The sleep habit that quietly raises your risk of heart disease | Dr Kristen Knutson

What It Covers

Dr Kristen Knutson, chair of the American Heart Association's scientific statement on circadian health, explains why sleep timing, regularity, and circadian alignment matter as much as duration for cardiometabolic risk. The episode covers light exposure, meal timing, exercise scheduling, melatonin use, and chronotype differences as tools for reducing disease risk.

Key Questions Answered

• •Circadian Regularity: Irregular sleep timing — going to bed at 10PM one night and 1AM the next — disrupts the body's internal clocks even without travel across time zones. This "social jet lag" impairs blood pressure dipping at night, a measurable cardiovascular risk marker. Aim for a consistent bedtime and wake time within roughly the same window every night, including weekends, to maintain synchronization.
• •Meal Timing and Weight: Eating at the wrong biological time increases fat storage independent of calorie intake. A Spanish weight-loss study found women eating lunch earlier lost more weight than those eating later, despite no measurable difference in food quantity or sleep. The practical rule: stop eating two to three hours before bed, and begin eating relatively soon after waking, ideally within thirty minutes of your habitual wake time.
• •Morning Light as the Primary Clock Signal: Morning light exposure is the strongest circadian synchronizer available, resetting the central brain clock daily. Artificial indoor light is sufficient if bright enough to see clearly. Evening light, particularly blue-green wavelengths from screens and overhead fixtures, delays the clock. Blue-light-blocking glasses or switching to red-spectrum lighting after sunset reduces this disruption without requiring complete darkness.
• •Melatonin Is a Timing Signal, Not a Sleep Aid: Melatonin naturally rises one to two hours before habitual bedtime and signals darkness to the brain rather than inducing sleep directly. Taking it immediately before bed, as most people do, misses this window. Supplementation is most useful for jet lag or genuine circadian misalignment. It also raises insulin resistance temporarily, so eating after taking it is inadvisable, particularly for those with metabolic risk.
• •Chronotype Shapes All Timing Recommendations: Roughly 9–10% of adults over 37 are evening types, with higher prevalence in younger populations. Chronotype is approximately 50% genetic. Night owls forced onto morning-person schedules experience chronic circadian disruption that drives cardiometabolic risk — not the chronotype itself. Light therapy to advance a delayed clock must be introduced gradually, starting at the person's current habitual wake time, not an arbitrarily early hour.
• •Sleep Is Multidimensional Beyond Eight Hours: Seven to seven-and-a-half hours is associated with the lowest disease burden, but duration alone is insufficient. REM sleep, concentrated in the second half of the night, supports memory and brain maintenance and rebounds after short-term restriction. Waking overnight is a normal part of sleep cycling. Anxiety about waking — including fixating on a low wearable sleep score — can itself perpetuate insomnia through hyperarousal.