Abstract
Abstract Research was started into a possible circadian periodicity in the behaviour of the threespined stickleback. The behaviour of solitary males in the sexual phase of their breeding cycle was studied under constant conditions, including illumination of constant brightness (LL). No, 'free-running', circadian rhythm was found. Both the frequency of nest-visits and the rate of creeping-through (which shows an ultradian periodicity) were remarkably constant over long periods. Also, operant conditioning experiments, in which the male could switch the lights on or off, did not reveal any circadian fluctuation in the preference for light or dark. In the course of the long-term recordings 'rest-periods' were found, lasting some 10 to 50 minutes, in which the fish was surprisingly inactive, and did not visit the nest. Although some characteristics of these 'rest-periods' seem to point to a 'sleep-like' nature, others seem to argue against this.
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