To find out which of daily oscillating environmental factors in the continuous daylight of arctic summer are responsable for synchronization of animal activity period with the rotation of the earth, in Spitsbergen (78° N) a whole summer continuous measurements of temperature (°C), light intensity (lux) and spectral composition of the light (colour temperature in °K) were done. With a daily amplitude of 4-5° C in the air, resp. 10° C at the ground surface, temperature can only act as a Zeitgeber for heterothermic animals. Measurements of light intensity (with a photocell directed vertical upward to the zenith) showed relatively large daily oscillations. But since light intensity depends on the cosine of the angle of incidence of the light (Lambert's cosine law) and in high latitudes the angle of incidence of the sun is always very great, in fact measurements have to be done with the photocell directed to the sun or, as the latter meets with mechanical problems, all dates got with the photocell vertical upward must be converted after Lambert's cosine law. Converted dates of light intensity oscillate not nearly as intense as measured dates. This small daily amplitude of light intensity, which in addition is strongly influenced by sudden changes of the wheather, can hardly come into consideration as a Zeitgeber of animal activity period. Colour temperature however showed large and regular daily oscillations and therefore could be a Zeitgeber of animal activity period in high arctic summer, particularly because in the laboratory the synchroniing effect of an artificial daily alternation of colour temperature is already proved. Nest (feeding) activity of local snowbuntings and perch hopping activity of greenfinches, brought from Germany, were registrated with light traps. Activity period of both bird species was clearly synchronized with the rotation of the earth. In a narrow valley the activity pattern of greenfinches was identical as in open country, in spite of temporarily total other conditions of light intensity (shading by mountains). Apparently even the activity period of non arctic animals can be synchronized by weak Zeitgebers like colour temperature.