Duffy, J. F., Rimmer, D. W., & Czeisler, C. A. (2001). Association of intrinsic circadian period with morningness-eveningness, usual wake time, and circadian phase. Behavioral Neuroscience, 115, 895-899.
Hedge, A. (1999). Biological rhythms: DEA 325. Retrieved January 18, 2005 from Cornell University
Butcher, J. N., Mineka, S., Hooley, J. M., (2004). Abnormal psychology (12th ed.). New York: Allyn & Bacon.
Ennis, E., & McConville, C. (2004). Stable characteristics of mood and seasonality. Personality and Individual Differences, 36, 1305-1315.