Men who are "night owls" have an increased risk of type 2 diabetes and general wasting of muscle mass than their "early-bird" counterparts, while women are more likely to have the metabolic syndrome if they are nocturnal than women who are not, new research shows.

The study was published in the April issue of the Journal of Clinical Endocrinology and Metabolism byJi Hee Yu, PhD, from Korea University College of Medicine, Ansan, and colleagues.

"To the best of our knowledge, this is the first study to reveal an association between chronotype and sarcopenia or metabolic disorders at the population level," they write.

Asked to comment, Endocrine Society spokesperson Orfeu Buxton, PhD, from Harvard Medical School, Boston, Massachusetts, told Medscape Medical News that it is well documented that shortened sleep duration reduces insulin sensitivity and promotes diabetes and obesity risk.

Sleep quality also influences diabetes and obesity risk, although probably through a different mechanism, he added.

"That's what's so fascinating about this paper — they showed that it's the later timing and poor-quality sleep that's related to diabetes and the metabolic syndrome," Dr Buxton said.

Gender Differences Apparent

In the study of 1620 middle-aged adults between the ages of 47 and 59, Dr Yu and colleagues found 29.6% of the cohort qualified as morning chronotype, or early birds, while 5.8% of the cohort qualified as evening chronotype, or night owls.

The majority of the cohort, 64.5%, was neither a morning nor evening chronotype.

In all participants, the risk for diabetes for evening types was 73% higher than it was for morning types at an odds ratio (OR) of 1.73 after researchers controlled for confounders including lifestyle factors.

The risk of the metabolic syndrome was 74% higher for evening types compared with morning types at an OR of 1.74, again after adjustment for confounding factors.

The risk of sarcopenia or general muscle-mass wasting in turn was over threefold higher for evening types compared with morning types (OR, 3.16).

In contrast, visceral obesity was not significantly associated with being either an evening or a morning person.

The investigators also saw gender differences in the associations between chronotype and metabolic disorders. In men, the risk for diabetes was almost threefold higher among the night owls compared with the early birds (OR, 2.98). And the risk of sarcopenia was almost fourfold higher among night-owl men compared with the early-bird men (OR, 3.89).

In contrast, the metabolic syndrome was not related to chronotype in men.

Women who were night owls, on the other hand, were over twice as likely to have the metabolic syndrome as women who were early birds (OR, 2.22).

In contrast to men, neither diabetes nor sarcopenia was associated with the evening chronotype in women.

Shortened Sleep Duration, Disruption, and Timing All Influence Risk

As the authors discuss, the evening chronotype is likely associated with a worse metabolic profile than others for multiple reasons, one of which may "sleep debt."

"Individuals with evening chronotype are more likely to suffer from sleep curtailment because of the discrepancy between intrinsic sleep-wake rhythm and actual bedtime, the former determined by biological clock and the latter influenced by social requirement," they observe.

Although average sleep duration was not different between night owls and early birds in their study, "evening persons are known to accumulate sleep debt on weekdays and then have extended catch-up sleep on the weekend," the authors point out.

The findings support the potential negative consequences of circadian disruptions in individuals with evening chronotype, they say.

At the recent ENDO 2015 annual meeting of the Endocrine Society in San Diego, Shahrad Taheri, PhD, from Weill Cornell Medical College in Doha, Qatar, and colleagues found that as little as 30 minutes a day of sleep debt can have significant effects on obesity and insulin resistance, increasing obesity by 17% and insulin resistance by 39% for every 30 minutes of weekday sleep debt at study end point 12 months later.

Dr Buxton has also reported that sleep deprivation and sleep disruption experienced by shift workers both increase obesity and diabetes risk, but they do so through different pathways.

"You can't take health behaviors, or for that matter poverty and socioeconomic status, out from these studies, as they are important factors as well," he observed.

"My point is that with respect to sleep duration, sleep quality, and sleep timing, all three components contribute to diabetes and obesity risk."

The study was supported by funds from the Korea Centers for Disease Control and Prevention and a Korea University Grant. The authors report no relevant financial relationships, as does Dr Buxton.

J Clin Endocrinol Metab. 2015;100:1494-1502. Available at: http://press.endocrine.org/journal/jcem.