Coordinated daily rhythms are evident in most aspects of our physiology, driven by internal timing systems known as circadian clocks. Our understanding of how biological clocks are built and function has grown exponentially over the past 20 years. With this has come an appreciation that disruption of the clock contributes to the pathophysiology of numerous diseases, from metabolic disease to ne...

Environmental time cues, such as photocycles (light/dark) and thermocycles (warm/cold), synchronize (entrain) endogenous biological clocks to local time. Although much is known about entrainment of the Arabidopsis thaliana clock to photocycles, the determinants of thermoperception and entrainment to thermocycles are not known. The Arabidopsis PSEUDO-RESPONSE REGULATOR (PRR) genes, including the...

Circadian (daily) biological clocks express characteristics that are difficult to explain by known biochemical mechanisms, and will ultimately require characterizing the structures, functions, and interactions of their molecular components. KaiC is an essential circadian protein in cyanobacteria that forms the core of the KaiABC clock protein complex. We report the crystal structure of the KaiC...

Circadian rhythms are entrained daily by environmental photic and non-photic cues. The present review describes the anatomy and functional characteristics of the three major input pathways to the circadian clock mediating entrainment: the retino-hypothalamic tract, the geniculo-hypothalamic tract and the midbrain raphe projection.

Temperature compensation is an important property of all biological clocks. In Neurospora crassa, negative-feedback regulation on the frequency (frq) gene's transcription by the FRQ protein plays a central role in the organism's circadian pacemaker. Earlier model calculations predicted that the stability of FRQ should determine the period length of Neurospora's circadian rhythm as well as the r...

BACKGROUND Tidal (12.4 hr) cycles of behavior and physiology adapt intertidal organisms to temporally complex coastal environments, yet their underlying mechanism is unknown. However, the very existence of an independent "circatidal" clock has been disputed, and it has been argued that tidal rhythms arise as a submultiple of a circadian clock, operating in dual oscillators whose outputs are hel...

The internal circadian clock adapts slowly, if at all, to rapid transitions between different shift schedules. This leads to misalignment (desynchrony) of rhythmic physiological systems, such as sleep, alertness, performance, metabolism and the hormones melatonin and cortisol, with the imposed work-rest schedule. Consequences include sleep deprivation and poor performance. Clock gene variants m...

BACKGROUND Coupled biological oscillators can tick with the same period. How this collective period is established is a key question in understanding biological clocks. We explore this question in the segmentation clock, a population of coupled cellular oscillators in the vertebrate embryo that sets the rhythm of somitogenesis, the morphological segmentation of the body axis. The oscillating ce...

Oscillations are found throughout the physical and biological worlds. Their interactions can result in a systematic process of synchronization called entrainment, which is distinct from a simple stimulus-response pattern. Oscillators respond to stimuli at some times in their cycle and may not respond at others. Oscillators can also be driven if the stimulus is strong (or if the oscillator is we...

Endogenous circadian clocks are robust regulators of physiology and behavior. Synchronization or entrainment of biological clocks to environmental time is adaptive and important for physiological homeostasis and for the proper timing of species-specific behaviors. We studied subjects in the laboratory for up to 55 days each to determine the ability to entrain the human clock to a weak circadian...