What does the model Cara Delevingne have in common with Leonardo da Vinci and John F Kennedy? The habit of napping. When US Vogue this month revealed that Delevingne had nodded off during an interview, she wasn't pleased, saying her sleepiness was evidence of how hard she worked. But napping isn't restricted to celebrities -– or even humans. More than 85 percent of mammals sleep for short periods during the day; humans are unusual in having distinct times for waking and sleeping. Children and elderly people usually nap.