Deep within the brain lives a collection of neurons called the suprachiasmatic nucleus, or SCN for short. The SCN is like a clock, helping every system, organ and cell in your body run on an optimal schedule. This master clock takes its cues from the 24-hour cycle of the sun and moon. Over millions of years, this constant cycling of light and dark has established a rhythm designed to optimize sleep, appetite, metabolism, mood and more.
The SCN isn’t the only clock in town, though. There are similar clocks in your heart, gut, nervous system, muscles and more, constantly turning to the SCN for guidance. The master clock tells organs when to churn out hormones or kick-start crucial behavioral chains at the right time of day or night.
Example: As the sun begins to rise, its bluish early-morning rays travel through your eyes (even when they’re closed) and trigger the SCN to rev things up. The pineal gland in the brain slows production of the sleep hormone melatonin…the adrenal glands churn out energizing cortisol and epinephrine…and your body temperature rises after dropping a degree or two overnight.
These sorts of 24-hour cycles, during which the brain and body undergo critical changes in biology, are known as the circadian rhythm.
It’s about More Than Sleep
The circadian rhythm often is associated with sleep, weight loss and metabolic health. It is the reason that intermittent fasting and other forms of time-restricted eating are successful—they involve eating when your organs are prepared to process nutrients, fats and sugars…and not eating when your organs are programmed to rest and rejuvenate. Eating at times that complement your circadian rhythm gives your body clocks what they need to thrive, resulting in better blood sugar regulation, enhanced weight maintenance and more.
Here are four more ways to tap into your natural rhythm…
Best time to build strength: Afternoon through early evening. Muscles live by their own circadian rhythm and are programmed for peak strength between 4 pm and 8 pm. This helps explain why more strength-based world records are broken during evening competitions…why tennis serves tend to be faster later in the day…and why swim strokes are more powerful. These sorts of moves rely heavily on fast-twitch muscle fibers, which are more responsive to circadian rhythms than slow-twitch muscle fibers. Slow-twitch muscle fibers rely on aerobic respiration, and so are used for long, endurance sports such as marathons. Generally speaking, a regular 6 pm strength-training routine will lead to bigger gains in muscle mass versus the same routine performed in the mornings. The new words “chrono-exercise” and “chrono-activity” refer to the practice of adjusting your workout routine to mesh with your body clocks.
This doesn’t mean you should skip morning workouts if that’s the only time of day you can fit them in. But if you’re looking for optimal results, save your push-ups, squats and lunges for later in the day. Reminder: Stop exercising at least three hours before bed—otherwise, your heart rate and body temperature may remain too elevated for sleep.
Bonus: If you have type 2 diabetes, afternoon strength workouts may be even more advantageous. Muscles use glucose, so any gains in strength benefit blood sugar control. If afternoon workouts help you gain even a little more muscle, that bodes well for blood-sugar management.
Best time for cardio: Between 8 am and 11 am (for women only). In a study in European Journal of Preventive Cardiology, researchers from the Netherlands followed nearly 87,000 adults between the ages of 42 and 78 for six years. They found that women who exercised mainly in the morning had the lowest risk for heart attack and stroke. Compared with women who fit in most of their physical activity later in the day, early-morning exercisers (between 8 am and 9 am) and late-morning exercisers (between 10 am and 11 am) had a 22% to 24% lower risk for heart disease, respectively.
Morning cardio’s benefit likely stems from several sources, one of which involves the sympathetic nervous system —the “fight-or-flight” branch of the autonomic nervous system responsible for all the thinking and doing that happen during the day and that kicks into high gear during moments of stress. Your sympathetic nervous system’s circadian rhythm maximizes its power during morning hours, so your run at 8 am, swim at 9 am or spin class at 10 am revs the engine at the optimal time. Cardio exercise later in the afternoon, when the heart has been go-go-going for hours and hours, might cause extra strain that hampers its functioning. Note: These study results were not as pronounced for men, though the researchers aren’t sure why.
Best time to nap: Between 1 pm and 3 pm. Prime napping time strikes six to eight hours after you’ve woken up. This is one of the two time periods during which circadian rhythms cause humans to be the sleepiest (the other is just after midnight). A 90-minute nap initiated between 1 pm and 3 pm is what neuroscientists call the “perfect nap”—it’s long enough to let your brain move through an entire cycle of sleep (including light sleep, slow-wave sleep and REM sleep). You’ll also spend similar amounts of time in each sleep stage as you would during nocturnal sleep, but in a fraction of the time. A 90-minute snooze at this time might yield benefits similar to a seven-hour sleep at night…and you will wake up feeling refreshed because you’ll have cycled through all the stages of sleep. Avoid naps between 30 and 60 minutes long, which put you into a deep sleep that can be hard to wake up from.
Don’t have 90 minutes? Try a 10- to 20-minute nap within the same early-afternoon time frame. That’s enough sleep to boost energy and alertness but you won’t nap long enough to reach the deep stages of sleep, so you’ll wake up ready to hit the ground running.
Best time to take heart medication: At bedtime. Chronotherapy or chronomedicine is an approach to disease management that incorporates circadian rhythms into treatment of illnesses and chronic health conditions such as heart disease, arthritis, allergies, migraines and cancer. Many of these conditions observe their own circadian rhythm, with symptoms intensifying at certain times of day or night…or the disease itself being more receptive to treatment. Case in point: With the cardiovascular system, chief processes such as heart rate, blood pressure and blood clotting undergo changes in the morning that increase risk for heart attack and stroke.
Recent finding: You are 50% more likely to have a stroke between 6 am and noon versus the rest of the day, per a 2023 meta-analysis by researchers from Rush Medical College and Rush–Presbyterian–St Luke’s Medical Center. You’re two to three times more likely to experience a heart attack in the morning than at night, and morning heart attacks tend to be more severe. Likely reasons: Circadian rhythms increase activity of clotting factors in the blood during the morning hours, and strokes and heart attacks occur when blood clots become dangerously lodged in the brain or heart.
For decades, doctors have prescribed short-acting statins (cholesterol-lowering drugs) such as simvastatin (Zocor) and lovastatin (Mevacor) to be taken at night because the cholesterol-producing liver enzyme HMG-CoA reductase is active in the evening so the medication has a greater impact. Several studies also indicate that antihypertensive medication (for high blood pressure) is more effective when taken at night, though a large British study suggested no difference between morning and evening administration. But even if evening antihypertensive dosing isn’t more effective than morning dosing, it’s still just as effective, plus it reduces the intensity of some negative side effects such as lightheadedness or leg swelling.