Science: Cave Men

For approximately one million years human beings have lived in a physiological rhythm determined by day and night—that 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...

Want the full story?

Subscribe Now

Subscribe
Subscribe

Get TIME the way you want it

  • One Week Digital Pass — $4.99
  • Monthly Pay-As-You-Go DIGITAL ACCESS$2.99
  • One Year ALL ACCESSJust $30!   Best Deal!
    Print Magazine + Digital Edition + Subscriber-only Content on TIME.com

Learn more about the benefits of being a TIME subscriber

If you are already a subscriber sign up — registration is free!