For approximately one million years human beings have lived in a physiological rhythm determined by day and nightthat is, a rhythm of about 24 hours. So ingrained is this habit that a daily temperature cycle occurs, body heat being lowest (among people who normally sleep at night) in the early morning, highest in the early afternoon. Some time ago Physiologist Nathaniel Kleitman of the University of Chicago determined to find out whether the human mechanism could break away from this ages-old habit, adapt itself to a cycle of different length.
On June...
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