The effects of sleep upon metabolic rate in the little penguin were examined at thermoneutral and low temperatures in day and night experiments. Little penguins show similar electrophysiological indices of sleep to other birds. The amount of sleep increased at night due to increased frequency of sleep periods and decreased with cold exposure due to a reduction in the length of sleep periods. Sleep was associated with a slight decrease (8%) in metabolic rate when compared to quiet wakefulness in all experimental conditions. This descrease, however, represents only a marginal reduction in daily energy costs. There would be a difference of only 2.4% in the hypothetical daily energy budget of a resting little penguin if it did not sleep at all.