The 24-h rhythms in sleep and temperature both change in Alzheimer's disease (AD). Characteristic changes consist of a more fragmented diurnal sleep profile with frequent nocturnal awakenings and daytime sleepiness, as well as a reduction in the amplitude of the 24-h rhythm in core body temperature (CBT). Although the 24-h rhythm in CBT is to a large extent the result of a 24-h rhythm in heat loss from the skin caused by pronounced changes in skin blood flow and consequently skin temperature (Ts), changes in the diurnal skin temperature profile in AD as compared to normal aging have remained unexplored. Because recent work indicates a causal contribution of fluctuations in skin temperature to daytime sleepiness and nocturnal sleep depth, the present study aimed to investigate the skin temperature rhythm in AD and its association with daytime sleepiness and nocturnal sleep. Ambulatory recorders were used to estimate sleep and 24-h rhythms in skin temperature in 55 AD patients and 26 matched non-demented elderly controls. Subjective sleep and daytime sleepiness were obtained using questionnaires. The results indicate that AD patients had a significantly higher daytime proximal skin temperature (PST) than elderly controls. In both AD patients and elderly controls, an elevated daytime PST was associated with more daytime sleepiness. The findings suggest a deficient downregulation of daytime proximal skin blood flow that might contribute to daytime sleepiness. Because daytime sleepiness contributes to cognitive dysfunction in AD, further research into the underlying mechanisms and possible reversibility is warranted.