15 Dawns in Space

The concept of a "day" aboard an orbiting spacecraft is a little abstract: every 24 hours, astronauts on board the ISS will experience 15 dawns as the station speeds around the world. But human beings have been conditioned by millions of years of evolution to a 24-hour daily cycle, and so-called circadian rhythms of waking and sleeping are hard-wired into our brains and bodies. So astronauts work and sleep to fixed schedules that match these ancient rhythms. Any other arrangement would soon have crews living in a state of permanent jet lag.