You probably look at clocks throughout the day to check the time—whether it’s the alarm clock by your bed, the watch on your wrist, or the clock on your smartphone. But you’re also looking at a clock when you look in the mirror.

Scientists have learned that your body is essentially a clock—that there are internal, 24-hour timetables that determine when every gene in your genome, every reaction-sparking enzyme, every hormone, and every chemical in your brain turns on and off.

Circadian rhythms

These timetables are called circadian rhythms. They are the master programs that instruct what time of day or night the body carries out its daily tasks, like getting energy from food, defending itself against threats like viruses, and repairing and rejuvenating cells.

Scientists used to think that the body’s circadian rhythms were controlled externally, by the day-night cycle of light and dark. But that’s only half the story. Now, it’s understood that the body’s circadian rhythms are also regulated internally, by hundreds of cell-specific and organ-specific “clock genes” that optimize the body’s ability to carry out its many tasks and functions.

When these gene-regulated clocks are working well—when you’re living in sync with the natural rhythms of sleeping, eating, and being active—you’re likely to be healthy. But when your circadian clocks are disrupted, you’re more likely to feel lousy.

Out of sync

Short-term—like when you have jet lag—you can suffer from insomnia, fatigue, headaches, irritability and moodiness, indigestion, constipation, susceptibility to infection, and muscle aches and pains. Long-term—because of an erratic sleep schedule, eating late at night, or being sedentary—you’re at greater risk for nearly every chronic disease and condition, including heart disease, diabetes, obesity, fatty liver disease, gut diseases, immune problems, depression and anxiety, Alzheimer’s disease, and various cancers.

Bottom line: Maintaining or restoring circadian rhythms is arguably the most important factor in maintaining or restoring good health.

The rhythmic lifestyle

There are simple guidelines to fix your circadian clock, nurture your circadian rhythms, and prevent, manage, and even reverse disease.

Guideline #1: Set a bedtime and be consistent with it. Your day begins the night before, with how you slept. The best way to ensure sufficient, restful sleep is to have a regular (in other words, rhythmic) bedtime. Whatever time you choose—with 10 p.m. to 11 p.m. being the physiological sweet spot—try to be in bed for at least eight consecutive hours, in order to get 6.5 to 7.5 hours of sleep, which science shows is just right for rest and rejuvenation.

Guideline #2: Reduce blue light at night. Another crucial circadian factor in getting a good night’s sleep is to reduce your exposure to light at night in the hours before bedtime. You particularly want to reduce blue light at night. During the daytime, this wavelength of light boosts attention, reaction time, and mood. But at night, blue light reduces production of melatonin, the sleep-inducing hormone. The main source of blue light at night is screens: your smartphone, tablet, laptop, computer, gaming console, and TV.

The best strategy is to minimize or eliminate all use of screens two hours before your set bedtime. But for most of us, that’s not realistic. An easy fix is to wear blue-light filtering glasses at night. Put them on right after dinner. Within 10 to 15 minutes, you’ll notice your eyes relax, you’ll experience less eyestrain, and your brain will adjust to the color.

You can also use an app on your smartphone that reduces blue light two hours before your preset bedtime, such as the Nightshift feature on the iPhone. Most new tablets and laptops also come with a function to reduce the brightness of the screen at a set time. After dinner, use the dimmer switch for LED lights in your home, which emit blue light.

Guideline #3: After waking up, wait one hour to eat breakfast. After you wake up, wait at least one hour before eating any calorie-containing food. (Two hours is even better.) Within one hour of waking up, your level of the stress hormone cortisol reaches its peak, and your level of sleep-inducing melatonin hasn’t returned to baseline. High cortisol and residual melatonin interfere with the regulation of glucose (blood sugar), and a steady level of glucose is crucial to maintaining your metabolic health. Favor noncaloric drinks during this time, like warm water, lemon water, or black coffee. (Noncaloric sweeteners are also okay.) People with heartburn should stick to plain water.

Guideline #4: Eat all your daily calories within a 12-hour time period. This approach is called time-restricted eating—and it works wonders to harmonize your circadian rhythms. (You also can choose a smaller window of eating opportunity, like eight, nine, 10, or 11 hours.) Most of us eat for 15 hours or more every day, but a shorter “feeding period” provides the digestive system the right amount of time to process food, uninterrupted by a new influx of nutrients. It also provides the gastrointestinal tract enough time to repair and rejuvenate, supporting the healthy bacteria that are the foundation of good digestion.

Research in animals and people shows that time-restricted eating stabilizes blood pressure, blood sugar, and blood fats, and helps prevent metabolic diseases like obesity, diabetes, heart disease, and stroke. To implement time-restricted eating, first set a breakfast time and stick with it. If breakfast begins at 8 a.m., dinner must end by 8 p.m. An earlier breakfast is better—because if you start early, you end early, before melatonin levels begin to rise, leading you towards sleep. Plus, once your body recognizes that no more food is coming in, it transitions to repair-and-rejuvenation mode. Alcohol is considered food, too. If you like a drink or two in the evening, have it either before dinner or with dinner.

Guideline #5: Don’t eat late at night. It’s the worst choice you can make. It disrupts the digestive clock, waking up the body when it is meant to be slowing down. For example, eating raises your core body temperature, but deep sleep requires a drop in core body temperature. And because your gut isn’t prepared for food, the food moves more slowly through your system, perhaps causing acid reflux (heartburn).

Guideline #6: Spend at least 30 minutes outdoors every day. This doesn’t mean only on sunny days. Spending time outdoors even on a cloudy day provides 4,000 to 5,000 lux, the standard measure of brightness. If you can’t get outside, spend time next to a large window, perhaps when you’re eating breakfast or lunch.

Guideline #7: If you can, combine light exposure with exercise. Exercise is healthy no matter when and where you do it. It builds muscle mass and strength, builds bone, burns fat, and tones your metabolism, strengthens your heart, and enlivens your brain. But exercising outdoors helps you meet two guidelines simultaneously.

Plus, most people who exercise outdoors walk—and walking is the most common physical activity for people who maintain a habit of physical activity.

If you’re prediabetic or diabetic, walking in the late afternoon or early evening is best to regulate blood sugar.

SCN: The Master Clock

The suprachiasmatic nucleus (SCN) is the body’s master clock. It receives information about light from the outside world through the retina and resets all the other clocks, including organ-specific clocks like the heart clock, the liver clock, and the gut clock. External light, the SCN, the organ clocks, and the genetic clocks form the three major rhythms that are key to health: sleep, nutrition, and activity.

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