After a long day at the zoo, sore feet and stiff legs might make you wonder how flamingos get any shuteye while precariously standing atop a single stiltlike leg. To figure out how they do it, scientists placed juvenile flamingos atop a force platform, essentially a hypersensitive bathroom scale, and studied how the birds’ tiny muscular movements allowed them to maintain balance over their bodies’ subtle sways. While standing and grooming, the sway was large, but it decreased sevenfold while the birds slept on one leg. It appeared that the birds simultaneously exerted less muscle force and held a steadier position while asleep, the researchers report today in Biology Letters. Next, the team used two fresh flamingo cadavers to find an anatomical explanation. Although birds have the same bones in their legs as humans, their thighs are oriented horizontally, so it looks like they are permanently holding the yoga chair pose. The scientists stably propped up the dead birds on one leg by placing the foot under the body’s center. This effectively locks the leg joints in place, preventing the knee from flexing while allowing the joint to remain fully extended. Bony articulations in the joints may explain the passive locking mechanism. And although the scientists are careful to note that this study didn’t directly measure muscle activity, it’s the first evidence to suggest that flamingos, and perhaps other birds, sleep on one leg because it simply takes less energy.