Brain areas involved in sleep and biological rhythm regulation are sensitive to light and temperature, which in an evolutionary sense are the oldest cyclically varying physical aspects of the environment. We have therefore proposed that the 24-cycle of sleep and wakefulness is likely to show a thermosensitivity that is firmly rooted at the systems level of the responsible network of brain areas (Van Someren, 2000; Van Someren, 2003; Van Someren, 2006). In contrast to previous work by other groups, we reasoned that skin temperature rather than the previously proposed brain temperature would be of major importance. We have built a dedicated experimental setup that allows for clamping of skin temperature while measuring brain activity and performance in humans. With the use of this setup we were able to confirm our hypothesis by showing that the induction of minute changes in skin temperature had pronounced effects on sleep and vigilance in healthy young and elderly people as well as in the sleep disorders of insomnia and narcolepsy (Raymann et al., 2005; Raymann and Van Someren, 2007; Raymann et al., 2007; Raymann et al., 2008; Raymann and Van Someren, 2008; Fronczek et al., 2008; Fronczek et al., 2008) Follow-up brain imaging studies will focus on the brain areas involved in the effects of skin temperature on sleep and vigilance.