Work Needed to Unravel Why Night Owls at Higher Metabolic Risk

Liam Davenport

December 06, 2018

Increasing evidence suggests that being an evening type raises an individual's risk of developing type 2 diabetes and cardiovascular disease, likely through a combination of poor diet, erratic eating patterns, and irregular sleeping hours, say European researchers.

They are calling for more research into the different aspects of this phenomenon so that any lessons learned can help in modifying the behavior of those at risk.

An individual's morning or evening preference is dictated by their chronotype, or internal circadian clock system.

Researchers led by Suzana Almoosawi, PhD, Brain, Performance, and Nutrition Research Center, Northumbria University, Newcastle upon Tyne, UK, reviewed the literature to determine the relationship between chronotype, lifestyle, and cardiometabolic disease risk.

They found that an individual's morning or evening preference is affected by a range of genetic, ethnic, and societal factors.

This, in turn, is associated with differences in dietary intake, timing of eating (or chrononutrition), and sleeping patterns, with night owls more likely to have an unhealthy, unstable lifestyle.

Consequently, the risk of type 2 diabetes is increased more than twofold in evening types versus early birds, an effect seen most starkly in rotating shift workers. The review, published online in Advances in Nutrition, also highlights evidence that poor diabetes control is more common in evening types.

"Scientific evidence is providing increasing insight into the relationship between your chronotype, diet, and cardiometabolic health," noted senior author Leonidas G. Karagounis, PhD, Nestlé Research Center, Institute of Nutritional Sciences, Lausanne, Switzerland, said in a press release issued by Northumbria University.

"Overall, cross-sectional studies suggest that an evening chronotype is associated with lower intake of fruits and vegetables, and higher intake of energy drinks, alcoholic, sugary, and caffeinated beverages, as well as higher energy intake from fat."

"The review has highlighted a major gap in our understanding as to how our biological clock affects food intake in infants, children, and the elderly," added Almoosawi. But more research is required, she stressed.

Noting that a large proportion of individuals shift to an evening preference at around 6 years of age, she noted: "Whether physiological changes, school timings, or social schedules determine this change, we do not know."

"In teenagers, we also find that evening chronotype is related to more erratic eating behavior and poorer diet. This could have important implications to health in adulthood as most dietary habits are established in adolescence."

Lifestyles Misaligned With Circadian Rhythms in Modern Society

The authors point out that, in modern societies, it is common for individuals to have lifestyles that are "misaligned" with their circadian clock system and the daily light-dark cycle, and to eat at "physiologically inappropriate times of the day."

This, they say, disrupts the "natural oscillations" of physiological processes such as glucose and lipid metabolism and blood pressure, which can affect the risk of developing type 2 diabetes and cardiovascular risk.

To examine the impact of chronotype on these processes and risks, the researchers conducted a nonsystematic literature review, without specifying how the studies were chosen.

Chronotype can be assessed by a range of measures, the authors explain, including dim-light melatonin onset and several self-report questionnaires, although the latter typically remain to be validated in large studies.

Nevertheless, the evidence suggests that there are several genetic and environmental factors that influence the distribution of chronotype within a given population.

Several specific mutations in circadian clock regulator genes have been identified, and it has also been shown that chronotype can be affected by ethnicity and societal norms, with Germans found to be more likely to have an "evening preference" than, for example, Indians and Slovakians.

Furthermore, there seen to be differences in evening and morning preferences between individuals living in urban and rural areas within any sample country, in addition to which the amount of time spent outdoors can affect on sleep patterns.

Morning and evening preferences also change during an individual's lifetime, with the morning chronotype more common in childhood, appearing as early as 3 weeks of age.

At 2 years of age, 90% of individuals have a morning preference, falling to 58% at 6 years of age, and then further as they go into adulthood, before the trend is reversed in middle to older age.

More Research Needed on Chrononutrition

Crucially, the authors point out that several studies have demonstrated that an evening chronotype is associated with an increased risk of both cardiovascular disease and type 2 diabetes.

One study indicated that an evening preference was associated with a 2.5-fold increased risk of type 2 diabetes compared with a morning preference, as well as poorer glycemic control among those who already have the disease.

This, the authors say, could be related to people who go to bed later tending to have unhealthier diets than early birds, with the former tending to have greater consumption of alcohol, sugar, caffeinated drinks, and fast foods.

Moreover, night owls have more erratic eating and sleeping patterns, as they typically miss breakfast and build up a "sleep debt" during the week that they compensate for at weekends.

One study showed, for example, that skipping breakfast was associated with higher HbA1c levels, even after adjusting for age, sex, race, body mass index, number of diabetes complications, insulin use, depressive symptoms, perceived sleep debt, and percentage of daily caloric intake at dinner.

In contrast, morning people eat slightly more fruit and vegetables per day, including grains and rye, than evening people, and have smaller, more frequent meals and more stable sleeping patterns. Eating later in the day could also affect glucose metabolism.

Glucose levels should decline naturally during the day and reach their lowest point at night before going to sleep.

People who eat late in the evening, however, have increased glucose levels just as they are going to bed, again potentially increasing the risk type 2 diabetes.

These patterns are seen most commonly in shift workers, particularly those who work rotating shifts, as they are forced to constantly adjust their body clock to fit their working hours.

This, the authors write, reduces insulin sensitivity and affects glucose tolerance, with a meta-analysis of 12 observational studies perhaps unsurprisingly showing that rotating shift work was associated with an increased risk of type 2 diabetes.

Acknowledging the numerous limitations of their review, the authors call for "further research on the best methods to assess chronotype...and to examine how chronotype may affect the association between chrononutrition and long-term cardiometabolic health."

Chrononutrition is an emerging research field in nutritional epidemiology that encompasses three dimensions of eating behavior: timing, frequency, and regularity, they note.

To date, few studies have investigated how an individual's circadian typology (one's chronotype) affects the association between chrononutrition and cardiometabolic health.

Generating more research in this field will potentially help "guide the development of health-promotion strategies aimed at preventing and treating chronic diseases based on an individual's chronotype," they conclude.

No funding was declared for this study. Karagounis and three other authors are employed by Nestlé.

Adv Nutr. Published online November 30, 2018. Full text

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