A recent study conducted in Auckland, New Zealand, has found that individuals who identify as "night owls"—those who stay up late and wake up later—may face increased risks related to metabolic health and body composition. The research, which focused on healthy women aged 18 to 45 from different ethnic backgrounds, provides new insights into how sleep timing may influence diet quality and health outcomes.

The study included 287 women, comprising 130 New Zealand Pacific women and 157 New Zealand European women. Researchers classified participants’ sleep patterns using the Munich Chronotype Questionnaire (MCTQ), categorizing them as morning, intermediate, or evening types based on their typical sleep and wake times. Women who regularly slept from 11 p.m. to 8 a.m. were considered intermediate types.

Over the course of the study, participants recorded all food and beverage intake across five non-consecutive days, encompassing both weekdays and weekends. Unlike many studies that rely solely on body mass index (BMI) to assess body composition, the research team employed dual-energy X-ray absorptiometry (DXA) scans to measure body fat percentage and its distribution, including fat accumulation around the waist and hips. Blood samples were also taken to evaluate metabolic markers such as blood sugar, cholesterol, triglycerides, insulin, and hormones connected to appetite regulation and fat storage.

Findings revealed that women with evening chronotypes consumed fewer nutrient-dense foods and tended to ingest a larger proportion of their daily calories in the evening compared to their morning and intermediate counterparts. This pattern was linked to higher body fat percentages and an elevated risk of obesity. Night owls generally ate less in the morning and concentrated most of their caloric intake later in the day, resulting in increased BMI and greater total body fat.

Blood analyses indicated that evening types exhibited higher levels of insulin and triglycerides alongside lower concentrations of high-density lipoprotein (HDL) cholesterol, commonly referred to as "good" cholesterol. They also had elevated leptin levels, a hormone associated with body fat, and reduced ghrelin levels, which stimulate hunger. These hormonal differences, coupled with dietary patterns, contributed to poorer metabolic health outcomes across nearly all measured indicators when compared with morning and intermediate types.

Additionally, night owls showed a reduced intake of dietary fiber, vitamins, and essential minerals despite consuming similar total calories and nutrients as the other groups. Excess fat stored around the waist, a characteristic more prevalent among night owls, is typically linked to a heightened risk for cardiovascular disease and other metabolic conditions.

Published in Frontiers in Nutrition, the study underscores the potential metabolic disadvantages of late sleeping patterns, highlighting the importance of chronotype as a factor influencing diet quality and health risks in women. Further research may help clarify whether modifying sleep patterns could offer a pathway to improved metabolic health.