Mindfulness, overall sleep quality and alcohol consumption could explain why people who stay up late have a higher risk of depression, according to a new study published in the open access journal PLOS One by Simon Evans of the University of Surrey, UK, and colleagues. Previous research has shown that night owls, who stay up late, so-called “evening chronotypes”, show more symptoms of depression than early risers, so-called “morning chronotypes”. In the new study, Evans and his colleagues collected data from 546 university students using an online questionnaire.

Night Owls are More Prone to Depression

The data included self-reported information about the students’ sleep patterns, mindfulness, rumination, alcohol consumption, and levels of depression and anxiety. The study confirmed that people with an evening chronotype have a significantly higher risk of depression and that this association can be explained by differences in mindfulness, sleep quality, and alcohol consumption. On average, evening chronotypes had poorer sleep quality, higher alcohol consumption, and lower mindfulness than morning chronotypes. The study was limited in its ability to demonstrate cause and effect due to its cross-sectional design, which relied on data collected at just one point in time.

Furthermore, the results may not be applicable to age groups other than the university students included in the study. With these caveats in mind, the authors conclude that interventions targeting mindfulness, sleep and alcohol consumption could have the potential to reduce the risk of depression, especially in young adults. The authors add, “Since poor mental health is common among young adults, these study results are especially important – many young adults tend to stay up late, and the results show how interventions could be implemented to lower their risk of depression.”

Late Chronotypes and Cardiovascular Diseases

However, night owls also have an increased risk of other health issues. According to a study by the University of Gothenburg in Sweden, atherosclerosis occurs almost twice as often in night owls as in early risers. Circadian function appears to be especially important in the early stages of cardiovascular disease. In atherosclerosis, fatty deposits build up on the inside of the arteries, impeding blood flow. The disease develops over a very long period and goes unnoticed until it causes blood clots that result in angina, heart attack or stroke. Previous research has indicated that people with late-night habits may be at increased risk for cardiovascular disease, but this is the first study to show how the circadian rhythm specifically affects arterial calcification.

The study, published in the journal “Sleep Medicine”, involved 771 men and women between the ages of 50 and 64 who were all part of the larger SCAPIS population study. The degree of hardening of the arteries in the coronary arteries was examined using computed tomography. The participants indicated their so-called chronotype on a five-point scale: extreme morning type, moderate morning type, intermediate type, moderate evening type or extreme evening type. Of the 771 participants, 144 were identified as extreme morning types and 128 as extreme evening types. In the group that was most awake in the morning, 22.2% had severe atherosclerosis – the lowest percentage of all five chronotypes. The group of extreme evening types had the highest prevalence of severe coronary artery calcification, at 40.6%. The statistical analysis took into account a number of other factors that can influence the risk of atherosclerosis, including blood pressure, blood lipids, weight, physical activity, stress levels, sleep and smoking. In addition to the already known factors, the individual circadian rhythm also appears to be an important risk factor for atherosclerosis.

Impaired Metabolism in Night Owls

Are night owls also more prone to metabolic disorders such as type 2 diabetes? Researchers at Rutgers University in New Jersey found that those who stay up later are less able to use fat for energy, meaning fats can accumulate in the body and increase the risk of type 2 diabetes. The differences in metabolism are related to how well each group can use insulin to promote glucose uptake by cells for storage and energy. People who are morning larks (i.e., individuals who prefer to be active in the morning) are more likely to use fat for energy and are more active and aerobic during the day than night owls. Night owls (i.e., people who prefer to be active late in the day or at night) use less fat as an energy source when they rest or exercise.

The researchers divided the participants (n=51) into two groups (early and late) based on their “chronotype” – our natural tendency to be active and sleep at different times. They used advanced imaging techniques to determine body mass and composition, insulin sensitivity, and breath samples to measure fat and carbohydrate metabolism. The participants were monitored for a week to assess their activity patterns throughout the day. They were on a calorie- and nutrient-controlled diet and had to fast overnight to minimize the effects of diet on the results. To examine their preferred energy source, they were tested at rest before undergoing two 15-minute bouts of exercise: one moderate and one high-intensity session on a treadmill. Aerobic fitness was tested by a step challenge in which the incline was increased by 2.5% every two minutes until the participant was exhausted.

Researchers found that early risers use more fat for energy, both at rest and during exercise, than night owls. Early risers were also more insulin sensitive. Night owls, on the other hand, are insulin resistant, meaning their bodies require more insulin to lower blood sugar levels and their bodies favor carbohydrates over fats for energy. The impaired ability of this group to respond to insulin to promote the utilization of fuels can be detrimental, as it suggests a higher risk of type 2 diabetes and/or heart disease. The cause of this shift in metabolic preference between early birds and night owls is still unknown and requires further investigation.