Relationship of endogenous circadian melatonin and temperature rhythms to self-reported preference for morning or evening activity in young and older people

Abstract

Background: Morningness-eveningness refers to interindividual differences in preferred timing of behavior (i.e., bed and wake times). Older people have earlier wake times and rate themselves as more morning-like than young adults. It has been reported that the phase of circadian rhythms is earlier in morning-types than in evening types, and that older people have earlier phases than young adults. These changes in phase have been considered to be the chronobiological basis of differences in preferred bed and wake times and age-related changes therein. Whether such differences in phase are associated with changes in the phase relationship between endogenous circadian rhythms and the sleep-wake cycle has not been investigated previously.

Methods: We investigated the association between circadian phase, the phase relationship between the sleep-wake cycle and circadian rhythms, and morningness-eveningness, and their interaction with aging. In this circadian rhythm study, 68 young and 40 older subjects participated.

Results: Among the young subjects, the phase of the melatonin and core temperature rhythms occurred earlier in morning than in evening types and the interval between circadian phase and usual wake time was longer in morning types. Thus, while evening types woke at a later clock hour than morning types, morning types actually woke at a later circadian phase. Comparing young and older morning types we found that older morning types had an earlier circadian phase and a shorter phase-wake time interval. The shorter phase-waketime interval in older "morning types" is opposite to the change associated with morningness in young people, and is more similar to young evening types.

Conclusions: These findings demonstrate an association between circadian phase, the relationship between the sleep-wake cycle and circadian phase, and morningness-eveningness in young adults. Furthermore, they demonstrate that age-related changes in phase angle cannot be attributed fully to an age-related shift toward morningness. These findings have important implications for understanding individual preferences in sleep-wake timing and age-related changes in the timing of sleep.

Figure 1.

Distribution of scores on the Morningness-Eveningness Questionnaire among the 68 young men (upper panel) and the 40 older men and women (lower panel). As indicated on the lower axis, individuals scoring 41 or below are classified as evening types and those scoring above 59 are classified as morning types .

Figure 2.

Score on the Morningness-Eveningness Questionnaire vs. circadian phase of the core body temperature minimum (CBT_min) for the 108 subjects. Filled squares: young subjects; open squares: older subjects. A significant correlation was found between morningness-eveningness score and CBT_min when all subjects were considered together. This correlation remained significant when young subjects were analyzed separately, but did not reach statistical significance when the older subjects were analyzed separately (see text for details).

Figure 3.

Clock hour of endogenous circadian phase in young morning and evening types. The time of the endogenous temperature minimum (circles) and the melatonin maximum (triangles) was significantly later in the 13 evening types than in the 10 morning types.

Figure 4.

Temperature and melatonin curves of morning and evening types from a constant routine (CR) averaged with respect to time of day. Upper panel: average endogenous temperature curves of morning (open circles) and evening (filled circles) types during the CR. Lower panel: average plasma melatonin curves of morning (open triangles) and evening (filled triangles) types during the CR. Average curves were complied with respect to actual time of day. The average hourly value for each individual was first calculated, and those hourly values were then averaged across subjects for each of the two groups.

Figure 5.

Temperature and melatonin z-score curves of morning and evening types averaged with respect to habitual waketime. Symbols as in Figure 4. Data for each subject were first expressed relative to the subject’s mean (= 0) and standard deviation (=1). Average curves were complied with respect to each subject’s habitual waketime, with habitual waketime set to a reference value of 08:00 for all subjects. The average hourly z-score for each individual was first calculated, and those hourly values were then averaged across subjects for each of the two groups.

Figure 6.

Average interval between endogenous circadian temperature phase and habitual waketime in young evening (upper bar) and morning (middle bar) types, and older morning (lower bar) types.