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 assess welfare. This study therefore investigated the nightly activity budget of 63 giraffe (Giraffa camelopardalis) from 13 EAZA institutions. The nightly behaviour was recorded and analysed in the winter seasons 2015–2018 over an observation period from 17:00 to 7:00 h for 10–14 nights, using infrared sensitive cameras. To analyse rest-activity rhythms of three age classes of giraffe during the dark phase of the night, linear mixed models were applied. Our results show that activity increases with increasing age of giraffe. Differences between age classes were further found for the time spent on rapid eye movement (REM) sleep, with juvenile and sub-adult giraffe spending more time in this specific state than adults. Analyses of behavioural rhythmicity indicated a rest-activity cycle length of about four hours, with age determining the rhythmic curve. Overall, this large-scale study confirms a strong age effect for both the nocturnal activity budget and the rest-activity rhythm in giraffe. The results of this study provide an important contribution to the continuous improvement of husbandry and management for zoo housed giraffe.