The interaction of internal clocks and environmental conditions determines the daily behavioural rhythm of an animal. Due to the strong influence of light, these circadian rhythms oscillate in mammals over a cycle length of about 24 h, equivalent to the daily light-dark cycle. The relation between activity and rest within this cycle is species-specific and age dependent. Since strong deviations from existing rhythms can harm health seriously, the observation of animal behaviour using activity budgets is a common tool to