Aging and Insomnia Introduction There Is a Close Relationship between the Cir- Cadian Rhythm of Core Body Temperature (cbt) and the Daily Variation in Vigilance, as Shown

THERE IS A CLOSE RELATIONSHIP BETWEEN THE CIRCADIAN RHYTHM OF CORE BODY TEMPERATURE (CBT) AND THE DAILY VARIATION IN VIGILANCE, AS SHOWN by Kleitman.1 Vigilance is optimal during the circadian phase of increased CBT.2-6 Under controlled conditions, the rhythm in CBT results to a large extent from the rhythm in skin blood flow, which determines skin temperature, and the resulting heat transfer gradient from the body to the environment. Skin temperature thus shows a circadian rhythm that is reciprocal to the CBT rhythm, i.e., low during the habitual wake period,7-9 although a reversal of the proximal skin temperature has been found under specific conditions.10 We have proposed that these intrinsic changes in both CBT and skin temperature could modulate neuronal activity in vigilance regulating brain areas.11 A likely brain structure linking vigilance with core body and skin temperature is the preoptic area/ anterior hypothalamus (POAH), the major thermoregulatory center of the mammalian brain as well as a key structure in vigilance state control. Both local brain temperature and skin temperature modulate the firing rate of thermosensitive neurons in the POAH and other brain areas involved in vigilance regulation.11,12 Support for a causal contribution of skin, but not core, temperature to vigilance regulation has been provided by a strictly controlled laboratory study, demonstrating accelerated sleep onset with very subtle warming of the skin in a comfortable and thermoneutral range.13 The simultaneous and independent induction of changes in core body temperature did not affect sleep propensity. These findings suggest that the previously reported positive phase relation of the circadian rhythm in core body temperature and the circadian rhythm in vigilance2-6 might also be an indirect reflection of a reciprocal phase relation of the circadian rhythm in skin temperature and the circadian rhythm in vigilance. A limited number of experimental studies have investigated the effect of skin temperature manipulation on vigilance. These studies manipulated ambient temperature to induce skin temperatures beyond the range of the normal diurnal fluctuations in skin and core body temperature. A recent meta-analysis on the effects of temperature manipulation on task performance indicates an inverted U-shape, not surprisingly demonstrating that both high and low temperatures that activate thermoregulatory responses adversely affect performance.14 Thus, both heat and cold stress negatively affected sustained vigilance.15,16 This makes sense from the point of view that the brain needs to readdress its resources from optimal task performance towards thermal stress defense. These findings cannot be generalized to the normal diurnal range of in skin temperature. Consequently, no conclusion can be drawn regarding a causal contribution of the circadian rhythm in skin temperature to the circadian rhythm in vigilance. The aim of the present study was to investigate whether the induction of changes in skin and core body temperature within the comfortable range of normal diurnal rhythm could causally affect vigilance. Our recently developed experimental set-up13 allowed us to induce subtle changes in core body temperature Time-on-task Impairment of Psychomotor Vigilance Is Affected by Mild Skin Warming and Changes With Aging and Insomnia