The eyes close in sleep, but the pillow remains awake.
The source for the proverb is David Crystal’s book, As They Say in Zanzibar: Proverbial Wisdom from Around the World (Oxford University Press: 2006). The saying means that even if you go to sleep, those around you (including the pillow!) are not asleep as well... and that means you need to expect the unexpected: while you snooze, they might get up to no good!
The image is by treehouse1977 at Flickr.
The poster is made with AutoMotivator.