Why does sickness make you sleepy?

In Brain General Structure and Function, Brain Science, Sleep by Brainy Days Ahead

It’s well known that humans and other animals are fatigued and sleepy when sick. Researchers at the Perelman School of Medicine at the University of Pennsylvania study a microscopic roundworm to reveal what may be the mechanism for this sleepiness. During sickness, cells are under stress, and organisms experience sleepiness to promote sleep and recover from the cellular stress. The researchers find that, in the worm, the sleepiness is caused by release from the ALA neuron of FLP-13 and other neuropeptides, a group of chemicals that send signals between brain neurons. The research suggests that similar signaling may happen in humans.