Circadian Rhythms: Lessons for Humans from Worms, Bacteria, and Mice

We share melatonin with primitive organisms. Two recent studies examining the roles of melatonin and circadian rhythms in basic life functions may improve our understanding of human circadian rhythms.

Tosches and colleagues found that melatonin regulates a circadian rhythm of cilia in a marine worm that swims toward the surface at night and sinks to the bottom during the day. Melatonin genes (which are linked to genes for light-reactive compounds similar to those in the vertebrate retina) were upregulated in the dark. Rising melatonin levels during darkness increased rhythmic activity of a cholinergic neuron that altered the coordinated motion of cilia.

Thaiss and colleagues demonstrated that intestinal bacteria in mice have a diurnal rhythm …