PDF Summary:The Circadian Code, by Satchin Panda

How often do you feel truly rested and energized throughout the day? If your answer is “sometimes” or “rarely,” it might be because your circadian rhythm—your body’s natural 24-hour schedule—has been disrupted. In The Circadian Code, Satchin Panda explains that modern habits, like irregular sleep schedules, late-night eating, and excessive exposure to artificial light, can throw off your circadian rhythm and cause various health problems. He suggests simple adjustments you can make to your daily habits to live in sync with your circadian rhythm and be your healthiest and most energized self.

In this guide, we’ll discuss how your circadian rhythm works, what happens when it’s disrupted, and ways you can sync your lifestyle habits to your circadian rhythm. Along the way, we’ll cite scientific research that supports or contradicts Panda’s ideas and bring in other experts’ advice on living in sync with your circadian rhythm.

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(Shortform note: Because sleep deprivation affects health and performance, it has a huge impact on the economy. One study found that sleep deprivation costs the US economy up to $411 billion annually (equivalent to 2.28% of the country's GDP) due to lost productivity. Additionally, sleep deprivation not only leads to the health issues that Panda describes—the study shows that people who sleep less than six hours a night have a 13% higher risk of mortality compared to those who sleep seven to nine hours.)

Panda explains that high-quality sleep acts as a cleaning process for your brain, removing waste and toxins that could impair your brain function. Consistent sleep patterns that follow your natural circadian rhythm help this cleaning process work efficiently.

(Shortform note: In Sleep Smarter, Shawn Stevenson adds that the brain’s waste removal system is 10 times more active when you’re asleep than when you’re awake. During sleep, your brain cells shrink by about 60%, which allows the clean-up process to work more efficiently. If you don’t get quality sleep, the waste products stay in your brain and can lead to cognitive diseases like Alzheimer’s.)

Panda recommends three sleep habits: going to bed early, having a regular sleep schedule, and creating a good sleep environment.

Habit #1: Go to Bed Early

First, Panda recommends you go to bed early so that you get the amount of sleep you need—at least seven hours a night for adults. Going to bed at a late hour makes it difficult for you to get this recommended amount of sleep. Now, this advice might seem challenging for people who consider themselves "night owls." However, Panda argues that the idea that some people are naturally night owls and others are early birds based on genetics is a myth. Rather, people who consider themselves night owls simply have habits that keep them awake longer—such as snacking, consuming caffeine late, or using bright lights in the evening.

(Shortform note: In Why We Sleep, Walker agrees that you should get at least seven hours of sleep, adding that only 1% of the population can get only six hours of sleep without showing any impairment. However, he disagrees with the idea that being a night owl is simply due to poor sleep habits. He argues that chronotypes—whether you’re a “morning person” or “night owl”—are real and are genetically determined. He explains that humans likely evolved to have different chronotypes to reduce the vulnerability of the population, with night owls watching out for threats in the evening and morning people keeping a lookout in the morning.)

In addition to giving your body adequate time to rest and repair, sleeping early has another benefit: It helps curb late-night snacking. Panda writes that hunger—especially for unhealthy, high-calorie foods—increases when you stay up late. When your body isn't sure how long it’ll stay awake, it “thinks” it needs more energy to support your activities and produces more of the hormone that makes you feel hungry (ghrelin) and less of the hormone that makes you feel full (leptin). This is why you crave midnight snacks, even when your body doesn’t need to eat.

Why You Crave Junk Food at Night

Research supports the idea that you crave unhealthier foods at night. Specifically, your preference for fatty, sugary, and salty foods peaks around midnight and stays high for a few hours afterward. But why do you reach for a bag of chips and not a cup of yogurt to satisfy your late-night cravings?

There are two main reasons. First, your body's hormone levels change throughout the day, affecting not only your hunger but also your food preferences. As night approaches, your cortisol levels naturally drop, signaling your body that it's time to sleep. Cortisol is a hormone that tells your liver to release sugar into your blood. If you stay awake when cortisol is low, your body experiences a drop in blood sugar. To make up for this, your body makes you crave high-calorie foods to give you energy.

Second, feeling tired at the end of the day can also increase your desire for unhealthy foods. Studies show that when you stay awake late into the night and your body produces ghrelin as Panda mentions, your brain’s reward centers respond more strongly to junk food than to healthy food. This makes it harder for you to make good decisions about what to eat. So while those midnight fast food runs may feel satisfying in the moment, they're really just your body's way of saying it needs rest.

Habit #2: Have a Regular Sleep Schedule

A second habit Panda recommends you adopt for quality sleep is keeping a consistent sleep schedule. When your sleep schedule is consistent, your body learns when to produce the sleep hormone melatonin, making it easier to fall asleep and wake up naturally.

(Shortform note: A consistent sleep schedule may be even more important for teenagers. During adolescence, the body starts producing melatonin about three hours later in the 24-hour sleep cycle compared to children or adults. This shift means teenagers naturally feel sleepy later at night and have a harder time waking up early in the morning, which explains why many teens struggle with early school start times. So, by setting a consistent sleep schedule, teenagers may find it easier to go to bed earlier and wake up in time for school.)

To keep a regular sleep schedule, go to bed and wake up at the same time each day, including on weekends. Changing your sleep schedule on the weekends throws off your circadian rhythm, which can lead to poor sleep quality, daytime fatigue, and mood changes. Panda adds that if you’re sleeping in much later on the weekends, chances are you’re probably not getting enough sleep during the week.

(Shortform note: Many experts agree with Panda on the importance of maintaining a regular sleep schedule, explaining that shifting your sleep schedule significantly on the weekends can have a similar effect on your body as flying across time zones every weekend. Ideally, aim to get sufficient sleep on a consistent schedule as much as possible. However, if you’re sleep-deprived, some experts argue that your priority should be increasing your total sleep time, even if it means varying your schedule. In these cases, catching up on sleep during weekends can be beneficial. However, it's still best to work toward a consistent sleep schedule in the long run.)

Habit #3: Create a Good Sleep Environment

Panda recommends you create a cool, dark, and quiet sleep environment to fall asleep more easily and stay asleep throughout the night. Your body temperature should decrease about one degree Fahrenheit to get the best sleep. This is also why you should avoid eating before you sleep: If you eat too close to bedtime, your body temperature will rise and keep you from reaching deep sleep. Finish eating two to four hours before you sleep to give your body time to cool off.

(Shortform note: Your body temperature rises when you eat because your metabolism generates heat as it breaks down food. Spicy, sugary, and fatty foods can have a bigger effect on your body temperature. Spicy foods contain compounds that trick your body into thinking it's hot, while sugary and fatty foods require more energy to digest, generating more heat in the process. By allowing a few hours between your last meal and bedtime, you give your body time to complete much of the digestion process and allow your temperature to naturally decrease.)

Panda also suggests it’s best to sleep in a dark room because any light, particularly blue light (from electronic devices and home lighting), can confuse your brain into thinking it’s still daytime. You can further improve your sleep quality by using white noise or fixing problems such as snoring or sleep apnea.

(Shortform note: In Why We Sleep, Walker provides more specific tips that can help you create the ideal sleep environment that Panda recommends. First, he recommends you lower your bedroom temperature to 65° F for the best sleep. If you’re struggling to stay cool, consider splashing water on your skin or sticking your hands and feet outside your blanket. Walker also recommends you create a dark room, arguing that even a bedside lamp can prevent you from feeling sleepy. He suggests dimming lights in rooms where you spend your evenings and using blackout curtains to maintain complete darkness throughout the night.)

If you still struggle to fall asleep after trying these strategies, Panda recommends trying natural supplements like melatonin before turning to sleep medications. He writes that sleep medications haven't been thoroughly examined for use beyond six months and aren’t intended to be a long-term solution.

Alternative Tips and Methods for Falling Asleep

Like Panda, Walker also advises, in Why We Sleep, against using sleep medications, writing that they can create a vicious cycle of dependence that's hard to break. Sleeping pills don't induce natural, restorative sleep because they put you into a state that lacks the deepest brain waves, leading to daytime drowsiness. This daytime drowsiness leads you to use more caffeine and take naps, which then makes it harder to sleep at night without pills.

As your body builds a tolerance, you need higher doses, which increases the risks of negative side effects like car accidents, infections, and possibly even cancer. Over time, you may develop a tolerance to the pills, and stopping them can lead to withdrawal insomnia, further perpetuating the cycle.

Instead of turning to medication, Walker suggests you consider cognitive behavioral therapy for insomnia (CBT-I). For this type of therapy, you’ll work with a therapist to change your thoughts and behaviors about sleep. Therapy can include practices like restricting how much time you spend in bed to build up your desire to sleep, removing clocks from your bedroom to reduce anxiety, and only going to bed when you're truly sleepy. Walker says that CBT-I has been shown to be more effective in the long run than sleeping pills. It helps you rebuild confidence in your ability to sleep naturally by addressing both the psychological and behavioral aspects of insomnia.

Although melatonin supplements are an option, in Sleep Smarter, Shawn Stevenson says you can fall asleep easier by going to bed before 10:00 PM, which is when your body produces the most melatonin. If you’re awake after 10:00 PM, you’ll get a burst of energy that makes it harder to sleep. While Stevenson doesn’t explicitly mention melatonin supplements, he does offer other diet and herbal supplement suggestions that can help you fall asleep easier. He suggests you get enough tryptophan, vitamin C, and omega-3s in your diet, as these three nutrients promote deeper, more restful sleep. You can also consider taking kava kava, chamomile, or valerian if you need extra help falling asleep at night.

Sync Your Light Exposure

In addition to syncing your eating and sleeping, you should also align your light exposure with your circadian rhythm. Panda explains that light serves as an important cue for your circadian rhythm, helping it stay in sync with the outside world.

(Shortform note: Environmental cues that help synchronize your circadian rhythm with the outside world are known as zeitgebers (German for “time givers”). While light exposure is the most powerful zeitgeber, other factors such as meal timing, physical activity, social interactions (like going to school or work), and even temperature changes all play a role in synchronizing your circadian rhythm.)

Panda explains that your body senses light through a protein in your eyes called melanopsin, which is different from the rod and cone cells that allow you to see. Even people who are blind or visually impaired feel the effects of light due to melanopsin. When light activates these proteins, your body recognizes that it’s daytime and prepares for waking activities like working and eating. When there’s no light, melanopsin remains unactivated, so your body concludes that it’s nighttime and prepares for sleep.

(Shortform note: Melanopsin is found in special cells called intrinsically photosensitive retinal ganglion cells (ipRGCs). These cells are responsible for non-image visual functions, which means they respond to light but don't contribute to what you actually see. IpRGCs start working much earlier in life than rods and cones. In fact, they're the only light-sensing cells that function in newborn mammals for about the first 10 days of life. However, although melanopsin may allow people who are blind or visually impaired to sense light, many of these individuals still suffer from health problems relating to circadian rhythm disruption, such as mood and sleep disorders.)

If you’re surrounded by artificial light (like most people), you may be activating melanopsin at the wrong times and disrupting your circadian rhythm. Bright lights at night trick your brain into thinking it’s daytime, making it harder to fall asleep and get quality rest. Additionally, if you stay indoors for most of the day, you aren’t getting enough light to trigger your melanopsin, leaving you feeling unenergized. Over time, this improper light exposure can lead to mental health issues like depression and anxiety.

(Shortform note: Not getting enough natural sunlight during the day not only affects your energy levels and mental health, but it can also lead to other health issues. In The Nature Fix, Florence Williams explains that when you don’t get enough sun exposure, your body can’t produce enough vitamin D, which is essential for strong bones. In fact, this has caused a resurgence of a bone disease called rickets in children living in some areas of the UK. Furthermore, a lack of sunlight may worsen your vision. Your eyes need natural light to develop properly, especially when you're young. Spending most of your time indoors, especially staring at digital screens, can increase your risk of developing myopia or nearsightedness.)

To improve your health, give your body the right amount of light at the right time. Panda provides two tips for syncing your light exposure to your circadian rhythm: avoiding blue light at night and getting plenty of bright light during the day.

Habit #1: Avoid Blue Light at Night

First, Panda suggests you reduce your blue light exposure at night to avoid confusing your circadian rhythm. He explains that your body reacts differently depending on the color of light. Blue light activates melanopsin and makes your brain think that it’s daytime, causing it to reset your internal clock to morning. Red or orange light, however, has less of an impact on melanopsin, allowing your brain to still think that it’s nighttime.

(Shortform note: Some research suggests that our heightened sensitivity to blue light stems from our evolutionary history. When life first emerged in the oceans, blue wavelengths of light penetrated the water more easily than other colors. As a result, organisms evolved photoreceptors that were particularly responsive to blue wavelengths. It’s believed that this adaptation persisted throughout evolutionary history, even as life moved onto land, because the sky's blue color reinforced the importance of blue light as a signal for daytime.)

Exposure to blue light at night suppresses melatonin production and harms your sleep quality. For this reason, Panda recommends you avoid screens before going to bed. Consider wearing blue-light-blocking glasses in the evening and using blue-light filters on your phones and other devices. You can even set up adjustable LED lights in your home that can shift to warmer tones in the evening.

(Shortform note: Consider implementing a digital sunset routine, where you gradually reduce your exposure to screens and artificial light sources in the hours leading up to bedtime. This can include dimming household lights, using candlelight or salt lamps for ambient lighting, and engaging in relaxing, screen-free activities like reading physical books, journaling, or light stretching. Turning Panda's blue light reduction strategies into a routine can help you get better sleep with less effort.)

Habit #2: Get a Lot of Bright Light During the Day

In addition to avoiding blue light at night, Panda encourages you to improve your circadian health by getting an abundance of light during the day. Exposing yourself to bright light as soon as you wake signals to your body that it’s daytime. Let in natural sunlight or take a short walk outside if you can.

Panda writes that being in bright light, especially sunlight, makes a big difference in how you feel and work. More exposure to natural light can make you happier, more alert, and perform better. While indoor lighting (even from the brightest LED bulbs) can't fully replicate the benefits of sunlight, being in a well-lit indoor area can still mimic some of the positive effects of outdoor light exposure.

(Shortform note: Light therapy lamps can be a valuable alternative when natural sunlight isn't available. These lamps simulate natural sunlight and can be especially helpful during darker winter months or for those who work in windowless environments. Some experts recommend you look for one that provides 10,000 lux of light and filters out UV rays to protect your eyes. Position the lamp 16 to 24 inches from your face and use it for about 20 to 30 minutes each morning.)

Sync Your Physical Activity

A fourth and final element of your lifestyle that Panda recommends you pay attention to is your physical activity. Regular exercise boosts your circadian health and overall well-being. Research shows that people who exercise during the day produce more of a hormone that promotes better sleep. Exercise also improves your mood and brain functioning, making it easier for you to relax, be productive, and feel happy.

(Shortform note: In Spark, John Ratey explains that exercise is especially beneficial for brain health because when our human ancestors hunted and gathered, their brains and bodies had to work in concert to survive. Because of this, our brains evolved to function best when paired with physical activity, and this connection remains even though our modern lives don't require the same physical demands. Ratey says that exercise enhances your brain structure and function in several ways: It stimulates your brain to produce proteins that strengthen neural connections and grow new brain cells, acts as a mild stressor that trains your body to handle stress better, and helps balance key neurotransmitters that influence mood, attention, and motivation.)

Panda recommends you exercise for at least 30 minutes a day for five days a week. You don’t have to exercise strenuously, either—any movement that burns calories works. If your schedule is tight, you can break up your physical activity into two or three smaller periods throughout the day and get the same benefits.

(Shortform note: You can divide your daily exercise into even smaller segments throughout the day. Some research suggests that short "exercise snacks" can give you similar benefits to longer workout sessions. Exercise snacks are brief, high-intensity movements lasting about two minutes that you can do throughout your day, such as taking the stairs instead of the elevator or doing a quick set of jumping jacks during a work break. These quick spurts of movement can improve your heart function, increase endurance, and enhance muscle strength.)

Because of your body’s natural rhythms, different types of exercises are better suited to different times of the day. Let’s look at some options you can consider.

Option #1: Morning aerobic exercise. Panda asserts that doing outdoor aerobic activities in the morning, such as walking, running, or swimming, can significantly boost your mood and energy levels. Getting outdoor morning exercise is also an easy way to get enough light exposure to help regulate your circadian rhythm.

(Shortform note: While walking, running, and swimming are good cardio options, Michael Easter recommends rucking as an outdoor exercise that aligns perfectly with our natural strengths. In The Comfort Crisis, Easter explains that rucking involves carrying a heavy backpack while walking long distances, which mimics the activities our ancestors performed regularly. It burns more calories than walking and provides health benefits similar to running but with less impact on your joints. Rucking also strengthens your core and glute muscles, which can help prevent back pain and improve posture. To incorporate rucking into your morning routine, simply add weight to your backpack during your regular walks.)

Option #2: Afternoon strength training. According to Panda, your muscle tone naturally increases in the afternoon, which makes it the best time for strength training. Scheduling more intense workouts like weight lifting during this time can lead to maximum gains with minimal risk of injury. Also, exercising in the late afternoon can reduce your appetite by evening, which can help you consume less at dinner.

(Shortform note: To maximize the benefits of your afternoon strength training, consider the timing of your protein intake. In Bigger Leaner Stronger, Michael Matthews says that consuming 30 to 40 grams of protein within a few hours before or after your workout can significantly boost muscle growth and recovery. Eating protein before exercising gives your body the nutrients it needs to start the muscle-building process. Having protein after training replenishes nutrients that were depleted and helps your body build more muscle than it broke down during the workout. Some dietitians add that eating protein makes you feel fuller than other nutrients like carbs and fats, so doing so can also help you eat less at dinner.)

Option #3: Exercise after dinner if it’s your only option. Panda writes that it’s better to exercise after dinner than to not exercise at all that day. However, avoid intense workouts and stay away from bright lights to avoid interfering with your sleep. For example, you might opt for a gentle yoga routine or a leisurely walk after dinner instead of a high-intensity workout.

(Shortform note: Some research suggests that evening exercise can actually help you fall asleep, as long as you keep the workout light to moderate in intensity (as Panda suggests) and finish at least an hour before bedtime. In addition to yoga and walking, other good evening exercise options include stretching, leisurely swimming or biking, and light to moderate weightlifting. Avoid vigorous activities like high-intensity interval training (HIIT), running, or heavy weightlifting close to bedtime, as these can stimulate your nervous system and raise your heart rate too much, making it harder to fall asleep.)