Chronobiology international
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Long-term, night shiftwork has been identified as a potential carcinogenic risk factor. It is hypothesized that increased light at night exposure during shiftwork reduces melatonin production, which is associated with increased cancer risk. Sleep duration has been hypothesized to influence both melatonin levels and cancer risk, and it has been suggested that sleep duration could be used as a proxy for melatonin production. ⋯ Analysis of salivary melatonin levels indicated that the circadian rhythms of night workers were not altered, meaning peak melatonin production occurred at night. This study indicates that two nights of rotating shift work may not change the timing of melatonin production to the day among those working at night. Additionally, in this study, sleep duration was not correlated with urinary melatonin levels, suggesting it may not be a good proxy for melatonin production.
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Comparative Study
Rhythmic cFos expression in the ventral subparaventricular zone influences general activity rhythms in the Nile grass rat, Arvicanthis niloticus.
Circadian rhythms in behavior and physiology are very different in diurnal and nocturnal rodents. A pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus is responsible for generating and maintaining circadian rhythms in mammals, and cellular and molecular rhythms within the SCN of diurnal and nocturnal rodents are very similar. The neural substrates determining whether an animal has a diurnal or nocturnal phase preference are thus likely to reside downstream of the SCN. ⋯ However, the number of cells expressing cFos in the vSPVZ was positively correlated with general activity during the subjective day relative to the subjective night when the animals were switched to DD, and this pattern persisted when a LD cycle was reinstated. Also, the number of cFos-ir cells in the vSPVZ was negatively correlated with the strength of rhythmicity in DD and the number of days required to re-entrain to a LD cycle following several weeks in DD. These data suggest that the vSPVZ emits signals important for the expression of stable diurnal activity patterns in grass rats, and that species differences in these signals may contribute to differences in behavioral and physiological rhythms of diurnal and nocturnal mammals. (Author correspondence: mschw009@umaryland.edu ).
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Australian sleepy lizards (Tiliqua rugosa) exhibit marked locomotor activity rhythms in the field and laboratory. Light-dark (LD) and temperature cycles (TCs) are considered important for the entrainment of circadian locomotor activity rhythms and for mediating seasonal adjustments in aspects of these rhythms, such as phase, amplitude, and activity pattern. The relative importance of 24 h LD and TCs in entraining the circadian locomotor activity rhythm in T. rugosa was examined in three experiments. ⋯ There was no difference in the phase relationships of lizard activity rhythms to the onset of the thermophase among the normal, delayed, and advanced TC groups, suggesting equally strong entrainment to the TC in each group. The results of this experiment excluded the possibility that masking effects were responsible for the locomotor activity responses of lizards to TCs. The three experiments demonstrated that TCs are important for entraining circadian locomotor activity rhythms of T. rugosa, even when photic cues are conflicting or absent, and that an interaction between LD cycles and TCs most accurately synchronizes this rhythm.