For generations, early to bed and early to rise was seen as the blueprint for a healthy life, and any departure from it was often considered unhealthy. Scientists, however, have discovered that whether someone is an early bird who wakes up early and starts the day with energy or a night owl who naturally stays up late and wakes up later is far more than a lifestyle choice. This pattern reflects the body’s natural preference for the timing of sleep and wake cycles within a 24-hour day.
A recent study, however, suggests that being a night owl might come with its own set of metabolic disadvantages.
Meals are also timing cues for the body’s internal clock, and when we eat can influence metabolism. In a recent paper, researchers explored whether a person’s chronotype is linked to eating habits, body composition and overall metabolic health.
The study followed a group of healthy women living in Auckland, New Zealand, and found that night owls consumed less energy and fewer micronutrient-dense foods than those with a morning-type chronotype. Higher calorie intake in the evening was associated with a higher body fat percentage and a greater risk of obesity.
Because most night owls ate very little in the morning and consumed a large portion of their food late at night, they tended to have a higher body mass index (BMI) and body fat percentage than early birds.
The findings were published in Frontiers in Nutrition.
Sleep, food and biomarkers
Studies suggest that chronotype influences more than when we sleep—it also shapes our eating habits. While most studies have focused on total food intake, they have paid less attention to the timing of calories and nutrients, even though both what we eat and when we eat can vary by chronotype.
Evening types are more likely to eat later in the day, make less healthy food choices and have poorer eating habits overall. They have also been associated with a higher BMI. However, BMI alone cannot distinguish between muscle mass and body fat. This is particularly relevant in New Zealand, where Pacific women typically have a larger body build and greater muscle mass than those of European descent.
In this study, the researchers went beyond BMI. The team applied the Munich Chronotype Questionnaire (MCTQ) to determine the chronotypes of 130 New Zealand Pacific women and 157 New Zealand European women ages 18 to 45. Based on their sleep-wake patterns, participants were classified as morning, intermediate or evening types. Women who typically went to sleep at 11 p.m. and woke up at 8 a.m. were categorized as intermediate types.
Participants then recorded everything they ate and drank over five nonconsecutive days—three weekdays and two weekend days. Dietitians later interviewed each participant to verify portion sizes and ensure the dietary records were accurate.
Instead of relying solely on body weight, the researchers used DXA scans, a type of X-ray, to measure how much body fat the women had and where it was stored, including around the belly and hips. They also measured blood biomarkers, including blood sugar, cholesterol and fat-related hormone levels.
Night owls had poorer results across nearly every health marker researchers measured, compared with morning and intermediate types. They had a higher average BMI and carried more body fat overall, with a higher android-to-gynoid ratio, indicating more weight around the midsection rather than the hips—a pattern known to raise the risk of health problems. They ate less healthy food, consumed less fiber and fell short on essential vitamins and minerals.
Blood work revealed that evening types had higher insulin and triglyceride levels and lower levels of protective HDL cholesterol. They also showed increased levels of leptin, a hormone linked to body fat, and lower levels of ghrelin, the hormone that signals hunger, compared with other chronotypes.
The findings show that although all chronotype groups consumed similar amounts of calories and macronutrients, evening-type women had less favorable metabolic biomarker, body fat and nutrient profiles.
The researchers say more research is needed on this link between chrono-nutrition and obesity, especially in ethnically diverse populations. This will help scientists understand chronotype’s true impact and eventually build personalized health interventions.