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 including the preoptic area and anterior hypothalamus (1-3). In contrast to previous work by other groups, we reasoned that skin temperature rather than the previously proposed local 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 within the thermoneutral and comfortable range had pronounced effects on sleep and vigilance in healthy young and elderly people as well as in the sleep disorder of insomnia (4-7). These studies concertedly provide the first and ample evidence for a causal role of skin temperature in vigilance state modification and call for applications to optimize e.g. bed-temperature for sleep and the temperature of chairs during work for vigilance. A recently investigated ensuing second line of research focuses on the spontaneous fluctuations in skin temperature within the comfortable range: our hypothesis would predict these to be informative for fluctuations in sleep depth and vigilance level. Indeed we just obtained data confirming that skin temperature measurements are of value in the detection of sleepiness and lapses of attention – a major cause of traffic and control accidents – during the day. Likewise, skin temperature measurements are of value in the detection of sleep versus immobile wakefulness during the night, yielding an essential possibility to improve ambulatory sleep monitoring devices like actigraphs.