It's harder to wake a penguin after lunch.
For penguins, the best time for a snooze is the afternoon. The birds sleep more deeply after lunch than during the morning rush hour, a French ecologist has found1.
Gérard Dewasmes crept up on sleeping king penguins (Aptenodytes patagonica) and tapped their shoulders with a weighted stick once a minute till they awoke.
"I have to approach the bird very slowly - it takes about ten minutes," says Dewasmes, who works at the University of Picardy in Amiens. "In many cases the bird wakes up first." He spent a month studying penguins living on the Crozet Archipelago, 1,000 km north of Antarctica.
It took around nine pokes rouse a bird from an afternoon nap - five to wake one sleeping in the morning. This is the first evidence for differing depths of sleep in a wild bird, says Dewasmes.
In the morning, penguin colonies bustle, as many birds head for the sea to feed. Stay-at-homes - birds tending chicks or eggs, for example - are bumped and jostled.
The finding suggests that birds' sleep fits their surroundings, and is not just influenced by them, claims Dewasmes. "I think that breeding birds must adapt their sleep strategies to surrounding social activity," he says. Light sleeping balances the needs for rest and vigilance, he says.
Penguins are less active in the afternoon, although Dewasmes couldn't tell if they were sleeping off their dinner or just resting.
Dewasmes, G. & Loos, N. Diurnal sleep depth changes in the king penguin (Aptenodytes patagonicus). Polar Biology, advance online publication 2002, doi:10.1007/s00300-002-0412-9 (2002).