Chapters by leading experts cover the whole range of biological rhythms, from infradian and circadian to the longer ultradian rhythms, in a wide variety of mammalian species. These are likely related to food preparation or other similar activities exposing the hands to cold. By examining 3 such groups in two continents over long periods of time, we were able to evaluate common elements and differences that provide insights into the nature of human sleep under natural conditions. Red arrows indicate onset of drop in finger temperature starting near the temperature nadir, indicative of peripheral vasoconstriction, serving to warm proximal regions with awakening. Note the much later sleep onset in the summer and the later wake onset in the summer relative to winter, despite shorter sleep times.
Note that the ambient temperature has a gradual fall at night and a rapid rise starting at sunrise, with sleep occurring during the period of slowly falling temperature.
Light and Biological Rhythms in Man
A recent study has shown a striking difference in the sleep onset and offset times as a function of light exposure in a comparison of two closely related traditional Argentinian hunter-gatherer populations[ 26 ]. Note the consistent pattern across groups and seasons. The Tsimané and San live far enough south of the equator to have substantial seasonal changes in day length and temperature. In the Tsimané, summer wake times were earlier and sleep onset times were later than in the winter, accounting for their reduced sleep duration; however, the San, despite their shorter sleep duration in the summer, as in the Tsimane, the time of awakening was significantly later, with the decreased sleep times being entirely the result of later sleep onset. Hunter-gatherer energetics and human obesity. Individuals in groups like those we observed may be less vulnerable to insomnia because they are exposed to a falling ambient temperature at the time of sleep onset and do not have to actively shed heat to achieve the body temperature reduction that accompanies sleep onset[ 35 — 37 ].