Chapter 8. Hormones and Sex
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Nicola Davis Death by cannibalism might seem like a high price to pay for a fleeting moment of passion, but male praying mantises are doing it for the kids, new research suggests. Scientists have discovered that female praying mantises who eat their mates after sex produce a greater number of eggs than those who do not, with the bodies of the ill-fated males used to aid their production. Of the species of praying mantises known to exhibit sexual cannibalism it is estimated up to 28% of males are eaten by their partner. After mating, the female stores the male’s sperm and later uses it to fertilise the eggs that she produces. The authors say the new study backs up a long-mooted theory that males could have evolved a behavioural trait of self-sacrifice to boost their reproductive success. “There is an obvious cost – you are dead, you have lost all future mating possibilities,” said William Brown, of the State University of New York at Fredonia, who co-authored the research. “We measure costs and benefit in terms of offspring production,” he added. If, by dying, the male can boost the number of offspring produced by one female, the theory goes, it could outweigh the downsides of missing out on future conquests. Published in the journal Proceedings of the Royal Society B by researchers in the US and Australia, the new study reveals how scientists unpicked the influence of cannibalism on the production of offspring in the praying mantis Tenodera sinensis, by tracking what happened to male ejaculate and bodily tissues after mating. © 2016 Guardian News and Media Limited
By Christie Aschwanden What is gender? It might sound like the kind of question that college students debate in a liberal arts class.1 But for the International Olympic Committee, it’s a practical question that demands a hard and fast answer. As at previous Olympic Games, athletes competing in Rio de Janeiro will be segregated into women’s events and men’s events, and that means the IOC needs a way to sort women from men. New IOC guidelines issued in November allow athletes who have transitioned to another gender to compete without sex reassignment surgery. The rules allow athletes who’d previously identified as female to compete in the male category without restriction, because they would not gain an advantage from their previous gender. Those who transition from male to female, on the other hand, must meet several requirements. The athlete must declare a female identity, and this identity cannot change for at least four years. The athlete must also document that her total serum testosterone levels have remained below a certain limit for a minimum of 12 months before competing, and these levels must remain under the threshold as long as she’s competing. The Olympic committee’s decision is a “huge step forward for everybody in the [transgender] community,” Caitlyn Jenner told me last week. “You can still have your old parts, which I think is very forward thinking.” Jenner is a trans woman who won the gold medal in the decathlon at the 1976 Olympics when she was Bruce Jenner, and she’s keeping the anatomical details of her own transition private. The public “is obsessed with — do you have it, or don’t have it?” she said, but “a trans person’s body parts is nobody’s business.”
Keyword: Sexual Behavior
Link ID: 22373 - Posted: 06.29.2016
By Elizabeth Pennisi Cave fish have long fascinated biologists because of their missing eyes and pale skin. Now, one researcher is studying them for another reason: Their behavior may provide clues to the genetic basis of some human psychiatric disorders. Last week at the 23rd International Conference on Subterranean Biology in Fayetteville, Arkansas, he demonstrated how drugs that help people with schizophrenia and autism similarly affect the fish. “I think there is a lot of potential” for these fish to teach us about mental disorders, says David Culver, an evolutionary biologist at American University in Washington, D.C., who was not involved in the study. Culver adds that—like other work on the cause of cave fish blindness—the new research may also have implications for human disease. A decade ago, the lead author on the new study, Masato Yoshizawa, wanted to understand brain evolution by investigating the effects of natural selection on behavior. The Mexican tetra (Astyanax mexicanus), a cave fish with very close surface relatives, seemed an excellent prospect for that work. Because the two populations can interbreed, it’s easier to pin down genes that might be related to the neural defects underlying behavioral differences. Such breeding studies are not possible in humans. The blind cave fish differ from their surface relatives in several notable ways. They don’t have a social structure and they don’t school. Instead, they lead solitary lives—a behavior that makes sense given their lack of natural predators. They also almost never sleep. They are hyperactive, and—unlike other fish—they are attracted to certain vibrations in the water. Finally, they tend to do the same behavior over and over again and seem to have higher anxiety than their surface relatives. © 2016 American Association for the Advancement of Science.
By Patrick Monahan The soft, blinking lights of fireflies aren’t just beautiful—they may also play a role in creating new species. A new study shows that using light-up powers for courtship makes species split off from each other at a faster pace, providing some of the clearest evidence yet that the struggle to find mates shapes the diversity of life. The firefly’s glow, like the enormous claws of fiddler crabs and the elaborate dances of manakins, was sculpted by the struggle for sex. Scientists have long thought that this kind of mating-driven natural selection—called “sexual selection”—could make species split into two. Say females in two populations prefer different color patterns in males: Even if the populations have the same needs in every other way, that simple preference could make them split into species with males of separate colors. “A lot of closely related species differ in sexual traits,” says Emily Ellis, an evolutionary biologist at the University of California (UC), Santa Barbara. But actually linking this kind of evolution to species proliferation is a hard idea to test. “So many people have looked at this and found differing results,” she says—possibly because they looked at smaller groups, like birds, rather than across the whole tree of life. That’s where bioluminescence comes in. Many groups of living organisms, from insects to fish to octopuses, emit light, whether to ward off predators, dazzle prey, or attract mates. It’s a trait that has evolved more than 40 times across the animal kingdom, Ellis says. © 2016 American Association for the Advancement of Science.
By Ruth Williams The offspring of certain mice fed a high-fat diet have altered gut microbiomes and may be prone to autism-like behaviors including social deficits, according to a study published today (June 16) in Cell. But treating these offspring with a specific microbial species they lack can rectify the animals’ social behavior. “There’s growing evidence that the microbiome, particularly early in life, can have long-term effects on brain development and behavior,” said anatomist and neuroscientist John Cryan of University College Cork in Ireland who was not involved in the study. “What this paper does is take advantage of the fact that we get our microbiome from our mums, and looks at what happens if the mum disturbs her microbiome during pregnancy.” According to the US Centers for Disease Control and Prevention, one in 68 U.S. children have autism spectrum disorder (ASD). Recent evidence suggests that the risk of ASD is increased for the offspring of mothers with obesity. In both humans and non-human primates, the offspring of obese mothers have also been shown to have abnormal microbiomes. And some people with ASD have imbalanced gut microbes, or dysbiosis. Baylor College of Medicine’s Mauro Costa-Mattioli and colleagues sought to better understand how maternal obesity, the microbiome, and ASD are interconnected. The team turned to mice for answers. The researchers gave female animals high-fat diets before setting up matings, later finding that a “large proportion” of the offspring exhibited ASD-like behaviors, including reduced social interaction, repetitive behaviors, and anxiety. The team analyzed the microbiomes of these offspring, finding that they differed from those of control animals. © 1986-2016 The Scientist
Link ID: 22336 - Posted: 06.18.2016
Susan Milius The Nyctibatrachus humayuni frogs live only in India’s Western Ghats, a region of still-unexplored biodiversity. Video now shows that the mating male of the species positions himself loosely on a female’s back, with his hands on the ground or leaves. From this position, called a dorsal straddle, the male then releases sperm directly onto the female’s back. Then, in an unusual move, he retreats before she lays the eggs. Sperm trickling down the female’s back and legs fertilize the eggs, an international research team reports June 14 in PeerJ. It’s the first time biologists have documented this loose straddling position. More typically, male frogs, which don’t deliver sperm into a female reproductive tract, hold tight and contact freshly deposited eggs to fertilize them. Bombay night frogs do on occasion crawl over their eggs, but researchers found the eggs are already fertilized. Biologists studying the challenges of external fertilization have previously cataloged six basic forms of male frog mating grasp, or amplexus. Four take some kind of back-hug approach or a head straddle. Other species position themselves rump-to-rump or, in what’s called glued amplexus, with the male dangling from a behemoth female. Position is hardly the only unexpected feature of courting Bombay night frogs. Females give courtship croaks, one of only a few dozen female-vocal species among the 6,500-plus known kinds of frogs. © Society for Science & the Public 2000 - 2016
Keyword: Sexual Behavior
Link ID: 22326 - Posted: 06.16.2016
by Laura Sanders Any parent trying to hustle a school-bound kid out the door in the morning knows that her child’s skull possesses a strange and powerful form of black magic: It can repel parents’ voices. Important messages like “find your shoes” bounce off the impenetrable fortress and drift unheeded to the floor. But when this perplexing force field is off, it turns out that mothers’ voices actually have profound effects on kids. Children’s brains practically buzz when they hear their moms’ voices, scientists report in the May 31 Proceedings of the National Academy of Sciences. (Fun and not surprising side note: Babies’ voices get into moms’ brains, too.) The parts of kids’ brains that handle emotions, face recognition and reward were prodded into action by mothers’ voices, brain scans of 24 children ages 7 to 12 revealed. And words were not required to get this big reaction. In the study, children listened to nonsense words said by either their mother or one of two unfamiliar women. Even when the words were fake, mothers’ voices still prompted lots of neural action. The study was done in older kids, but children are known to tune into their mothers’ voices early. Really early, in fact. One study found that fetuses’ heart rates change when they hear their moms read a story. For a fetus crammed into a dark, muffled cabin, voices may take on outsized importance. |© Society for Science & the Public 2000 - 2016.
By Teal Burrell Sociability may be skin deep. The social impairments and high anxiety seen in people with autism or related disorders may be partly due to a disruption in the nerves of the skin that sense touch, a new study in mice suggests. Autism spectrum disorders are primarily thought of as disorders of the brain, generally characterized by repetitive behaviors and deficits in communication skills and social interaction. But a majority of people with autism spectrum disorders also have an altered tactile sense; they are often hypersensitive to light touch and can be overwhelmed by certain textures. “They tend to be very wary of social touch [like a hug or handshake], or if they go outside and feel a gust of wind, it can be very unnerving,” says neuroscientist Lauren Orefice from Harvard Medical School in Boston. An appreciation for this sensory aspect of autism has grown in recent years. The newest version of psychiatry’s bible, the Diagnostic and Statistical Manual of Mental Disorders, includes the sensory abnormalities of autism as core features of the disease. “That was a big nod and a recognition that this is a really important aspect of autism,” says Kevin Pelphrey, a cognitive neuroscientist at The George Washington University in Washington, D.C., who was not involved in the work. The sensation of touch starts in the peripheral nervous system—in receptors at the surface of the skin—and travels along nerves that connect into the central nervous system. Whereas many autism researchers focus on the end of the pathway—the brain—Orefice and colleagues wondered about the first leg of the trip. So the group introduced mutations that silenced genes associated with autism spectrum disorders in mice, adding them in a way that restricted the effects to peripheral nerve cells, they report today in Cell. The team singled out the gene Mecp2, which encodes a protein that regulates the expression of genes that help forge connections between nerve cells. © 2016 American Association for the Advancement of Science
By Linda Marsa| Helen Epstein felt deeply isolated and alone. Haunted by her parents’ harrowing experiences in Nazi concentration camps in World War II, she was troubled as a child by images of piles of skeletons and barbed wire, and, in her words, “a floating sense of danger and incipient harm.” But her Czech-born parents’ defense against the horrific memories was to detach. “Their survival strategy in the war was denial and dissociation, and that carried into their behavior afterward,” recalls Epstein, who was born shortly after the war and grew up in Manhattan. “They believed in action over reflection. Introspection was not encouraged, but a full schedule of activities was.” It was only when she was a student at Israel’s Hebrew University in the late 1960s that she realized she was part of a community that shared a cultural and historical legacy that included both pain and fear. “I met dozens of kids of survivors,” she says, “one after the other who shared certain characteristics: preoccupation with a family past and Israel, and who spoke several middle European languages — just like me.” Epstein’s 1979 book about her observations, Children of the Holocaust, gave voice to that sense of alienation and free-floating anxiety. In the years since, mental health professionals have largely attributed the second generation’s moodiness, hypervigilance and depression to learned behavior. It is only now, more than three decades later, that science has the tools to see that this legacy of trauma becomes etched in our DNA — a process known as epigenetics, in which environmental factors trigger genetic changes that may be passed on, just as surely as blue eyes and crooked smiles.
By Virginia Morell Sex is never simple—even among lizards. Unlike mammals, the sex of central bearded dragons, large lizards found in eastern Australia, is determined by their chromosomes and the environment. If the eggs are incubated in high temperatures, male embryos turn into females. Such sex-reversed lizards still retain the chromosomal makeup of a male, but they develop into functional superfemales, whose output of eggs exceeds that of the regular females. Now, a new study predicts that—in some cases—these superfemales may be able to drive regular ones to extinction. That’s because superfemales not only produce more eggs, but they’re also exceptionally bold. Looking at the shape, physiology, and behavior of 20 sex-reversed females, 55 males, and 40 regular females, scientists found that the sex-reversed dragons were physically similar to regular males: They had a male dragon’s long tail and high body temperature. They were also behaviorally similar, acting like bold, active males—even as they produced viable eggs. Indeed, the scientists report in the current issue of the Proceedings of the Royal Society B that these sex-reversed females were behaviorally more malelike than the genetic males. Because of these advantages, this third sex could reproductively outcompete normal females, the scientists say, possibly causing some populations to lose the female sex chromosome. (Females are the heterogametic sex, like human males.) In such a population, the dragons’ sex would then be determined solely by temperature instead of genetics—something that’s occurred in the lab within a single generation. Could it happen in the wild? The scientists are still investigating. © 2016 American Association for the Advancement of Science
By Sarah DeWeerdt, Spectrum Brains from people with autism show patterns of gene expression similar to those from people with schizophrenia, according to a new analysis. The findings, published May 24 in Translational Psychiatry, deepen the connections between the two conditions, says study leader Dan Arking, associate professor of genetic medicine at Johns Hopkins University in Baltimore, Maryland. People who have either autism or schizophrenia share features such as language problems and difficulty understanding other people’s thoughts and feelings. They also have genetic risk factors in common. “And now I think we can show that they share overlap in gene expression,” Arking says. The study builds on previous work, in which Arking’s team characterized gene expression in postmortem brain tissue from 32 individuals with autism and 40 controls. In the new analysis, the researchers made use of that dataset as well as one from the Stanley Medical Research Institute that looked at 31 people with schizophrenia, 25 with bipolar disorder and 26 controls3. They found 106 genes expressed at lower levels in autism and schizophrenia brains than in controls. These genes are involved in the development of neurons, especially the formation of the long projections that carry nerve signals and the development of the junctions, or synapses, between one cell and the next. The results are consistent with those from previous studies indicating a role for genes involved in brain development in both conditions. “On the one hand, it’s exciting because it tells us that there’s a lot of overlap,” says Jeremy Willsey, assistant professor of psychiatry at the University of California, San Francisco, who was not involved in the work. “On the other hand, these are fairly general things that are overlapping.” © 2016 Scientific American
By Perri Klass, M.D. When girls come in for their physical exams, one of the questions I routinely ask is “Do you get your period?” I try to ask before I expect the answer to be yes, so that if a girl doesn’t seem to know about the changes of puberty that lie ahead, I can encourage her to talk about them with her mother, and offer to help answer questions. And I often point out that even those who have not yet embarked on puberty themselves are likely to have classmates who are going through these changes, so, again, it’s important to let kids know that their questions are welcome, and will be answered accurately. But like everybody else who deals with girls, I’m aware that this means bringing up the topic when girls are pretty young. Puberty is now coming earlier for many girls, with bodies changing in the third and fourth grade, and there is a complicated discussion about the reasons, from obesity and family stress to chemicals in the environment that may disrupt the normal effects of hormones. I’m not going to try to delineate that discussion here — though it’s an important one — because I want to concentrate on the effect, rather than the cause, of reaching puberty early. A large study published in May in the journal Pediatrics looked at a group of 8,327 children born in Hong Kong in April and May of 1997, for whom a great deal of health data has been collected. The researchers had access to the children’s health records, showing how their doctors had documented their physical maturity, according to what are known as the Tanner stages, for the standardized pediatric index of sexual maturation. Before children enter puberty, we call it Tanner I; for girls, Tanner II is the beginning of breast development, while for boys, it’s the enlargement of the scrotum and testes and the reddening and changing of the scrotum skin. Boys and girls then progress through the intermediate changes to stage V, full physical maturity. © 2016 The New York Times Company
By Ann Griswold, Women who develop infections during pregnancy run an increased risk of having a child with autism. Most data indicate that an overactive maternal immune response underlies the risk. But a new analysis runs contrary to this view: It ties high levels of an inflammatory protein in pregnant women to a low risk of autism in their children, suggesting that a strong immune response is protective. Researchers looked at 1,315 mother-child pairs, including 500 children with autism and 235 with developmental delay. They found that healthy pregnant women with high levels of C-reactive protein (CRP), a marker of inflammation, are less likely to have a child with autism than are women with typical levels of the protein. The findings contradict a 2013 report from a large Finnish cohort that tied high CRP levels during pregnancy to an increased risk of having a child with autism. “It was the opposite of what we expected to find,” says senior researcher Lisa Croen, director of the Autism Research Program at Kaiser Permanente in Oakland, California. The work appeared in April in Translational Psychiatry. The results suggest that the strength of a woman’s immune system, rather than its response to infection, is the important factor in determining autism risk. Moderate or low baseline levels of CRP might indicate a relatively weak ability to fight off infection. And a less vigorous immune response might boost the risk in some women, the researchers say. © 2016 Scientific American,
By Ann Lukits Teens who baby-sit may not only gain confidence in caring for young children, they may also alter their brain chemistry in a way that could make them better parents, suggests an animal study in Developmental Psychobiology. Young female rats housed with various groups of unrelated rat pups had fully developed mothering skills as adults, compared with control rats without caregiving, or alloparenting, experience. The early caregivers had significantly higher concentrations of tryptophan hydroxylase-2 (TPH2) in the brain, an enzyme associated with increased production of serotonin, a chemical involved in mood and social behavior. Previous research has associated baby-sitting experience in humans with greater confidence in new mothers, researchers said. Experiments at Michigan State University involved two groups of juvenile or adolescent female rats from 16 litters. In one group, 24 rats were housed in separate cages with a different group of week-old pups each day. A second group of 24 controls were given pink pup-size pencil erasers. The experiments continued for 14 days. Eight mature rats from both groups were subsequently exposed to new groups of pups. Six rats with alloparenting experience acted maternally toward the pups, whereas none of the control rats exhibited maternal behavior. Rats with alloparenting experience also displayed less anxiety during behavioral testing. The animals were euthanized after testing and TPH2 levels measured in a section of the brain called the dorsal raphe nucleus. ©2016 Dow Jones & Company, Inc
Keyword: Sexual Behavior
Link ID: 22273 - Posted: 06.01.2016
Susan Milius Forget it, peacocks. Nice try, elk. Sure, sexy feathers and antlers are showy, but the sperm of a fruit fly could be the most over-the-top, exaggerated male ornamentation of all. In certain fruit fly species, such as Drosophila bifurca, males measuring just a few millimeters produce sperm with a tail as long as 5.8-centimeters, researchers report May 25 in Nature. Adjusted for body size, the disproportionately supersized sperm outdoes such exuberant body parts as pheasant display feathers, deer antlers, scarab beetle horns and the forward-grasping forceps of earwigs. Fruit flies’ giant sperm have been challenging to explain, says study coauthor Scott Pitnick of Syracuse University in New York. Now he and his colleagues propose that a complex interplay of male and female benefits has accelerated sperm length in a runaway-train scenario. Males with longer sperm deliver fewer sperm, bucking a more-is-better trend. Yet, they still manage to transfer a few dozen to a few hundred per mating. And as newly arrived sperm compete to displace those already waiting in a female’s storage organ, longer is better. Fewer sperm per mating means females tend to mate more often, intensifying the sperm-vs.-sperm competition. Females that have the longest storage organs, which favor the longest sperm, benefit too: Males producing megasperm, the researchers found, tend to be the ones with good genes likely to produce robust offspring. “Sex,” says Pitnick, “is a powerful force.” © Society for Science & the Public 2000 - 2016
By Christie Aschwanden When concussions make the news, it’s usually about football. But head injuries happen in other sports too, and not just to men. During a congressional hearing on concussions in youth sports on Friday, Dawn Comstock, an epidemiologist who studies sports injuries, told a House Energy and Commerce subcommittee that in sports like soccer and basketball in which girls and boys play by the same rules, with the same equipment and the same facilities, “girls have higher concussion rates than boys.” Comstock, a researcher at the Colorado School of Public Health, is the first author of a 2015 study published in JAMA Pediatrics that quantified concussions in high school soccer and found that they were about one and a half times more common in girls than in boys. When U.S. Rep. Diana DeGette, D-Colo., asked whether more data was needed to show that girls have higher concussion rates, Comstock replied, “We already have the data that’s consistently shown this gender difference.” What we don’t have, she said, is a proven explanation for the discrepancy. Some researchers have wondered whether women and girls are simply more likely to report their symptoms than men and boys are. “It’s a sexist way to say that they’re not as tough,” said Katherine Price Snedaker, executive director of Pink Concussions,1 an organization that is seeking answers to how concussions affect women and girls. The group recently held a summit on female concussion and traumatic brain injuries at Georgetown University, and one of the speakers was Shannon Bauman, a sports physician who presented data from 207 athletes — both male and female — who’d been evaluated at her specialty concussion clinic in Barrie, Ontario, between September 2014 and January 2016.
By Simon Oxenham The “cuddle chemical”. The “moral molecule”. Oxytocin has quite a reputation – but much of what we thought about the so-called “love hormone” may be wrong. Oxytocin is made by the hypothalamus and acts on the brain, playing a role in bonding, sex and pregnancy. But findings that a sniff of the hormone is enough to make people trust each other more are being called into question after a string of studies failed to replicate classic experiments. Paul Zak at the Centre for Neuroeconomic Studies in Claremont, California, made his moral molecule hypothesis famous in 2011 when he memorably squirted a syringe of the hormone into the air while delivering a TED talk. When people sniff oxytocin before playing a money-lending game, it increases how much they trust each other, he explained. But several teams have been unable to replicate his finding. Last November, Gideon Nave at the California Institute of Technology in Pasadena and his colleagues reviewed studies of oxytocin, and concluded that the effect of nasal squirts of the hormone on trust are not reliably different from zero. Nave’s team aren’t the only ones calling the moral molecule hypothesis into question. In 2012, Moïra Mikolajczak at the Catholic University of Louvain (UCL) in Belgium and her colleagues published their own seminal findings backing a link between trust and oxytocin. They found that when people filled out an anonymous questionnaire about their sex lives and fantasies, they were less likely to seal the envelopes they returned them in if given a nasal dose of oxytocin beforehand. © Copyright Reed Business Information Ltd.
Laura Glynn, Pregnancy brain typically refers to lapses in attention and memory. About 80 percent of new mothers report difficulties remembering things that once came naturally, and although not all studies support this, the weight of the evidence shows that during pregnancy, women exhibit measurable declines in important cognitive skills. But it's not all bad news. The maternal brain also features important enhancements. Mother rats score higher in tests of attention, foraging and planning than peers who have never given birth. These gains most likely render them better able to defend and provide for their pups. The benefits for human moms are less clear, but data are emerging that suggest human pregnancies initiate neural restructuring. A 2010 study found that in the first few months after giving birth, human females show changes in several key brain regions. Specifically, they often exhibit increased volume in the hypothalamus, striatum and amygdala—areas essential for emotional regulation and parental motivation—as well as in regions governing decision making and protective instincts. We can glean further evidence from behavioral changes during pregnancy. Many women exhibit blunted physiological and psychological responses to stress, which may afford mother and fetus protection from the potentially adverse effects of taxing situations. And in the postpartum period, the hormones that sustain breast-feeding maintain these dampened stress responses. © 2016 Scientific American
Sara Reardon As a medical student in Paris in the 1980s, Eric Vilain found himself pondering the differences between men and women. What causes them to develop differently, and what happens when the process goes awry? At the time, he was encountering babies that defied simple classification as a boy or girl. Born with disorders of sex development (DSDs), many had intermediate genitalia — an overlarge clitoris, an undersized penis or features of both sexes. Then, as now, the usual practice was to operate. And the decision of whether a child would be left with male or female genitalia was often made not on scientific evidence, says Vilain, but on practicality: an oft-repeated, if insensitive, line has it that “it's easier to dig a hole than build a pole”. Vilain found the approach disturbing. “I was fascinated and shocked by how the medical team was making decisions.” Vilain has spent the better part of his career studying the ambiguities of sex. Now a paediatrician and geneticist at the University of California, Los Angeles (UCLA), he is one of the world's foremost experts on the genetic determinants of DSDs. He has worked closely with intersex advocacy groups that campaign for recognition and better medical treatment — a movement that has recently gained momentum. And in 2011, he established a major longitudinal study to track the psychological and medical well-being of hundreds of children with DSDs. © 2016 Nature Publishing Group
By David Shultz Did you sleep well? The answer may depend on your age, location and gender. A survey of 5000 sleepers from across the world has revealed that women get the most sleep, particularly those under the age of 25. Daniel Forger at the University of Michigan in Ann Arbor and his team were able to get their huge dataset thanks to Entrain, a smartphone app that people use to track their sleep. With their consent, Forger’s team accessed users’ data on their wake time, bed time, time zone and how much light they were exposed to during the day. Analysing this information, they found that middle-aged men sleep the least, while women under the age of 25 sleep the most. As a whole, women appear to sleep on average for 30 minutes longer than men, thanks to going to bed slightly earlier and waking up slightly later. For an individual, the time they woke up had the strongest link to how much sleep they got, suggesting that having a job that starts early every day can mean that you get less sleep than someone who starts work at a later hour. There were also differences between countries. People in Singapore, for example, sleep for an average of 7.5 hours a night, while Australians get 8.1 hours. Late bedtimes seem to be to blame – people in Singapore tended to stay up until after 11.45 pm each night, while people in Australia were likely to hit the hay closer to 10.45 pm. The team found that, in general, national wake-up times were linked more to daylight hours than bedtimes. This could be because bedtimes are more affected by social factors. © Copyright Reed Business Information Ltd.