People who take longer midday naps are at a higher risk for obesity and high blood pressure, according to Boston researchers who found that “not all siestas are the same.”

The Brigham and Women’s Hospital scientists looked at more than 3,000 adults from a Mediterranean population, examining the relationship of midday naps with obesity and other conditions tied to heart disease and diabetes.

The researchers found that those who took siestas of 30 minutes or longer (considered long siestas) were more likely to have a higher body mass index, elevated blood pressure, and a cluster of other conditions associated with heart disease and diabetes (metabolic syndrome) — compared with those who didn’t take long siestas.

However, those who took short siestas, also known as “power naps,” did not see an increased risk for obesity and other conditions. In contrast, short siesta-takers were less likely to have elevated systolic blood pressure than those who took no siestas, according to the Brigham researchers.

“Not all siestas are the same,” said senior author Marta Garaulet, a visiting professor in the Division of Sleep and Circadian Disorders at the Brigham. “The length of time, position of sleep, and other specific factors can affect the health outcomes of a nap.

“A previous study that we conducted in a large study population in the UK had found that siestas were associated with an increased risk of obesity,” Garaulet said. “We wanted to determine whether this would hold true in a country where siestas are more culturally embedded, in this case Spain, as well as how the length of time for siestas is related to metabolic health.”

The researchers examined data from 3,275 adults in a Mediterranean population, specifically people from the Spanish region of Murcia.

The scientists found that long siesta-takers had a higher body mass index and were more likely to have metabolic syndrome than those who did not take siestas. Also, compared with the no-siesta group, the long siesta group had higher values of waist circumference, fasting glucose levels, systolic blood pressure, and diastolic blood pressure.

The researchers found that long siestas were linked with later nightly sleep timing and food timing, and with increased energy intake at lunch and cigarette smoking.

“This study shows the importance of considering siesta length and raises the question whether short naps may offer unique benefits,” said co-author Frank Scheer, a senior neuroscientist and professor in the Medical Chronobiology Program in the Brigham’s Division of Sleep and Circadian Disorders.

Scheer added, “If future studies further substantiate the advantages of shorter siestas, I think that that could be the driving force behind the uncovering of optimal nap durations, and a cultural shift in the recognition of the long-term health effects and productivity increases that can amount from this lifestyle behavior.”