Ed Yong in The Atlantic:
In 1909, the Japanese scientist Kuniomi Ishimori collected spinal fluid from sleep-deprived dogs and injected it into active, rested pooches. Within hours, the latter fell into a deep sleep. By coincidence, a pair of French researchers did the same experiments a few years later and got the same results. These studies, and others like them, suggested that the blood of sleepy animals contains some kind of soporific secret sauce of chemicals. Ishimori called these “hypogenic substances.” Others labeled them “somnogens.”
The sources of these sleep-inducing chemicals have proved surprisingly elusive, and scientists have found only a few that fit the bill. Now Hirofumi Toda from the University of Pennsylvania has discovered another—a gene called nemuri that triggers sleep, at least in fruit flies. Unexpectedly, it alsobecomes active during infections and acts to kill incoming microbes. It seems to be part of a self-regulating system, analogous enough to an internal thermostat that we might call it a sleep-o-stat. It can send animals to sleep when they most need shut-eye, whether because they’re sick or because they just haven’t slept enough.
This sleep-o-stat works separately from the daily body clocks that make us feel more tired at night.