Changes

The frequency of heart attacks peaks between 6 a.m. and noon<ref>Muller, J.E., Stone, P.H., Turin, Z.G., et al., “The Milis Study Group: Circadian Variation in the Frequency of Onset of Acute Myocardial Infarction,” New England Journal of Medicine 313:1315-

The frequency of heart attacks peaks between 6 a.m. and noon<ref>Muller, J.E., Stone, P.H., Turin, Z.G., et al., “The Milis Study Group: Circadian Variation in the Frequency of Onset of Acute Myocardial Infarction,” New England Journal of Medicine 313:1315-

1322, 1985.</ref> asthma attacks are most prevalent at night, human babies are born predominantly in the early morning hours. While these patterns do not necessarily indicate that the events are driven by the circadian pacemaker, they do suggest temporal order in the functioning of the human body.  

1322, 1985.</ref> asthma attacks are most prevalent at night, human babies are born predominantly in the early morning hours.<ref>Glattre, E., and Bjerkedal, T., “The 24-Hour Rhythmicity of Birth,” Acta Obstetrics and Gynecology, Scandinavia 62:31-36, 1983.</ref>,<ref>Kaiser, H., and Halberg, F., “Circadian Periodic Aspects of Birth,” Annals of the New York Academy of Sciences 98:1056-1068, 1962.</ref> While these patterns do not necessarily indicate that the events are driven by the circadian pacemaker, they do suggest temporal order in the functioning of the human body.  

This temporal organization appears to be beneficial; the human body is prepared for routine changes in state, such as awakening each morning, rather than simply reacting after shifts in demand. In addition, these regular cycles in the body present considerations for diagnosis of health problems and for the timing of medical treatment  

This temporal organization appears to be beneficial; the human body is prepared for routine changes in state, such as awakening each morning, rather than simply reacting after shifts in demand.<ref>Moore-Ede, M.C., “Physiology of the Circadian Timing System: Predictive vs. Reactive Homeostasis,” American Journal of Physiology, 250:R737- R752, 1986.</ref> In addition, these regular cycles in the body present considerations for diagnosis of health problems and for the timing of medical treatment.<ref>Halberg, F., “Implications of Biological Rhythms for Clinical Practice,” Hospital Practice 12:139-149, 1977.</ref> 

The circadian rhythms of various functions in humans, such as hormone production, body temperature, and sleepiness, are normally coordinated— i.e., they bear a specific relationship to each other. This temporal organization suggests that some biological timekeeping device must drive, regulate, or at least integrate various circadian rhythms. In mammals, considerable experimental evidence indicates that a region of the brain called the suprachiasmatic nucleus (SCN) is the circadian pacemaker. The SCN, composed of a cluster of thousands of small nerve cells, is located within a region of the brain, the hypothalamus, that controls such basic functions as food intake and body temperature. [297]

The circadian rhythms of various functions in humans, such as hormone production, body temperature, and sleepiness, are normally coordinated— i.e., they bear a specific relationship to each other. This temporal organization suggests that some biological timekeeping device must drive, regulate, or at least integrate various circadian rhythms. In mammals, considerable experimental evidence indicates that a region of the brain called the suprachiasmatic nucleus (SCN) is the circadian pacemaker.<ref>Meijer, J.H., and Rietveld, W.J., “Neurophysiology of the Suprachiasmatic Circadian Pacemaker in Rodents,” Physiological Review 69:671-707, 1989.</ref> The SCN, composed of a cluster of thousands of small nerve cells, is located within a region of the brain, the hypothalamus, that controls such basic functions as food intake and body temperature. [297]

The secretion of cortisol, a steroid hormone important for metabolism and responses to stress, fluctuates daily, peaking in the very early morning hours and falling to a negligible amount by the end of the day (181). Besides its use as a marker for the internal pacemaker, the circadian rhythm of cortisol secretion may drive other rhythms in the body and has important clinical implications.

The secretion of cortisol, a steroid hormone important for metabolism and responses to stress, fluctuates daily, peaking in the very early morning hours and falling to a negligible amount by the end of the day (181). Besides its use as a marker for the internal pacemaker, the circadian rhythm of cortisol secretion may drive other rhythms in the body and has important clinical implications.