Women, try not to think of this if you lie awake at night: having trouble sleeping means you're likely to gain weight.
As if simply getting older weren't hard enough, new research shows that middle-aged and older women who have trouble falling or staying asleep may pack on more pounds than their well-rested contemporaries.
A number of studies have found that sleep-deprived children and adults are more likely to be overweight than those who usually get a full night's rest. But many of those studies assessed people at one point in time, so it was hard to know which came first, the sleep problems or the excess pounds.
A few studies have followed people over time, but they've disagreed about whether poor sleep is linked to expanding waistlines.
The new findings, reported in the International Journal of Obesity, strengthen the evidence that sleep problems are related to weight gain. In this case, the study design allowed the researchers to show that sleep problems came before substantial weight gain in some participants.
Finnish researchers followed more than 7,300 40- to 60-year-old adults for seven years. They found that women who reported significant sleep problems at the outset generally put on more weight over time than women who slept well.
Roughly one-third of women with frequent sleep problems gained at least 11 pounds, versus about a fifth of women with no sleep difficulties at the outset.
Men were spared, however. Their sleep problems were not related to weight gain.
The link in the women persisted even when the investigators accounted for a number of factors that can affect both sleep quality and weight gain -- including participants' body weight at the study's start, their exercise habits and their general physical and mental health.
While the findings do not prove cause-and-effect, they raise the possibility that improving sleep quality might help stave off excess weight gain, lead researcher Peppi Lyytikainen, of the University of Helsinki, told Reuters Health by e-mail.
The 7,332 men and women in the study were first surveyed between 2000 and 2002. Those who said they'd had trouble falling asleep or staying asleep on at least 14 nights in the past month were classified as having "frequent" sleep problems. The study participants also reported their weight and height during the first survey, then again five to seven years later.
At the outset, 20 percent of women had frequent sleep problems. Overall, the study found, those women were more likely to report a "major" weight gain -- 11 pounds or more -- by the study's end compared to women who slept well.
But the 17 percent of men who reported sleep problems were no more likely to gain weight than those who slept without difficulty.
The reason for the disparate findings for men and women is unclear, according to Lyytikainen's team. But it might be related to the fact that the study included a smaller number of men than women -- 1,300 versus more than 5,700 -- which may have made any potential effect among men harder to detect.
This type of study, however -- in which researchers observe people over time -- can't prove cause-and-effect. While the researchers accounted for a number of variables related to sleep and weight -- like self-reported general health and exercise and other lifestyle habits -- they cannot rule out the possibility that factors other than sleep problems account for the higher risk of substantial weight.
Other research does suggest that sleep deprivation may affect the body in ways that contribute to weight gain, Lyytikainen said.
There is evidence, for example, that sleep loss alters people's levels of the appetite-regulating hormones leptin and ghrelin -- which could, in theory, spur them to overeat.
It is unknown, however, whether treating insomnia and other sleep disturbances has any added benefit for people's waistlines.