How can some people get by on six hours of sleep, while others struggle with anything less than nine? Why does depression often go hand in hand with poor sleep? And why is good sleep so critical to overall metabolism?
The answers to all of these questions may lie in our genes.
Although the science of sleep is still young, scientists have uncovered a number of surprising ways that genes might affect your sleep. Here are five:
1. "Short-sleepers" can get by on just 4 to 6 hours a night.
In 2009, researchers at the University of California, San Francisco discovered a gene mutation that allows some people to feel refreshed on much less sleep than the normal population -- as little as 4 hours a night, the Wall Street Journal reported.
2. There's a gene linked to both seasonal depression and poor sleep.
A two-for-one special that no one wants: Two rare variants of the PERIOD3 (PER3) gene are linked to both Seasonal Affective Disorder -- depression related to change in the seasons -- and poor sleeping patterns.
That's because PER3 helps regulate the sleep-wake cycle and the relationship between moods and natural light, according to a study published in January in the Proceedings of the National Academy of Sciences.
"This is the first human mutation directly linked to seasonal affective disorder, and the first clear sign of a mechanism that could link sleep to mood disorders," Dr. Louis Ptáček, professor of neurology at UC San Francisco and a co-author of the study, said in a statement.
3. DNA may be why sleep and metabolism are associated.
In 2013, scientists identified a "gene region" that's linked with longer sleep, better glucose metabolism and a lower likelihood of ADHD. The region is located near a gene called PAX8, LiveScience reported, which helps regulate thyroid hormone levels. The thyroid can impact sleep cycles: People with inadequate thyroid hormones often sleep excessively, and people with too much of it can be hyperactive.
4. An overactive gene is linked to severe insomnia.
The gene known as neuromedin U (Nmu) has been called "nature's stimulant" for its apparent role in wakefulness and, by extension, insomnia. A new study, published this month in the journal Neuron, showed that zebrafish with over-expressed Nmu became more active both day and night and displayed a "profound form of insomnia."
The researchers concluded that Nmu may be applied in developing new therapies to address sleep disorders in the future, but more research is needed.
"Sleep is a mysterious process," Dr. David Prober, assistant professor of biology at the California Institute of Technology and a co-author of the study, said in a statement. "We spend a third of our lives doing it, and every animal with a complex nervous system seems to do it, so it must be important. But we still don't understand why we do it or how it's regulated."
5. Being a "morning person" could be part of your DNA.
Scientists recently isolated 15 areas in the human genome that are associated with the tendency toward "morningness," or a preference for rising early. They also found that morning people were less likely to suffer from insomnia or depression, and had lower average BMIs than so-called "night owls."
With so many correlated benefits to early rising, one might despair at the thought that their genes don't set them up for it. But the scientists who conducted the study said genetics are only one factor in being an early bird; lifestyle factors and individual choices are just as important.