Circadian/diurnal rhythms

Suprachiasmatic nucleus A hypothalamic nucleus involved in the maintenance of circadian (diurnal) rhythms. 

FIGU RE 6.21.8 Proposed model of the circadian (diurnal) rhythm of a cell. A biochemical pacemaker maintains the basic rhythm, which is then entrained by exposure to sunlight. (From Johnson, C.H. and Hastings, J.W., Am. ScL, 74, 29, 1986. With permission.)... 

Shapiro DJ, Rodwell VW 1969 Diurnal variation and cholesterol regulation of hepatic HMG-CoA reductase activity. Biochem Biophys Res Commun 37 867—872 Silver R, LeSauter J, Tresco PA, Lehman MN 1996 A diffusible coupling signal from the transplanted suprachiasmatic nucleus controlling circadian locomotor rhythms. Nature 382 810-813... 

Fort A, Mills JN. Influence of sleep, lack of sleep and circadian rhythm on short psychometrics tests. In Colquhoun WP, ed. Aspects of Human Efficiency Diurnal Rhythm and Loss of Sleep. London English University Press, 1972 115-127. 

The timing of corticosteroid administration has been pinpointed to interact with the diurnal rhythm of the hypothalamic-pituitary-adrenal axis in humans (Meibohm et al 1997). Dose timing has a pivotal effect on the safety profile and consequently the risk-benefit ratio of inhaled corticosteroids. In humans, maximum adrenal suppression occurs with the administration of aerosolized corticosteroids in the early morning hours, whereas endogenous cortisol production is least disrupted by administration in the afternoon. The circadian pattern results from the... 

C ,)max The peak plasma concentration reached duringr constant infusion of a drug, circadian rhythm (diurnal rhythm) The intrinsic rhythmic changes with a periodicity of approximately 24 hours in an organism. 

Beyond the two-hour ultradian rhythms, our lives are also set to the circadian or diurnal rhythm of the daily cycle that regulates sleep and wakefulness. In fact, our biological clock is set not to a twenty-four-hour day but to one that lasts closer to twenty-five hours. Presumably that gap be-... 

Physiological clocks like this are widely distributed in living creatures of all kinds. They are coupled to the most varied rhythms. As far as 24 hour rhythms are concerned, one used to speak of endogenous diurnal rhythms. It was soon noticed, however, that these rhythms usually show periods of 18-21 hours, not exactly 24, under constant external conditions. For this reason the designation circadian rhythms (circa = about, dies = day) is preferred nowadays. 

It is most probable that sleep and waking stem from an inherent cycle of neuronal activity that can be influenced dramatically by changes in sensory stimulation. This is demonstrable not only in humans and laboratory animals, but also in invertebrates. Thus, while we cannot be sure that other animals sleep in the same way that we do, they do show a circadian cycle of motor activity. In some (nocturnal) species, such as the rat, this activity is actually highest during darkness. Even aplysia, the sea hare, has such a rhythm but this is more like that of humans in being maximally active during daylight (diurnal). 

Melatonin secretion is synchronized to the light/dark (LD) cycle, with a nocturnal maximum (in young humans, about 200 pg/ml plasma) and low diurnal baseline levels (about 10 pg/ml plasma). Studies have supported the value of the exogenous administration of melatonin in circadian rhythm sleep disorders, insomnia, cancer, neurodegenerative diseases, disorders of the immune function, and oxidative damage (Karasek et al. 2002 Pandi-Perumal et al. 2005, 2006 Srinivasan et al. 2005a,b, 2006 Hardeland et al. 2006). 

Desynchronization of circadian or seasonal rhythms cause diurnal variations in mood and sleep patterns and can result in seasonal recurrences of mood episodes. 

If the model for the oscillator is correct. Per and Cry expression will define solar/ circadian time, driving the activity—rest cycle rather than just being a passive reflection of the activity cycle. Therefore, their expression patterns should exhibit the same phase in the SCN of nocturnal and diurnal species. This is confirmed by examination of Per expression in the SCN of the diurnally active ground squirrel, Spermophilus (Mrosovsky et al 2001). The rhythm of Per1 and Per2 expression in... 

Maywood ES, O Brien JA, Hastings MH 2003 Expression of mCLOCK and other circadian clock-relevant proteins in the mouse suprachiasmatic nuclei. J Neuroendocrinol 15 329-334 Mrosovsky N, Edelstein K, Hastings MH, Maywood ES 2001 Cycle ofperiod gene expression in a diurnal mammal Spermophilus tridecemlineatus) implications for nonphotic phase shifting. J Biol Rhythms 16 471-478... 

Melatonin (lV-acetyl-5-methoxytryptamine) is a neuro-hormone secreted by the pineal gland from the amino acid precursor [,-tryptophan. Its endogenous secretion is photosensitive and has a circadian rhythm—plasma melatonin concentrations are highest at night in both diurnal and nocturnal animals, and fall with age (1). The nocturnal melatonin peak coincides with a drop in body temperature and increased sleepiness in healthy humans. Oral melatonin has a short half-life (30-50 minutes) and extensive first-pass metabolism. Its clearance is reduced in severe liver disease (2). 

People and animals follow a natural circadian rhythm. Humans and many animals tend to be diurnal. They get up in the morning, are active during the day, and sleep at night. Disruptions in this natural cycle can be difficult. With the advent of air travel, people could... 

It is a well-known fact that the circadian rhythm is entrained for diurnal cues to approximately 24 hours. However, a non-entrained rhythm, which operates in the absence of external cues, lasts between 25 and 27 hours. Thus the human sleep-wake cycle normally shows a 24-hour rhythm but not all physiological processes (for example, body temperature) follow the sleep-wake cycle. 

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