Abstract: Disturbed light–dark (LD) cycles are associated with circadian disruption of physiological and behavioural rhythms and in turn with an increased risk of disease development. However, direct causal links and underlying mechanisms leading to negative health consequences still need to be revealed. In the present study, we exposed male Wistar rats to repeated phase shifts of LD cycle and analysed their ability to cope with mild emotional stressors. In experiment 1, rats were submitted to either a regular 12:12 LD cycle (CTRL rats) or 8-h phase delay shifts applied every 2 days for 5 weeks (SHIFT rats). Subsequently, the behaviour was examined in the open-field, black–white box and elevated plus maze tests. In experiment 2, changes in blood pressure (BP), heart rate (HR) as well as the activity of autonomic nervous system were measured in telemeterised rats in response to open-field and black–white box tests before and after 5-week exposure to shifted LD regime. Locomotor activity was consistently higher in SHIFT than CTRL rats in in the open-field and black–white box tests. Interestingly, in the elevated plus maze, SHIFT rats displayed increased risk assessment and decreased grooming compared to CTRL rats. Anxiety measures were affected only in the black–white box, where SHIFT rats displayed reduced anxiety-like behaviour compared to CTRL rats. Differences in behavioural reactivity between SHIFT and CTRL rats did not correspond with BP and HR changes. However, exposure to phase shifts increased the sympathovagal reactivity in the black–white box. Together, our results demonstrated that disturbed LD conditions decreased emotional reactivity of rats and affected their ability to cope with emotional stressors denoting an additional risk mechanism linking disrupted circadian organisation to adverse health effects.

Abstract: Light at night alters behavior and cognitive performances in rodents, the variations of which in gender and stages of reproductive cycle in females are elusive. Young mice habituated in light:dark (12:12h) cycle were given a single exposure of light (100lx) at early night for one hour duration followed by experimentations in open field (closed wall with circular big arena), elevated plus maze and square habituated field for memory performance using novel object recognition task. Light effects were compared with results found during without light conditions. Proestrous females appeared to have greater locomotor activity, less anxiety and better memory performance compared to the diestrous females at night without light exposure. The status of locomotor activity, anxiety and memory performance of male mice at night without light exposure appeared to be comparable to females where the stage of estrous cycle is important to characterize the nocturnal behavior of male mice. Light maximally affected proestrous females with decrease in locomotor activity, increase in anxiety and failure of memory performance. Male and diestrous female mice performed memory performance without alteration of locomotor activity and anxiety after exposure to light where males performed better memory performance with greater locomotor activity and more anxiety compared to that of diestrous females. The present study characterizes the mice nocturnal behavior with and without a single exposure to light stimuli with its gender features and estrous cycle variation. In addition, the study indicates an association of memory performance with locomotor activity and anxiety in mice nocturnal behavior.

Abstract: Circadian rhythms are generated by the autonomous circadian clock, the suprachiasmatic nucleus (SCN), and clock genes that are present in all tissues. The SCN times these peripheral clocks, as well as behavioral and physiological processes. Recent studies show that frequent violations of conditions set by our biological clock, such as shift work, jet lag, sleep deprivation, or simply eating at the wrong time of the day, may have deleterious effects on health. This infringement, also known as circadian desynchronization, is associated with chronic diseases like diabetes, hypertension, cancer, and psychiatric disorders. In this review, we will evaluate evidence that these diseases stem from the need of the SCN for peripheral feedback to fine-tune its output and adjust physiological processes to the requirements of the moment. This feedback can vary from neuronal or hormonal signals from the liver to changes in blood pressure. Desynchronization renders the circadian network dysfunctional, resulting in a breakdown of many functions driven by the SCN, disrupting core clock rhythms in the periphery and disorganizing cellular processes that are normally driven by the synchrony between behavior and peripheral signals with neuronal and humoral output of the hypothalamus. Consequently, we propose that the loss of synchrony between the different elements of this circadian network as may occur during shiftwork and jet lag is the reason for the occurrence of health problems.