Women who are natural night owls carry more body fat and show less favourable blood sugar and cholesterol levels than women who prefer earlier schedules, even when both groups consume roughly the same number of calories each day, new research has found.
The study of nearly 300 women in New Zealand identified timing as the decisive factor. The night owls ate little in the morning and pushed their biggest meals into the late evening, and that late-night eating tracked closely with higher body fat and poorer metabolic health.
Early bird or night owl?
Whether someone is an early bird or a night owl comes down to chronotype, the body's built-in preference for when to sleep, wake and feel alert. To classify participants, the researchers used a questionnaire pinpointing the midpoint of each person's sleep on work-free days, dividing them into evening types, morning types and an intermediate group.
Rozanne Kruger, professor of nutrition and dietetics at Griffith University, led the analysis, which drew on a larger women's health study run at Massey University in Auckland. All 287 participants were healthy women aged 18 to 45, free of the chronic conditions that usually complicate such research.
A clear metabolic pattern
The evening types stood apart. They carried a higher body mass index, 31.4 compared with 26.1 among earlier risers, a larger proportion of body fat and more fat packed around the abdomen. Their blood told a similar story, with higher fasting insulin, triglycerides and long-term blood sugar.
When the eating happens
Across a full day, morning and evening types took in almost identical amounts of energy, with the evening group ahead by roughly 50 calories, a rounding error. Where the two groups split was the clock.
Morning and intermediate types front-loaded their eating, taking in about 15 per cent of the day's energy before ten in the morning. The evening types managed only about nine per cent that early, then made up for it after dark. After eight in the evening, the pattern flipped: evening types ate close to a quarter of their daily energy in the late-night window, roughly 2.5 times their morning share, while the earlier group took in about 11 per cent. The night-time food also leaned towards carbohydrate and fat.
"Both morning-types and evening-types consumed similar amounts of food or energy across the day, but it was the timing of eating that was crucial," said Kruger.
Belly fat and biomarkers
To see where the late eating landed on the body, the team measured the ratio of fat stored around the abdomen to fat on the hips and thighs, a gauge of belly fat that flags heart and metabolic risk more sharply than weight alone. No previous study had tracked this measure across chronotypes.
The contrast was sharpest there. Among the women with the most abdominal fat, the evening types' late eating stood out most clearly. Their blood work aligned with the pattern, running higher on insulin, triglycerides and glycated haemoglobin, a marker of average blood sugar, and lower on protective HDL cholesterol. A larger review pooling dozens of studies has reached the same conclusion, finding that evening types carry worse markers of heart and metabolic health even when they eat no more than early risers.
Why late eating can backfire
The body handles food differently depending on the hour. In the morning it is primed to burn energy, with insulin working efficiently and blood sugar remaining under control. As night falls, that machinery winds down in preparation for the overnight fast. A large meal close to bedtime arrives when the body has already turned towards storage rather than use.
Over months and years, the authors argue, that mismatch can push energy into fat instead of fuelling activity, especially around the middle. A trial has shown the effect directly: when volunteers ate later in the day, they burned fewer calories, felt hungrier, and their fat tissue tilted towards storing energy rather than releasing it.
Skipping breakfast may set the trap, as going without an early meal tends to leave people hungrier by evening, leading to overeating or heavy snacking at night, exactly the pattern the evening types showed.
The caveats
Most of the evening types were younger Pacific women from more deprived areas, so chronotype cannot be considered in isolation from diet, culture and socioeconomic factors. The researchers adjusted for those factors but could not fully separate them, and the findings need testing in more varied groups. The study also captured a single moment in time, so it cannot prove that late eating caused the extra fat rather than the reverse.
Even so, the work is among the first to show, in healthy young women, that meal timing rather than total intake lines up with body fat, central fat and metabolic markers. Nudging the biggest meals earlier, and easing off late-night eating, could give night owls a way to lower their risk without eating less overall.
"The research highlights that when people eat may be just as important as what they eat," said Kruger. The study is published in the journal Frontiers in Nutrition.
Source: Earth.com
