Means, standard deviation and range of continuous demographic variables.

aAssessed immediately after study enrolment, from the Horne Ostberg questionnaire [70] in all 170 subjects.bAssessed immediately after study enrolment, from the Epworth Sleepiness Scale [71] in 132 subjects. Scores of greater than 10 are suggestive of high daytime sleepiness.cAssessed immediately after study enrolment, from the Pittsburgh Sleep Quality Index [72] in 132 subjects. A score of greater than 5 is suggestive of a sleep disorder.dNumber of days on fixed sleep schedule prior to phase assessment.eSunrise to sunset times in Chicago (US Naval Observatory).fSelf-reported habitual use prior to start of study.