We humans spend a third of our lives asleep, oblivious to our surroundings and temporarily paralyzed. It’s a vulnerability that would seem to diminish our odds of survival, so evolutionarily speaking it must also somehow confer tremendous benefits. Yet our best guesses about what those benefits are tend to come from observing what happens when sleep is curtailed. As far as we know, all animals sleep in some way; deprive most of them of it for long enough, and they will die, but exactly why is unclear. In 2015, the American Academy of Sleep Medicine and the Sleep Research Society published a joint statement, based on a comprehensive review of research, saying that “sleeping less than seven hours per night on a regular basis” — which is the case for an estimated 35 to 40 percent of Americans during the workweek — is associated with adverse health outcomes. These include weight gain and obesity, diabetes, hypertension, heart disease and stroke, depression, impaired immune function, increased pain, greater likelihood of accidents and “increased risk of death.” The National Institutes of Health reported last year that sleep deficits may increase the beta-amyloid proteins in the brain linked with Alzheimer’s disease. But when it comes to “what sleep is, how much you need and what it’s for,” says Louis Ptacek, a professor of neurology at the University of California, San Francisco, “we know almost nothing — other than it’s bad not to get enough of it.” Indeed, says David Dinges, one of the statement’s authors and a professor of psychiatry at the University of Pennsylvania, “All of this makes it really tough to send out simple messages to the public about when you should sleep and how much you should sleep.”

Scientists believe that there are two separate but interrelated internal systems that regulate sleep. The first is the circadian system that tells our body when to sleep. Medicine already knows a great deal about how it works: Approximately every 24 hours, the suprachiasmatic nucleus, a small region in the hypothalamus, orchestrates physiological changes to prepare us for sleep, like lowering body temperature and releasing dopamine. But the second system — the one that tells our body the amount of sleep it needs — is still mysterious. One way to elucidate it would be to find genes that govern how long or deeply people sleep and observe where those genes are active. This fall, Ptacek, Ying-Hui Fu and other colleagues announced, in the journals Neuron and Science Translational Medicine, the discovery of two genetic mutations that seem to cause certain people to sleep far less than average. This brought the number of genes known to be involved in sleep duration to just three.

To learn what genes underlie a given trait or behavior, researchers look for anomalies. If you sequenced the genes of any two random people, you would find millions of variations between them, and there would be no telling which variation contributed to what traits. But diseases or disorders have discrete and relatively uncommon collections of symptoms that, when seen in families, suggest a genetic cause; if you look at the genes of five relatives who all have a particular type of early-onset Alzheimer’s disease, for example, and find a mutation that they all share, it’s much likelier to be related to their symptoms. Scientists can then engineer the same mutation in mice to see if it produces a similar effect in them. If it does, by observing where the gene is active in the brain, researchers can begin to study the processes that give rise to that specific collection of symptoms; the hope is that those processes will also offer clues to what neural mechanisms initiate more prevalent forms of the disease.