What are circadian rhythms?
Humans and many other organisms operate on a 24-hour clock. Certain physical, emotional, and behavioral factors change in more or less predictable ways over a 24-hour cycle. Various organs in humans have their own circadian rhythms. Light and dark exert major influence on circadian rhythms. Other factors, such eating, stress, physical activity, social interactions, and temperature affect circadian rhythms, which affect aspects of our daily life including sleep, body temperature, and the release of hormones. Departures from circadian rhythms can affect the risk of type 2 diabetes and other chronic diseases.
Chrononutrition
Chrononutrition refers to meal timing, its interactions with circadian rhythms, and the resulting effects on health. A recent review of the links between chrononutrition and diabetes identified three key messages: 1) meal timing, in addition to meal content, affects type 2 diabetes, 2) a carbohydrate-rich meal at night increases post-meal blood sugar more than the same meal consumed in the morning, and 3) diabetics should be encouraged to consume meals early, rather than late, in the day. My recent decision to stop eating at 6:30 pm after dinner (rather than continue snacking until bedtime) should help keep my blood sugar in a healthy range and reduce my chances of developing type 2 diabetes.
The timing of food intake seems to influence metabolic variables, such as insulin secretion and blood glucose concentration. Yet studies of intermittent fasting and time-restricted eating are controversial, with some studies showing significant effects of metabolic factors and other studies showing no effects.
Morning or evening chronotype
Chronotype refers to a person’s preference for morning or evening activity. Researchers in Boston used data from 63,676 women with an average age of 54 years at baseline in the Nurses’ Health Study II to determine if the evening chronotype predicted increased risk of developing type 2 diabetes during follow-up. Chronotype was assessed with the answer to the following: One hears about morning and evening types of people. Which one of these types do you consider yourself to be? Answers were classified as either definite morning, or intermediate, or definite evening. The study collected data on 6 lifestyle factors (diet, alcohol use, body mass index, physical activity, smoking status, sleep duration). Participants were awarded one point for each healthy lifestyle factor they achieved with an overall score (ranging from 0-6) of 3 or less denoting an unhealthy lifestyle.
The presence of 5 of the 6 unhealthy lifestyle factors (excluding alcohol use) predicted significantly higher odds of participants having a definite evening chronotype compared to a definite morning chronotype after adjusting for confounding factors. In addition, compared to the definite morning chronotype and after adjusting for sociodemographic factors, both the intermediate and the definite evening chronotype predicted a significant 72 percent higher risk of developing type 2 diabetes during an average follow-up of 4.7 years. Further adjustment for the 6 lifestyle factors substantially reduced the risk to 21 percent higher but it remained statistically significant. Thus, the definite evening chronotype, along with unhealthy lifestyle factors, predicted a much higher risk of developing type 2 diabetes compared to the definite morning chronotype.
Chrononutrition and cardiometabolic health
The timing and amount of food consumption relative to daily circadian rhythms appears to affect several measures of health. A recent review arrived at 5 tentative conclusions. 1) Skipping breakfast may increase coronary heart disease, type 2 diabetes, hypertension, and obesity risk factors. 2) Eating breakfast earlier rather than later may reduce odds of the metabolic syndrome. 3) Earlier timing of lunch may lead to better blood sugar control and lower insulin resistance. 4) Later timing of dinner and greater caloric intake at dinner may increase risk of cardiovascular disease and the metabolic syndrome. 5) Increased day-to-day variability of the timing of breakfast and dinner may lead to higher body mass index, blood sugar, and inflammation.
Based on these findings, here’s my approach to healthy meal timing and amount. Eat a substantial breakfast at 7 am after arising, eat a more substantial lunch (the biggest meal of the day) at noon, and eat a modest dinner (the smallest meal of the day) at 5:30 pm. No food after dinner until breakfast. Alas, I’m still tempted to snack after dinner.
Chronotype, sleep, and cancer
A 2025 systematic review included 22 studies that related chronotype, sleep timing, sleep regularity, and cancer. Eighteen of the studies investigated chronotype. Six of the studies reported elevated cancer risk for evening compared to morning chronotype. Yet, 10 studies reported no significant associations. The review found “evidence linking late chronotype, later sleep midpoint, increased social jet lag or weekend catch-up sleep to risk of cancer.” Large differences among studies with respect to sleep measures, terminology, and cancer sites clouded interpretation of study results. The review concluded with the following sentence: “Current evidence linking sleep timing, sleep regularity, and chronotype with cancer risk remains inconclusive.”
Circadian health, cardiometabolic health, and disease risk
In 2025, the American Heart Association issued a Scientific Statement regarding the role of circadian health and cardiometabolic health and disease risk. Circadian health refers to the optimal function, rhythmicity, and alignment of central and peripheral circadian clocks in the body. Circadian rhythms operate on approximately 24-hour cycles and regulate physiological and behavioral processes that synchronize to environmental cues, such as light and food intake. Bodily processes that operate via circadian rhythms include blood pressure, heart rate, body temperature, and blood vessel function, among others. Disruption (misalignment) of circadian rhythms seems to foster adverse health outcomes such as weight gain, type 2 diabetes, hypertension, and cardiovascular disease.
Causes of circadian disruption include shift work, irregular sleep timing, social jet lag (changes in behaviors from weekday to weekend), insufficient early morning light, bright nighttime light, and mistimed eating and physical activity. Individual humans can have different chronotypes, approximated as morning or evening persons. Studies suggest that improved circadian health can arise from improved sleep regularity (including week days and weekends), avoiding bright light (especially in green and blue wavelengths) at night, experiencing bright morning light, eating larger meals earlier in the day, not eating following dinner until breakfast, and aligning physical activity with one’s chronotype. While there seems to be little doubt that circadian health promotes cardiometabolic health, the research base needs to expand substantially to support specific recommendations to achieve optimal circadian health.
What to do
Maintaining regular daily schedules of eating and sleeping might help reduce your risks of certain chronic diseases, including cardiovascular disease and type 2 diabetes.
