Circadian Rhythms
Please note: numbers used in the text indicate scientific references that are located at the bottom of each page of this website. These references can each be clicked to bring up the abstract of the article.

What are they?
The near-daily pattern of changes that takes place in various aspects of our functioning. These include:

Physiologic rhythms:
For example, the up and down movement of our core body temperature throughout the day; the rise and fall of cortisol levels, and the onset and offset of melatonin secretion.

Behavioral rhythms:
This includes the ebb and flow of our level of mental alertness, appetite, motor activity, and the progression of our sleep-wake cycle over a near-24 hour period. [64, 66]

Mood rhythms:
Recently, it has been determined that even emotional state changes in a predictable manner as we move through our day. [48, 65]

Source of Circadian Rhythms:
These periodic changes in biologic rhythms are driven by a specific group of cells in the hypothalamic area of the brain, the suprachiasmatic nuclei.

The suprachiasmatic nuclei acts like the conductor of an orchestra, setting and synchronizing the tempo of activity of each of the major organ systems of the body.

When experimentally isolated, the circadian clock runs on a period that lasts slightly longer than the standard 24-hour day. Hence, the term circadian ('circa' for almost and 'dian' for day).

Entrainment: The process of keeping our internal, circadian time in step with the cycle of day and night.

Because the circadian period for most people is longer than the 24-hour solar day, our internal clock requires regular resetting in order to not become desynchronized with the environment. If uncorrected, we would, for example, go to sleep later and later each day.

Light is the major resynchronizing agent for the human circadian system. Put differently, exposure to light, whether natural or artificial, keeps us coupled with the outside cycle of light and darkness. Melatonin is another factor that is capable of resetting our circadian timing. This process of resynchronization is called entrainment

Circadian Disruption:

In states of health, our circadian rhythms remain in fixed patterns, in terms of their own timing, their relation to other biorhythms, and in relation to the 24 hour period of light and darkness.

In different situations, this overall synchronization can change. The peak time of hormone secretion can change. The point of lowest body temperature can become advanced or delayed. Misalignments can occur, where the rhythm of one variable falls out of synchrony with another or out of synchrony with the 24 hour solar day.

Changes of this kind can produce disturbances in physiology, behavior or mood.[67, 68] There is growing evidence that circadian disruption plays a role in the cause and maintenance of depressive states. [69, 70, These types of circadian disruption are thought to contribute to jet lag, seasonal affective disorder, and other sleep and psychiatric conditions. [49, 50, 51] There is growing evidence that circadian disruption plays a role in the cause and maintenance of depressive states. [69, 70, 71]


48. Boivin, D.B., et al., Complex Interaction of the Sleep-Wake Cycle and Circadian Phase Modulates Mood in Healthy Subjects. Archives of General Psychiatry, 1997. 54(2): p. 145-152.

49. Wirz-Justice, A., Bromundt, V, Cajochen C., Circadian Disruption and Psychiatric Disorders: The Importance of Entrainment. Sleep Medicine Clinics, 2009. 4: p. 273-284.

50. Albrecht, U. and U. Albrecht, Circadian clocks in mood-related behaviors. Annals of Medicine, 2010. 42(4): p. 241-51.

51. Monteleone, P., et al., The circadian basis of mood disorders: recent developments and treatment implications. European Neuropsychopharmacology, 2008. 18(10): p. 701-11.

64. Munch, M.Y., et al., Biological Rhythms Workshop IC: sleep and rhythms. Cold Spring Harbor Symposia on Quantitative Biology, 2007. 72: p. 35-46.

65. Birchler-Pedross, A., et al., Subjective Well-Being Is Modulated by Circadian Phase, Sleep Pressure, Age, and Gender. Journal of Biological Rhythms, 2009. 24(3): p. 232-242.

66. Saper, C.B., et al., Homeostatic, circadian, and emotional regulation of sleep. Journal of Comparative Neurology, 2005. 493(1): p. 92-8.

67. Erren, T.C., et al., Defining chronodisruption. Journal of Pineal Research, 2009. 46(3): p. 245-7.

68. Bechtold, D.A., et al., Circadian dysfunction in disease. Trends in Pharmacological Sciences, 2010. 31(5): p. 191-8.

69. Emens, J., et al., Circadian misalignment in major depressive disorder. Psychiatry Research, 2009. 168(3): p. 259-261.

70. Lewy, A.J. and A.J. Lewy, Circadian misalignment in mood disturbances. Current Psychiatry Reports, 2009. 11(6): p. 459-65.

71. Hasler, B.P., et al., Phase relationships between core body temperature, melatonin, and sleep are associated with depression severity: Further evidence for circadian misalignment in non-seasonal depression. Psychiatry Research, 2010. 178(1): p. 205-207.