The daily variations found in many aspects of physiology are collectively known as circadian rhythm (from "circa" meaning "about" and "dien" meaning "day"). Circadian oscillation in clock gene expression can generate quantitative or functional variations of the molecules directly involved in many physiological functions. This paper reviews the molecular mechanisms of the circadian clock, the tr...

The circadian clock interacts with other regulatory pathways to tune physiology to predictable daily changes and unexpected environmental fluctuations. However, the complexity of circadian clocks in higher organisms has prevented a clear understanding of how natural environmental conditions affect circadian clocks and their physiological outputs. Here, we dissect the interaction between circadi...

As totally blind people cannot perceive the light-dark cycle (the major synchroniser of the circadian pacemaker) their circadian rhythms often "free run" on a cycle slightly longer than 24 h. When the free-running sleep propensity rhythm passes out of phase with the desired time for sleep, night-time insomnia and daytime sleepiness result. It has recently been shown that daily melatonin adminis...

The increasing demand for improved agricultural production will require more efficient breeding for traits that maintain yield under heterogeneous environments. The internal circadian oscillator is essential for perceiving and coordinating environmental cues such as day length, temperature, and abiotic stress responses within physiological processes. To investigate the contribution of the circa...

The circadian rhythm of pineal melatonin is the best marker of internal time under low ambient light levels. The endogenous melatonin rhythm exhibits a close association with the endogenous circadian component of the sleep propensity rhythm. This has led to the idea that melatonin is an internal sleep "facilitator" in humans, and therefore useful in the treatment of insomnia and the readjustmen...

Sleep is an evolutionarily conserved behavioral state whose regulation is poorly understood. A classical model posits that sleep is regulated by homeostatic and circadian mechanisms. Several factors have been implicated in mediating the homeostatic regulation of sleep, but molecules underlying the circadian mechanism are unknown. Here we use animals lacking melatonin due to mutation of arylalky...

We have used an in vitro model system of the circadian clock, dispersed chick pineal cells, to examine the effects of temperature on the circadian clock of a homeotherm. This preparation enabled us to isolate a circadian clock from in vivo homeostatic temperature regulation and expose cells to both constant temperatures and abrupt temperature changes. By manipulating the temperature of the pine...

Circadian rhythm disturbances are frequently reported in patients recovering from traumatic brain injury (TBI). Since circadian clock output is mediated by some of the same molecular signaling cascades that regulate memory formation (cAMP/MAPK/CREB), cognitive problems reported by TBI survivors may be related to injury-induced dysregulation of the circadian clock. In laboratory animals, aberran...

AIM To analyse how age-related losses in crystalline lens transmittance and pupillary area affect circadian photoreception and compare the circadian performance of phakic and pseudophakic individuals of the same age. METHODS The spectral sensitivity of circadian photoreception peaks in the blue part of the spectrum at approximately 460 nm. Photosensitive retinal ganglion cells send unconsciou...

The circadian clock increases organisms' fitness by regulating physiological responses(1). In mammals, the circadian clock in the suprachiasmatic nucleus (SCN) governs daily behavioural rhythms(2). Similarly, in Arabidopsis, tissue-specific circadian clock functions have emerged, and the importance of the vasculature clock for photoperiodic flowering has been demonstrated(3-5). However, it rema...