Introduction: Vigilance is directly related to core and skin temperatures (TC and Tsk). Biological day reflects a high TC and alertness; night reflects vice versa. At rest, TC is regulated largely by controlling blood flow (and hence Tsk) in extremities; their vasodilation strongly predicts reduced vigilance (Raymann et al., 2007b) and faster sleep onset (Kräuchi et al., 1999). In narcolepsy, high daytime extremity temperatures and a smaller distal-to-proximal gradient (DPG) indicates higher sleep propensity (Fronczek et al., 2006b). Cool extremities have been linked observationally to delayed sleep onset in the elderly, and experimentally shown to reduce sleep propensity in narcolepsy (Fronczek et al., 2008). Therefore, the aim of this study was to test the hypothesis that cooling the feet would maintain vigilance during extended wakefulness in healthy adults.Methods: A randomised cross-over experiment was completed using nine healthy young adult participants with normal sleep patterns. After providing informed consent, and a daytime familiarisation, they undertook three 4-h laboratory sessions in which water-perfused booties were used to provide Mild cooling, Moderate cooling or no cooling (Control). Sessions were in a dimly-lit room, beginning at 2230. Each 30 min consisted of quiet rest interspersed with a 10- min psychomotor vigilance task (PVT), 7-min Karolinska Drowsiness Test (KDT), and ratings of sleepiness, perceived body temperature and thermal discomfort. EEG spectral powers (theta, alpha and beta) were determined within the PVT and KDT. Analyses were by repeated measures ANOVA (α=0.05) with post-hoc contrasts.Results: Foot temperatures in Control and Mild and Moderate cooling averaged 34.5 ±0.5, 30.8±0.2 and 26.4 ±0.1 OC (all P<0.01). Yet, the upper-limb DPG remained stable (at ~0.3 OC) regardless of condition (P=0.57). The decline in TC (~0.35 OC) was also unaffected by condition (P=0.84), as was vigilance (interaction for response speed: P=0.45). A small and transient reduction in sleepiness was evident with cooling (P=0.046); otherwise sleepiness and vigilance deteriorated in conjunction with the fall in TC in each condition (r>0.80). Participants felt cooler throughout both cooling trials, but thermal comfort was unaffected (P=0.43), as were almost all EEG parameters during the KDT. All dependent measures were affected by time.Discussion and Conclusion: In healthy, young adults, core temperature and vigilance decline during the period of normal sleep onset and early sleep regardless of mild or moderate cooling of the feet, and any effect on sleepiness is small and transient.
PDF
download
878987797
028-85220240
