^ Keckeis, Marietta; Lattova, Zuzana; Maurovich-Horvat, Eszter; Beitinger, Pierre A.; Birkmann, Steffen; Lauer, Christoph J.; Wetter, Thomas C.; Wilde-Frenz, Johanna; Pollmächer, Thomas (2010). Finkelstein, David (ed.). "Impaired Glucose Tolerance in Sleep Disorders". PLoS ONE. 5 (3): e9444. doi:10.1371/journal.pone.0009444. PMC 2830474. PMID 20209158.
The natural internal clock that controls our 24-hour cycle of sleep and waking, circadian rhythms are easily upset by changes in schedule, and they're greatly affected by light and darkness. Jet lag is the best known circadian rhythm disorder, but this sensitive inner clock can also be disrupted by changes in routine resulting in an erratic sleep schedule.