People who have repeatedly changing sleep and wake times and get different amounts of sleep each night are more likely to have metabolic health conditions. That is the finding of a new study.

For years, lack of sleep has been linked to a wide collection of metabolic conditions, including obesity, high blood pressure, and diabetes. But until the recent study, health researchers did not know much about the effects of inconsistent sleep, including nightly changes in sleep amount and timing.

Tianyi Huang is among the writers of the study. Huang is with the Brigham and Women's Hospital and Harvard Medical School in Boston.

In an email to Reuters News Agency, Huang said that more inconsistent sleep times are associated with higher metabolic disease risk. And that is "no matter if one has short or long sleep duration or has good or poor sleep quality."

The researcher explained that night-to-night differences in sleep, either duration or timing, are associated with high risk of having several metabolic problems at the same time.

Huang added that these effects cannot be avoided by having a longer sleep duration on some nights.

For the study, 2,003 patients did home-based sleep studies for one week. They used devices known as actigraphs, which measure nighttime movements and sleep-wake cycles. The study was published in Diabetes Care.

On average, these people got about 7.15 hours of sleep each night and went to bed at around 11:40 p.m. Around two-thirds of them had more than one hour of change in sleep duration. And 45 percent of them had more than one hour of change in their bedtimes.

A total of 707 patients, or 35 percent, had metabolic syndrome – several types of metabolic problems that increase heart disease risks. They included increased blood pressure, high blood sugar, more fat around the stomach, and abnormal levels of some body chemicals.

Compared to people who had less than one hour of change in sleep duration, people whose sleep duration changed by 60 to 90 minutes were 27 percent more likely to have metabolic syndrome. The increase rose to 41 percent for people with 90 to 120 minutes of change in sleep duration. It rose to 57 percent with more than two hours of change in sleep duration.

Compared with people with no more than half an hour of change in their nightly bedtime, people whose bedtime changed by 30 to 60 minutes were similarly associated with metabolic syndrome. But it was 14 percent higher when bedtimes changed by 60 to 90 minutes. It was 58 percent higher when bedtimes changed by more than 90 minutes.

The study was not designed to prove whether or how changes in sleep duration or bedtimes might cause metabolic syndrome. Instead, it showed an association between inconsistent sleep behavior and metabolic diseases.

Kristen Knutson is a researcher at Northwestern University Feinberg School of Medicine in Chicago. She said the reason high inconsistency in sleep affects metabolic health may be about our biological clocks. Knutson was not involved in the study.

She said in an email that the human body has 24-hour rhythms and these rhythms must work together and with the environment for good health.

If a person sleeps at different times and different amounts, she said, the body's clocks may have difficulty "staying synchronized" which may lead to harm.

One limitation of the study is that researchers only studied sleep for one week. So the patients' longer-term sleep behavior is unknown. Researchers also lacked information on things that affect sleep consistency, like eating breakfast and meal timing, both of which also affect metabolic health.

This week-long study was part of a longer-term study on sleep consistency and metabolic problems financed by the National Institutes of Health.

Health experts say most adults need at least seven hours of sleep each night.

For the right amount and to avoid sleep-related problems, the Centers for Disease Control and Prevention offers guidelines. They include setting a consistent bedtime, sleeping in a dark room without electronics and avoiding large meals, caffeine and alcohol before bed.

I'm Alice Bryant. And I'm Jill Robbins.

Lisa Rapaport wrote this report for Reuters News Agency. Alice Bryant adapted it for Learning English. Caty Weaver was the editor.

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metabolic – adj. related to the chemical processes by which a plant or an animal uses food or water to grow, heal and make energy

obesity – n. the state of being very fat in a way that is unhealthy

inconsistent – adj. not continuing to happen or develop in the same way

associate – v. to connect someone or something with something else

cycle – n. a set of events or actions that happen again and again in the same order

rhythm – n. a regular, repeated pattern of events, changes or activities

synchronize – v. to happen at the same time or speed