Behavioral sleep was assessed for 152 nights in 5 adult, 2 immature, and 1 juvenile giraffes at a zoological garden, using continuous time-lapse video recording. Sleep occurred while the giraffes were standing (SS) and in recumbency (RS). Paradoxical sleep (PS) was recognized by the peculiar positioning of the head on the croup and by phasic events. The 24-h sleep profile had a main bimodal nocturnal sleep period between 20.00 and 07.00 hours, with a trough between 02.00 and 04.00 hours, and several short naps between 12.00 and 16.00 hours. Total sleep time (TST), excluding the juvenile, was 4.6 h, whereby PS compromised only 4.7%. TST was not age dependent, but the lowest amount of RS and the highest amount of S occurred in the oldest and the two oldest animals, respectively. Sleep was fragmented, as indicated by the predominance of RS episodes lasting less than 11 min. Sleep cycle duration was very variable with the most values between 1 and 35 min (when no waking or RS was allowed within PS episodes), or 6-35 min (when the criteria for ending a PS episode allowed 1-2 min interruptions by RS).
There were several indications for sleep regulations: (i) RS and SS complemented each other to yield a relatively stable daily value of TST; (ii) sleep was redistributed on nights following a day when the giraffes spent a few hours in an outside enclosure. The first peak of the bimodal sleep profile was absent and RS was more prominent in the second half of the night compared with nights following days spent in the barn; and (iii) napping was followed by a minor reduction of RS and an increase in SS in the subsequent night compared with nights following days without naps.