Being a night owl may shape more than bedtime. In a study of 287 women, those who naturally preferred later sleep and wake times ate less earlier in the day, consumed more food at night and had less favorable measures of body composition and metabolic health than women with earlier or intermediate schedules.
The findings, published in Frontiers in Nutrition, came from a cross-sectional observational study. Researchers measured sleep timing, food intake, body composition and blood markers during the same general period, so the results cannot establish that eating later caused higher body fat, poorer glucose regulation or changes in blood lipids.
“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 Rozanne Kruger, a professor in Griffith University’s School of Allied Health, Sport and Social Work.
The study included European and Pacific New Zealand women ages 18 to 45. Researchers classified participants by chronotype, which describes a person’s natural preference for earlier or later sleep and activity.
Chronotype is partly influenced by biology, but it can also intersect with work schedules, family responsibilities, culture, light exposure and other aspects of daily life. Someone with an evening chronotype may naturally feel more alert later at night and prefer to wake later in the morning.
Participants completed five-day food records, and researchers assessed when they consumed energy, protein, carbohydrates and fat. Body composition was measured using dual-energy X-ray absorptiometry, commonly called DXA, and participants provided fasting blood samples for tests related to glucose regulation and cardiovascular health.
Women with evening chronotypes consumed less food between 3 a.m. and 9:59 a.m. and more between 8 p.m. and 2:59 a.m. Morning and intermediate types showed the opposite pattern.
Evening types consumed about 22.6% of their daily energy after 8 p.m., compared with about 11% among women in the combined morning and intermediate group. They also ate less protein and energy earlier in the day, then consumed more energy, carbohydrates and fat at night.
The overall difference in estimated daily energy intake was relatively small, at roughly 51 calories. That suggests the timing and distribution of food may have differed more than the total amount consumed.
Women with evening chronotypes also had higher average body mass index, body fat percentage and central fat distribution. Their average triglycerides, insulin and HbA1c were higher, while their HDL cholesterol was lower.
However, the pattern was not uniformly worse across every metabolic measure. Fasting glucose did not differ significantly, and total and LDL cholesterol were somewhat lower among evening types, although their lower HDL cholesterol still contributed to a less favorable overall lipid profile.
The study also found that evening types reported lower intake of fiber and several micronutrients. That raises the possibility that food quality, rather than timing alone, contributed to the differences.
“The research highlights that when people eat may be just as important as what they eat,” Kruger said.
The results do not prove that food eaten after a certain clock time is automatically stored as fat. The body’s response to food may vary across the day, but an evening meal does not affect everyone in the same way.
For someone who goes to bed at 10 p.m., eating at 9 p.m. may occur close to their biological night. For a person who naturally sleeps much later, the same meal may fall at a different point in their internal schedule.
Researchers used fixed clock-time windows rather than biological measures of circadian timing. They therefore could not determine whether participants were eating at times that aligned or conflicted with their individual body clocks.
The study also could not determine which came first. Later eating might contribute to metabolic differences, but higher body fat, blood sugar regulation, work schedules, stress or other factors could also influence when people sleep and eat.
Ethnicity and socioeconomic conditions further complicated the results. Evening chronotypes were more common among Pacific women and among participants living with greater socioeconomic deprivation.
Researchers adjusted some analyses for ethnicity, age and deprivation, but statistical adjustments cannot fully separate chronotype from factors such as shift work, food access, cultural eating patterns or household schedules.
The sample was also selective. It included healthy, nonpregnant women from New Zealand, and women in the overweight BMI range were generally excluded from the original recruitment design. The findings may not apply to men, older adults, adolescents, pregnant women or people with existing metabolic disease.
Only 35 participants were classified as morning types, so the researchers combined them with intermediate types for several comparisons. The study is therefore better understood as a comparison between women with evening chronotypes and everyone else, rather than a precise examination of three separate groups.
“Targeting meal timing, particularly reducing late-night eating, could be an important strategy for improving health amongst people with an evening chronotype,” Kruger said.
That possibility still needs to be tested in intervention studies. This research did not ask participants to move meals earlier or examine whether doing so changed body composition, blood sugar or cholesterol.
For night owls, the findings do not necessarily mean forcing down a large breakfast or never eating after 8 p.m. A more realistic question may be whether routinely consuming much of the day’s food late at night leaves too little room for balanced meals earlier, especially when evening intake tends to be higher in energy and lower in fiber or other nutrients.
Future research will need to separate the effects of meal timing, sleep timing, diet quality and social schedules.
The study was supported by a Health Research Council of New Zealand grant. One author also held a doctoral scholarship funded by Massey University from 2019 to 2022.