In a human, various endogeneous rhythms of different periods are observed ranging from microsecond of molecular oscillation, about one second of the heart rhythm, about 24 hours of the circadian rhythm stood for by the sleep-wake cycle, to a hundred years of life time. How are the biological rhythms generated? Do the rhythms interact with each other? How are the living organisms with circadian rhythms influenced by the environment with period of 24 hours. The rhythmic behavior is common to most of the living organisms. In this talk, we consider a model of the biological rhythm and its stability to an externally applied perturbation. Firstly, we show that a system described by a set of nonlinear differential equations with a stable limit cycle provides an appropriate model of the biological rhythm. We explain a notion of phase reset and its role in stabilizing the rhythmic motion when exposed to perturbation, using a simplest model of the limit cycle, called the RIC. Then, synchronization or entrainment of the intrinsic rhythmic behavior, for example, the circadian rhythm, caused by an external oscillation, like the rotation of the earth with period of 24 hours is explained using the phase transtion curve of a limit cycle oscillator to periodic perturbance. Using a musculo-skeletal model of human walking, the stumbling reactions in response to an external perturbation are considered in the walking movement, in which the walking phase reset plays an important role to stabilize the walking motion. In fact, it is shown that appropriate amounts of the phase reset can prevent the model from falling, even for the perturbation that induces falling in the case without the phase reset. Furthermore, motor coordination of human lower limbs was also presented during pedalling a special kind of ergometer which allows its left and right pedals to rotate independently. In particular, relative phase between left and right rotational-velocity waveforms of the pedals and their amplitude modulation have been analyzed for patients with Parkinson's disease. Finally roles of endogeneous and exogeneous fluctuations in the living organisms is briefly discussed.