperature or melatonin rhythms) that are about 1 or 2 h earlier than those of the young (Duffy et al., 2002; Monk et al., 1995). This naturally raises the question of whether some of the sleep disruption of the elderly might be the result of bedtime choices at times that may be inappropriate for (i.e. at the wrong phase angle to) the timing of their endogenous circadian pacemaker (ECP). Thus, seniors might choose their habitual bedtime to be at a time that is at a different phase angle to their dim light melatonin onset (DLMO) or core body temperature minimum (Tmin), for example, than that chosen by younger adults (Carrier et al., 1999; Duffy et al., 2002). This hypothesis led to a body of work by which evening bright lights were used to delay the timing of the circadian pacemakers of those seniors experiencing unwanted early morning awakenings. Several authors have shown short-term beneficial effects on sleep of evening bright light treatment designed to accomplish such a phase delay (summarized in Campbell et al., 1995). Unfortunately, though, most older people do not like the light therapy, and a recent report has shown that maintenance light therapy for such patients is unlikely to work in the long term (Suhner et al., 2002). An alternative might be to leave the timing of an older person's circadian pacemaker alone, but to instead change the phase angle by changing the timing of bedtime. Before advocating such an approach, it would make sense to determine whether naturally occurring inter-individual and intra-individual differences in bedtime selection might be lawfully related to the amount of sleep that is obtained. That is the aim of the present study.
In an earlier study of morningness–eveningness in the elderly (Monk et al., 1991) we have shown not only that on average, older adults (80–91 years) had higher (Horne and Ostberg, 1976) morningness scores than younger (21–30 years) adults, but also that within the older group there was a significantly positive correlation (ρ = 0.38, n = 34, P < 0.05) between morningness score and total sleep time from polysomnography (PSG) indicating that more sleep was obtained in those scoring higher on morningness. PSG sleep was recorded (second night) at the subject's own habitual bedtime and wake time as calculated from a 2-week sleep diary before the laboratory sleep study. Thus, within the elderly group, the more morning-type the orientation, the more sleep was obtained. Of course, a morning-type orientation suggests, among other behavioral indices, an earlier bedtime. Therefore, one plausible explanation for this finding is that those elderly people who made bedtime choices appropriate to their earlier phasing ECP thus obtained more sleep than did those who failed to accommodate the ECP changes and made the ‘wrong’ timing-of-bedtime choices. This also suggests that even if the precise phase of the ECP (e.g. from DLMO or Tmin) is not known, diary data might allow the question of the relation between bedtime choices and sleep duration to be examined.
The present report considers diary data collected using the Pittsburgh Sleep Diary [PghSD] (Monk et al., 1994) which allows the examination of not only the timing of sleep, but also allows the derivation of subjective measures of sleep latency (SL) and wake after sleep onset (WASO). Thus one can obtain (admittedly inexact and subjective) estimates of time in bed (TIB) and total sleep time (TST) [TST = TIB − SL − WASO] from the PghSD, and determine their relation to the clock time at which bedtime was selected on that night. By accessing our database of diary sleep measures collected over several years, we were able to assemble a sample of 128 healthy seniors (63f, 65m, 70–92 years) who had completed the PghSD for a full week, and to use those data to determine the relationship between clock time of bedtime to TIB and TST on a subject-night basis.