The Power of When

Discover Your Chronotype--and the Best Time to Eat Lunch, Ask for a Raise, Have Sex, Write a Novel, Take Your Meds, and More


By Michael Breus, PhD

Foreword by Mehmet C. Oz, MD

Read by Michael Breus, PhD

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Learn the best time to do everything — from drink your coffee to have sex or go for a run — according to your body’s chronotype.

Most advice centers on what to do, or how to do it, and ignores the when of success. But exciting new research proves there is a right time to do just about everything, based on our biology and hormones.

As Dr. Michael Breus proves in The Power Of When, working with your body’s inner clock for maximum health, happiness, and productivity is easy, exciting, and fun. The Power Of When presents a groundbreaking program for getting back in sync with your natural rhythm by making minor changes to your daily routine.

After you’ve taken Dr. Breus’s comprehensive Bio-Time Quiz to figure out your chronotype (are you a Bear, Lion, Dolphin or Wolf?), you’ll find out the best time to do over 50 different activities. Featuring a foreword by Mehmet C. Oz, MD, and packed with fascinating facts, fun personality quizzes, and easy-to-follow guidelines, The Power Of When is the ultimate “lifehack” to help you achieve your goals.


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Dr. Breus has been a friend and colleague of mine since my show began. His tireless enthusiasm for learning, and for educating the public and being out in front of cutting-edge information about sleep and sleep disorders, has made him one of the core experts in many of my ventures.

I became interested in the healing power of circadian rhythms when I was in a meeting room with Dr. Breus. We were discussing the future of sleep medicine and why sleep deprivation was the most underappreciated health and wellness problem in America. I wanted to know what the next big thing in sleep would be.

Dr. Breus explained that the circadian system, also known as your biological clock, affects every area of functioning in the body, controlling everything from the multiplication of cancer cells to the integrity of the immune system. It became clear to me that there was a wealth of research on the topic but little exposure to it among the general public. I knew that people needed to be educated about it in a meaningful way, and so I encouraged Dr. Breus to write The Power of When.

The more you understand circadian dyssynchrony—a concept presented in the book in a highly accessible way—the more you will improve your life. For example, the gut has a circadian pacemaker of its own. When your gut is not on its biological clock schedule, hormone disruption causes increased levels of inflammation, inefficient metabolism, even decreased effectiveness of many prescription therapies.

The quiz at the beginning of the book helps you figure out which one of four distinct chronorhythm groups you fall into. Then you get a basic understanding of what a typical day for a Lion, Bear, Wolf, or Dolphin looks like and "when" the best time to do many basic tasks is.

As many of you know, I am a big proponent of regular bowel movements, and I speak about them frequently on my show. One of my favorite chapters in the book explains "when" to have one; it does not get much easier than that! Another chapter that I paid specific attention to was "when" to take medication effectively so that it can improve your life—overnight. And what about physical activity? Dr. Breus has dedicated an entire section of this book to identifying times "when" you will get the most benefit and pleasure from being active.

Circadian science will advance medical testing. Testing can now become more precise through the time-stamping of specimen collection and the comparison of results to time-based norms. Clinicians can get more accurate results. What if your blood was drawn to look at your thyroid levels in the morning versus the evening: Could the results differ to the point of diagnosis? It appears so.

Based on a simple understanding of your biology and scheduling, you will learn "when" you can get the most out of yourself and your significant relationships, in areas like sex, love, planning an event, and talking to your kids. "When" you see improvement in these areas, you can enhance your health and your life in ways you could never have imagined.

Of course, we cannot forget about work, which occupies so much of our time. Knowing and understanding "when" you function best, and "when" others function best, allows you to put your best ideas forward, be most creative, and be open to instruction. This book will allow you to learn "when" you can truly be your best on many levels.

I took the quiz and discovered I am a Lion. I identified well with this chronotype's characteristics, and realized that I had unknowingly created a schedule that worked for me, with many areas right on target. But I decided to change the timing of napping in my life to see if I could make this particular area more efficient. I was amazed at the impact this one simple change had on my health. And so it gives me great pleasure to write this foreword to tell all of you how this book can help you, your family, your career, and your health.

—Mehmet C. Oz, MD


Timing Is Everything

Do you want a simple, straightforward life hack that requires little effort and gets you closer to happiness and success? Of course you do! This might sound like a promise waiting to be broken. It's not.

You've probably already seen a lot of tricks and tips about the "what" and "how" of success.

How to lose weight.

How to please a sexual partner.

What to say to your boss to get a pay increase.

How to raise your kids.

What to eat.

How to work out.

What to think.

How to dream.

"What" and "how" are excellent and necessary questions. But there is another crucial question that must be addressed in order to make fast, dramatic, lasting improvements in the quality of your life across the board.

That question is "when."

"When" is the ultimate life hack.

It's the foundation of success, the key that unlocks a faster, smarter, better, and stronger you.

Knowing "when" enables you to perform "what" and "how" to your maximum potential. If you didn't change a thing about what you do and how you do it, and only made micro-adjustments to when you do it, you'd be healthier, happier, and more productive, starting… right now.

"When" really is that simple, and that powerful.

Just by making small tweaks to your schedule—such as when to have the first cup of coffee, when to answer emails, when to nap—you'll nudge the rhythm of your day back in sync with the rhythm of your biology, and then everything will start to feel easier and flow naturally.

What do I mean by the "rhythm of your biology"?

Contrary to what you might have heard, there is a perfect time to do just about anything. Good timing isn't something you choose, guess, or have to figure out. It's already happening inside you, in your DNA, from the minute you wake up to the minute you fall asleep, and every minute in between. An inner clock embedded inside your brain has been ticking away, keeping perfect time, since you were three months old.

This precisely engineered timekeeper is called your circadian pacemaker, or biological clock. Specifically, it's a group of nerves called the suprachiasmatic nucleus (SCN), in the hypothalamus, right above the pituitary gland.

In the morning, sunlight comes into your eyeballs, travels along the optic nerve, and activates the SCN to begin each day's circadian (Latin for "around a day") rhythm. The SCN is the master clock that controls dozens of other clocks throughout your body. Over the course of the day, your core temperature, blood pressure, cognition, hormonal flow, alertness, energy, digestion, hunger, metabolism, creativity, sociability, and athleticism, and your ability to heal, memorize, and sleep, among many other functions, fluctuate according to and are governed by the commands of your inner clocks. Everything you can do or want to do is controlled by physiological rhythms, whether you realize it or not.

For fifty thousand years, our ancestors organized their daily schedules around their inner clocks. They ate, hunted, gathered, socialized, rose, rested, procreated, and healed on perfect bio-time, or biological time. I'm not saying life was fantastic in prehistoric, biblical, or medieval times, but as a species, we thrived by rising with the sun, spending most of the day outdoors, and sleeping in total darkness. We created civilization and societies and made incredible advances that, ironically and effectively, turned our finely tuned and evolved inner clocks against us.

This is an example of several circadian rhythms going on inside you RIGHT NOW!

The most disruptive event in the history of bio-time occurred on December 31, 1879. At his research lab in Menlo Park, New Jersey, Thomas Edison introduced the long-lasting incandescent lightbulb to the world. He famously said, "We will make electricity so cheap that only the rich will burn candles." Within a decade, night, for all intents and purposes, became optional. We no longer rose at dawn and slept in total darkness. We once worked from dawn to dusk and ate our last meal in twilight. Working hours and dinnertime shifted later and later. We spent more time indoors exposed to artificial light and less time outdoors under the sun.

In an 1889 interview with Scientific American, Edison said, "I hardly ever sleep more than four hours per day, and I could keep this up for a year."1 In 1914, on the thirty-fifth anniversary of incandescent light, Edison used the occasion to identify sleep as a "bad habit." He proposed that all Americans sleep far fewer hours per day, and predicted a future of sleeplessness. "Everything which decreases the sum total of man's sleep increases the sum total of man's capabilities," he said. "There really is no reason why men should go to bed at all, and the man of the future will spend far less time in bed."2

The second major disruption of your biological time was transportation advances. Cars and planes allowed people to travel great distances rapidly. It takes a day for the body to adjust to a one-hour time zone difference, and, on horseback or in a coach, it'd take about that long to go that far. Starting in the mid-twentieth century, in the blink of an eye, evolutionarily speaking, we could travel multiple time zones in a few hours, leaving bio-time lagging behind.

Computer technology brought us to where we are now, in a 24/7 smartphone culture of perpetual dusk where we work, play, and eat around the clock.

It took only 125 years to undo 50,000 years of perfect bio-timekeeping. Saying that our physiology hasn't evolved as quickly as our technology is the understatement of the millennium. As a result, our "when" is way, way off.

Being out of sync with bio-time is devastating to one's physical, mental, and emotional well-being. The phenomenon is called chrono-misalignment ("chrono" means time). In the last fifteen years, scientists have been connecting the so-called diseases of civilization (mood disorders, heart disease, diabetes, cancer, and obesity) with chrono-misalignment. Symptoms include insomnia and sleep deprivation, which lead to depression, anxiety, and accidents, to say nothing of what feeling overwhelmed and exhausted does to relationships, careers, and health. Unless you turn off every screen and light at 6:00 p.m., you are likely to deal with chrono-misalignment in one way or another, whether it's morning fogginess, extra weight, feeling stressed out, or not performing to your potential. (It's unrealistic to power down at dusk, of course. But you can turn off screens a bit earlier than usual, and dim the lights as the night goes on.)

A sparrow doesn't rush to work by 9:00 a.m. on a coffee buzz while fighting traffic. A salmon doesn't attend a midnight concert. A deer doesn't binge-watch House of Cards all weekend. Imagine a house cat napping, playing, and cleaning on a societal schedule. It would never happen. Animals heed their inner clocks. Humans, with our big, superior brains, willfully ignore ours, cramming our circadian rhythms into a "social rhythm," often in direct opposition to what our bodies are supposed to be doing at that time.


I became board-certified in clinical sleep medicine fifteen years ago, around the same time that chronobiology (the study of circadian rhythms) became the hot topic in my field. The study of circadian rhythms in humans was virtually nonexistent before the 1970s and is still unknown to the general public. Why? For starters, most primary care doctors haven't heard of chronobiology, either. It isn't taught in medical school, except when it comes to a few rare sleep disorders. There's no official drug to prescribe for being out of sync with your inner clock (unless you count caffeine, the most abused substance on the planet!), but there are many drugs and nutraceuticals—foods that have a medicinal value—that have a detrimental effect on your bio-time. (For a list of such drugs, go to here.)

When a number of my patients weren't responding to standard insomnia therapy, I became interested in, and then fascinated with, chronobiology. I had to branch out and find new ways to help them, and I started using chronotherapy techniques—exposure to light boxes at certain times of the day, replacing lightbulbs in the bedroom with "sleep-friendly models" (check out Lighting Science, and recommending the "sleep hormone" melatonin at specific times within the circadian cycle—with some measure of success.

But then I wondered if my patients would get even better results if they adjusted their daily schedules to inch closer to their natural bio-time. I asked them to make minor changes to when they ate and exercised, spent time with friends and watched TV, and were exposed to artificial light. When they did, we started to see remarkable improvement, not only in their sleep but in their general health, mood, memory, concentration, fitness, and weight. Good timing is so powerful, I realized, that it could change anyone's life in almost every way.

I was hooked. I read everything I could find in medical journals about the profound benefits of being in sync with your bio-time. As I mentioned, the field has exploded as a research topic, so I have been very busy keeping up. Here's just a small sample of the top studies to make circadian breakthroughs in the last few years:

Treating a disease such as cancer on bio-time can save your life. In 2009, researchers from the University of North Carolina School of Medicine experimented with mice to determine if timing of medication affected the speed of DNA repair to damaged cells. They took extracts of the mouse brains at various times and found that when medication was taken at night, DNA repaired itself seven times faster, in correspondence with the circadian rising and falling levels of a certain enzyme. The researchers theorized that, to minimize side effects and maximize effectiveness, chemo drugs should be given to patients when their cells are better able to repair themselves.

Thinking on bio-time can make you smarter and more creative. In 2011, a team of psychologists from Michigan State University and Albion College asked their study subjects to solve problems, some analytical and some that required insight, at different times throughout the day. The subjects solved creative problems better during their non-optimal times, when they were tired and groggy. They solved analytical problems at their optimal times, when they were wide-awake and alert. The researchers concluded that creative and analytical thinking operates on bio-time. If you set out to solve a certain type of problem, you'll do better at certain times.

Eating on bio-time can help you manage weight. In a 2013 study of 420 overweight or obese men and women, researchers at the University of Murcia, Spain, put the subjects on a diet of 1,400 calories per day for twenty weeks. Half of the subjects were "early eaters," having their biggest meal of the day before 3:00 p.m. The other half, the "late eaters," had their biggest meal after 3:00 p.m. The two groups ate the same quantities of the same food, exercised at similar intensity and frequency, slept the same number of hours, and had comparable appetite hormones and gene function. Which group lost more weight? The early eaters lost twenty-two pounds, on average; the late eaters lost, on average, seventeen pounds, a 25 percent difference. The late eaters were more likely to skip breakfast.

Living on bio-time can make you happier. In 2015, researchers at Copenhagen University Hospital, in Denmark, treated seventy-five patients with major depression through the use of either daily chronotherapy (bright light exposure and a consistent wake time) or exercise. Sixty-two percent of the chronotherapy patients went into remission in six months. Only 38 percent of the exercisers did.

Running on bio-time can make you faster. In 2015, a team from the University of Birmingham, England, set out to find a connection between athletes' performance and whether they felt alert and active in the morning (morningness) or alert and active in the evening (eveningness). There is one, indeed. The number of hours between a runner's wake time and race time had a huge impact on performance. If the late risers ran in the evening, they were much faster than if they ran in the morning, for example. The differences in speed were significant, measuring up to 26 percent.

In the pages ahead, you'll read more about these and many other studies. They offer proof of the importance of keeping good bio-time, and demonstrate the dangers of ignoring it. The scientific fact is, if you are time-wise, your life will tick along like clockwork.

If you are out of sync with your inner timing, you're working against your own biology. When has that ever been a good idea?

I'm not an Edison hater. I'm not about to say you have to throw away your iPhone or go live in a cave. If not for science and technology, we wouldn't have the proof of just how profound bio-timing is to health and productivity. We can use the research and technology to help us keep near-perfect bio-time and still stay on a social schedule. That's the beauty of it: You don't have to overhaul your life to tap into the power of when. You only have to shift a few things around, set up some alarms on your phone, download my free app, and watch your life change for the better.




What's Your Chronotype?

Every person has a master biological clock ticking away inside his or her brain, and dozens of smaller biological clocks throughout his or her body.

But not every person's biological clocks keep the same time. Your friend's inner clocks might run at a different pace than yours, or your partner's, or your kids'. You know this already; you've observed that some people wake early, or don't feel hungry when you do, or are full of energy when you are winding down. Different people fall into different classifications, called "chronotypes," based on general morningness and eveningness preferences.

According to conventional wisdom and historical definition, there are three chronotypes:

1. Larks, the early risers

2. Hummingbirds, neither early nor late risers

3. Owls, the late risers

Psychologists and sleep doctors have long used a standard Morningness-Eveningness Questionnaire (MEQ) to determine an individual's chronotype. Having worked with patients and studied in the field for over fifteen years, I'd always been bothered by the three categories and how they were determined. By only assessing an individual's sleep/wake/activity preferences, the MEQ didn't match the patient population in my clinical practice at all.

The established chronotype assessment didn't include both measures of the two-step system for sleep. Along with wake preference, there is "sleep drive"—your need for sleep. Some people have higher sleep drives than others, just as some have stronger sex drives than others.

Your sleep drive is genetic, and it determines how much sleep you need and your depth of sleep.

Those with low sleep drive don't need a lot of sleep, so the night seems very long to them. Low sleep drive people are easily woken up by sound and light disturbance, and they wake up feeling less than refreshed.

Those with high sleep drive need more hours of sleep, so the night feels too short for them. High sleep drive people sleep deeply, but they wake up feeling less than refreshed no matter how much sleep they get.

Those with medium sleep drive sleep somewhat deeply and are satisfied and refreshed by seven hours of continuous rest.

The MEQ wasn't designed to take into account the individual's personality. But personality turns out to be incredibly important for figuring out chronotype. For example, morning types tend to be more health-conscious. Evening types tend to be impulsive. Neither type tends to be easygoing. This has been confirmed in dozens of studies. In a comprehensive evaluation of chronotype, personality is too big and relevant to ignore.

My second issue was that classic definitions didn't match up with my patient population. The three established types excluded 10 percent or more of the general population: insomniacs. Although bad sleepers can be found among early, late, and normal risers, true insomniacs—those who chronically struggle to fall and/or stay asleep and usually get less than six hours per night—I believe are a distinct chronotype, with a wake/sleep preference, sleep drive, and personality profile that are distinct from those of the classic three categories.

I decided to redefine the groups and write a questionnaire of my own that took all the important factors into account. I also renamed the chronotypes. Humans are mammals, not birds, and we share behaviors with other mammals. My chronotype names reflect that. I looked for mammals that accurately represented the four categories as I see them, and found exactly what I was looking for:

1. Dolphins. Real dolphins sleep with only half of their brain at a time (which is why they're called unihemispheric sleepers). The other half is awake and alert, concentrating on swimming and looking for predators. This name fits insomniacs well: intelligent, neurotic light sleepers with a low sleep drive.

2. Lions. Real lions are morning hunters at the top of the food chain. This name fits morning-oriented driven optimists with a medium sleep drive.

3. Bears. Real bears are go-with-the-flow ramblers, good sleepers, and anytime hunters. This name fits fun-loving, outgoing people who prefer a solar-based schedule and have a high sleep drive.

4. Wolves. Real wolves are nocturnal hunters. This name fits night-oriented creative extroverts with a medium sleep drive.

If you don't recognize yourself in the short descriptions above, perhaps you recognize one of your parents. Remember, your chronotype is genetic—determined specifically by the PER3 gene. If you have a long PER3 gene, you need at least seven hours of deep sleep to function, and tend to be an early riser. If you have a short PER3, you can get by on light or little sleep, and you tend to be a late riser. It's likely that at least one of your parents had the same chronotype as you.

Why so many types? Why is there variation at all? Since the dawn of man, a range of chronotypes has been necessary for species survival. Each chronotype had its purpose and contributed to the larger group's security. Bio-time had to be diverse for the larger group to stay safe over the long night. Although we don't stand watch over the cave opening anymore, our genetic structure hasn't changed all that much since prehistoric times, and neither have the following ratios:

Dolphins account for 10 percent of the population. Light sleepers, they rouse at the smallest noise to wake and warn the group of danger.

Lions account for 15 to 20 percent. They rise early, taking the morning shift of guarding the group and watching out for roving predators.

Bears account for 50 percent. Their cycles match the rise and fall of the sun; they hunt and gather in daylight.

Wolves account for 15 to 20 percent. They take the late shift to guard the group, drifting off when the most extreme Lions start to stir.


  • "The Power of When prepares you to take full advantage of our adaptation to the rhythms of nature. After reading this book, your decision-making ability will forever be improved. This is a new horizon in our understanding of human behavior, and Michael Breus masterfully presents the science as a recipe for self-improvement."
    David Perlmutter, MD, author of Grain Brain and Brain Maker
  • "Dr. Breus has succeeded in making the complex science of chronobiology accessible and engaging for all readers. Whether you're looking to improve your relationships, get a raise at work, or simply lead a more fulfilling life, THE POWER OF WHEN is the guidebook you've been waiting for."—Mark Hyman, MD, Director, Cleveland Clinic Center for Functional Medicine

On Sale
Sep 20, 2016
Hachette Audio

Michael Breus, PhD

About the Author

Michael J. Breus, PhD, is a Clinical Psychologist and both a Diplomate of the American Board of Sleep Medicine and a Fellow of The American Academy of Sleep Medicine. Dr. Breus has been in practice for 23 years, has been featured on The Dr. Oz Show more than 40 times, writes regularly for Psychology Today, and was recently names the top sleep specialist in California by Readers Digest. He is the author of three Amazon Bestsellers: The Power of When (2016), The Sleep Doctor’s Diet Plan (2011), and Beauty Sleep (2006). | Twitter: @thesleepdoctor | | Instagram: @thesleepdoctor
Stacey Griffith is the founding Senior Master Instructor at SoulCycle, the bestselling author of Two Turns from Zero, and a co-host of the popular podcast “The Way” with Sara Wragge. Griffith has been featured in top tier national media outlets including the New York TimesThe Wall Street Journal, VoguePeople, Town and Country, Vanity Fair, Self, Shape, Women’s Health, New York, and the New York Post, among others, and she is a highly sought-after motivational speaker. | Instagram: @staceygnyc | Twitter: @staceygnyc

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