Accelerometry reveals nocturnal biphasic sleep behavior in wild giraffe

Three giraffes had a midnight intermission. Their nights did not simply get quieter until morning: a more active spell separated two waves of rest. Giraffes sleep in short episodes between activities such as feeding and moving, so discovering a nightly routine means looking beyond any single nap.

Anna Lena Burger-Schulz and colleagues fitted three young adult Angolan giraffes at Namibia's Okapuka Ranch with head-mounted movement sensors and recorded their nights over four months. Video helped the researchers recognise a distinctive posture in the sensor readings: the giraffe lying down, neck curled backward, head resting on its flank or the ground. That was a clue to sleep, not a brain recording that could confirm it. Alongside this posture, the team tracked periods when the head barely moved.

In all three animals, stillness and the sleeping posture peaked around 9–11 pm and again around 3–5 am, with more activity near midnight. The sensors could not tell whether that activity was walking or feeding. Nor were these exactly two naps: each quieter window contained separate episodes. The discovery was in how those episodes accumulated. Follow one brief rest and it looks like a nap; follow months of nights and a two-part routine emerges.