Enhancing Circadian Zeitgebers

THE LINK BETWEEN CIRCADIAN RHYTHMS AND SLEEP IS CLEARLY AN IMPORTANT ONE. THOSE OF US WHO STUDY THE EFFECTS OF AGING ON HUMAN circadian rhythms often seek to forge a link between the effects of aging on circadian rhythms (which most older persons and policy makers don't care about) and the effects of aging on sleep (which many older persons and policy makers do care about). Very often, the raison d'être for the study of circadian rhythms in the aged (either explicitly stated or merely implied) is that such experiments might allow us to glean appropriate therapies for age-related circadian dysfunction. The hope is that by fixing older people's circadian rhythms we can fix their sleep, thus saving them from distress and, in some cases, inappropriately applied hypnotic medications.

In this issue of SLEEP, Zisberg and colleagues¹ present compelling evidence from a study of 89 Israeli seniors (both genders, mean age 75 y) to suggest that there are indeed certain circadian behaviors which may be associated with improved sleep. In particular, Zisberg, et al. used block multiple regression analyses to show that increased stability in daily routine (as measured by two separate instruments), predicted shorter sleep latency, higher sleep efficiency and increased overall sleep quality (using the Pittsburgh Sleep Quality Index).² These effects were observed over and beyond those of functional status, comorbidities, and age. One instrument, the Scale of Older Adults' Routine (SOAR)³ involved three structured interviews spaced 2 weeks apart and yielded measures separately related to basic activities such as eating and sleeping, and to instrumental activities such as shopping and public transportation use. The other instrument, the Social Rhythm Metric (SRM)⁴ involved a prospective 2-week diary of the timing of five major daily events relating to sleep, meals and social contacts. Both instruments essentially yielded numbers representing the level of regularity of the individual's daily routine, and both were significantly correlated with self-rated sleep variables (and with each other).

Circadian rhythms are driven by endogenous processes, are self-sustaining, and rely upon circadian time cues (zeitgebers) to remain appropriately oriented to the individual's environment and desired routine.⁵ The gold-standard measures of human circadian rhythms have been core body temperature and salivary or plasma melatonin levels. However, one can also make the case that the behavioral circadian rhythms related to the timing of sleep, meals, work and social interactions are just as valid circadian rhythms as the physiological ones. Moreover, these are the rhythms most salient to the individual himself or herself.

The strength of behavioral circadian rhythms comprises the concept underlying the daily regularity scores, as measured by the SOAR and SRM in Zisberg et al.'s study(1). However, behavioral circadian rhythms are not only circadian rhythms in their own right, but also serve as gatekeepers to the zeitgebers that impinge on the individual and thus serve to entrain his/her circadian rhythms to a 24-h period. The most powerful of such zeitgebers are the photic ones resulting from sleeping in the dark, for example.⁵ There is, however, also evidence for non-photic zeitgebers such as exercise and social contacts.⁶ Thus the improved sleep observed by Zisberg et al.¹ in seniors with a more regular daily lifestyle, may result both from the action of stronger behavioral circadian rhythms per se, and from those strong behavioral rhythms providing more coherent zeitgebers to the circadian system in general. Most behavioral treatments for insomnia⁷ include advice to the patient to be regular in their sleep timing (particularly in regard to rise-time), thus enhancing the strength of the patient's behavioral circadian rhythms. In both adults and college students, high SRM scores (greater daily regularity) have been shown to be associated with better subjective sleep.^8,9

Perhaps more salient, though, is the issue of zeitgeber enhancement. A number of different studies^10–12 have shown that enhancement of zeitgebers in elderly nursing home residents (e.g., by providing brighter light and/or daytime activities) can have beneficial effects on their sleep. Healthy seniors tend to have higher (i.e., more regular) lifestyles than do younger adults.¹³ In the auditory domain, those of us who work with the elderly often become accustomed to raising our voices slightly to accommodate their hearing loss. It may well be that seniors have found that a highly regular lifestyle has an equivalent effect in the circadian domain. By adopting a highly regular lifestyle it is possible that zeitgeber effects are amplified, perhaps accommodating age-related declines in endogenous circadian entrainment mechanisms. The Zisberg et al. findings¹ raise the hope that simple behavioral therapies may, for some seniors, serve to improve their sleep. Future work needs to measure the physiological circadian rhythms and zeitgeber exposure of high- versus low-regularity seniors, in order to provide good theoretical underpinnings to such therapies.

DISCLOSURE STATEMENT

The author has indicated no financial conflicts of interest.