A short review of hypotheses on human time perception emphasises the distinction between endogenous and exogenous concepts. Methods are described, and the related problem of stationarity is discussed. A hypothesis is formulated which assumes oscillatory processes as the basis for the perception of short temporal intervals. Several experiments are described which test this hypothesis. The results of the first experiment suggest a temporal constant in the range of the conscious present. Periodic components in subjective random series which are demonstrated in another experiment, indicate a sequence of temporal units. The influence of physiological factors on time perception is illustrated; diurnal variations are discussed as an example. However, it is shown that the observed variations in time perception are not dependent on body temperature alone. The importance of informational cues is noted and an experiment is described which indicates the role of individual differences.