AbstractDaily rhythms enable organisms to adapt to daily fluctuations in environmental factors. Do organisms still exhibit 24-h rhythms when living in habitats without obvious daily cycles in external signals? To answer this question, we measured the heart rates of six Adélie penguin (Pygoscelis adeliae) chicks on Inexpressible Island during the polar day between 15 and 21 January 2019. Averaged heart rates were between 186 and 233 beats/min for individual chicks. Both fast Fourier transformation and autocorrelation were employed to assess the daily rhythmicity. Based on fast Fourier transformation, a significant contribution of daily rhythm in heart rate variation was found only in one individual. Small effect size of significant autocorrelation coefficients was found in two individuals, while there was no significant autocorrelation coefficient for 24-h time lag in four other individuals. In summary, no prevailing daily rhythm of heart rate was found in these Adélie penguin chicks. We propose that the lack of daily rhythm in Adélie penguin chicks could be an adaptation to the local environment in the polar regions, but that the adaptive value thereof remains to be investigated.