ABSTRACT
All these observations pose two distinct problems. First, to what extent are the diurnal rhythms evoked by external signals such as light and darkness, or even more subtle influences such as the magnetic field of the earth? Conversely, how far are the rhythms the manifestation of intra-cellular biological clocks which go on working even though the environment is changed to perpetual light or perpetual darkness, or otherwise uniformised? There are many results which can only be explained on the latter assumption. In Spitzbergen's' white nights' the excretory rhythms of potassium and other ions in man would not adapt to constancy; they could be trained to as short an interval as 16 hr and to as long a one as 28 hr but not beyond those limits.b And secondly, what exactly are these internal biological clocks? As yet, no one knows.
