Weekly phase shifts of rhythms self-reported by almost feral human students in the USA and Spain

Eleven students in the USA and fifteen students in Spain wrote their wake-up and to-sleep times on forms for a month. The subjects reported large day-to-day phase shifts of magnitude comparable to those of travelers and shiftworkers. All individuals exhibited some large phase shifts of their day-to-day wake-up and to-sleep times (largest shift average, 4.54 hours; range, -12.7 to +13.9 hours). The students had daily cycles and weekly cycles in their wake-up and to-sleep times. Synchrony (entrainment) with the 24-hour environment was achieved by large weekly phase advances (individual averages ranged 1.2-4.8 hours) rather than by small daily adjustments. There was considerable variation in awake time: there were 2-3 hour differences between individuals, and there were 3.2 hour differences depending on day of the week. Alarm use appeared to be the Zeitgeber, in contrast to laboratory studies which show light and social Zeitgebers for humans. There was a 4 hour difference between the Spanish and USA students which was attributed to the 5 hour difference in time zones. Wake-up time and prior to-sleep time were interdependent.