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Science 25 June 1999:
Vol. 284. no. 5423, pp. 2177 - 2181
DOI: 10.1126/science.284.5423.2177
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Reports

Stability, Precision, and Near-24-Hour Period of the Human Circadian Pacemaker

Charles A. Czeisler, *1 Jeanne F. Duffy, 1 Theresa L. Shanahan, 1 Emery N. Brown, 2 Jude F. Mitchell, 1 David W. Rimmer, 1 Joseph M. Ronda, 1 Edward J. Silva, 1 James S. Allan, 1 Jonathan S. Emens, 1 Derk-Jan Dijk, 1 Richard E. Kronauer 3

Regulation of circadian period in humans was thought to differ from that of other species, with the period of the activity rhythm reported to range from 13 to 65 hours (median 25.2 hours) and the period of the body temperature rhythm reported to average 25 hours in adulthood, and to shorten with age. However, those observations were based on studies of humans exposed to light levels sufficient to confound circadian period estimation. Precise estimation of the periods of the endogenous circadian rhythms of melatonin, core body temperature, and cortisol in healthy young and older individuals living in carefully controlled lighting conditions has now revealed that the intrinsic period of the human circadian pacemaker averages 24.18 hours in both age groups, with a tight distribution consistent with other species. These findings have important implications for understanding the pathophysiology of disrupted sleep in older people.

1 Circadian, Neuroendocrine, and Sleep Disorders Section, Division of Endocrinology, Department of Medicine, Harvard Medical School, Brigham and Women's Hospital, 221 Longwood Avenue, Boston, MA 02115, USA.
2 Department of Anesthesia and Critical Care, Harvard Medical School, Massachusetts General Hospital, Boston, MA 02114, USA.
3 Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA.
*   To whom correspondence should be addressed.

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