Tc is one of the most powerful and stable indicators of circadian synchrony, reflecting activity of the circadian rhythm's "strong oscillator." In agreement with animal data (18), some human studies suggest that aging is associated with flatter and earlier phasing rhythms, even with self-regulated diurnal activity (56). Although neither the mean 24-h Tc nor the mean Tc during sleep was dependent on age, the mean Tc during the time period of 02:00 to 08:00 was significantly higher in older (69 ± 2 yr) subjects (56).
On the other hand, Monk et al. (38) showed no evidence that subjects >77 yr old had circadian temperature [rectal temperature (Tre)] rhythm amplitudes that were different from young adults. Whereas some investigations have shown that older men seem to differ more often from their younger counterparts, older women typically show no alterations in circadian Tc rhythm when compared with young men or women (38, 55, 56). Thus subtle changes in the circadian Tc rhythm of older adults may primarily reflect a gender effect. Figure 1 summarizes the age-related changes in thermoregulation at rest during normothermia.