How do you know what time it is? By your watch or the clock on your smartphone; by where you are—work or home or in bed; by what you're doing—brushing your teeth or ordering a drink at a bar; by what's on TV. Science journalist and former New Yorker senior editor Alan Burdick long thought that time was a construct, an external imposition against which he could rebel. But when he decided to reconcile that fraught relationship—as writers often do, by writing about it—he discovered a far more complex and mysterious set of relationships than he could have imagined. The result is his new book, Why Time Flies? Burdick writes about time at social, cellular, physiological, and psychological levels—it's a masterful interweaving of hardcore science journalism and lyrical personal writing. 

How do you know what time it is? Close your eyes. Are you alert or drowsy, are you hungry or content, warm or cold, energized or still? Beneath what you can sense, your liver, kidneys, and even hair follicles know the time, too. A bird knows when to sing, a crab knows when to change color, a cyanobacterium knows when to divide so that its DNA will be least vulnerable to the disruptive, mutagenic UV rays of the sun. In his book Burdick writes, "A clock is a thing that ticks." Add our bodies and cells and almost every organism on the planet to that list.

Our internal clocks tick out their own beats, but they continuously calibrate themselves to a host of external signals. Sunlight, of course, but also: food. When, what, and how we eat in the morning has a powerful impact on our bodies' sense of time. I spoke with Burdick about what science can tell us about that, and how writing this book changed his own sense of time.

Extra Crispy: To start at the very beginning, how do our bodies know that it’s morning? What does that mean physiologically?
Alan Burdick: In all of our cells, we have a 24-hour clock. We can get into the details of how that clock works—it's genes moving around and stuff—but basically every cell beats out a 24-hour rhythm. Not exactly the same length as the solar day, a 24.2-hour rhythm. And those cells together form larger collectives like your kidneys and your liver. And those are basically clocks too.

Your liver is more active during the day than it is at night. The same is true with your kidneys. And all of these clocks are kept in sync with each other by a master clock in your brain, a structure called the suprachiasmatic nucleus. Leaving daylight out of it for a second, that system will beat out a rhythm that governs the rises and falls in the activity of your physiology. So you metabolize alcohol worse at 10:00 or 11:00 o'clock at night [than earlier in the day] because that's when your liver is less active. And your hair cells grow faster during the day than at night because they're on a 24-hour clock. It's kind of like a symphony. You've got the brass and you've got the strings and the woodwind and not everybody is at their peak at the same time, but they're all kept in sync by this conductor in the brain. 

If you were kept in a closet for a couple of weeks, [your internal clock] would still be at a 24.2-hour rhythm, and slowly it would drift out of sync with the actual solar day. So you have this massive clock in the brain that basically collects sunlight in the morning and at night and uses that signal to reset itself. It propagates that signal to all the other clocks in the body, and that's what keeps all the clocks in sync with the solar day. 

But that means you need a regular sleeping schedule and a regular eating schedule to keep all the clocks in sync [with each other]. If you don't, things happen like your blood pressure starts to rise over a period of days and your hunger hormones are thrown off and your blood sugar levels start to go south. Over time you develop things like weight gain and then type 2 diabetes. 

So if the things that you have a choice about—like when you go to sleep, when you wake up, when you eat—if those are out of sync with the solar day, all of those health problems start cropping up?

Well more to the point, if it's out of sync with your circadian day, with your internal clock—which, if you’re keeping a steady sleep/wake cycle, is in sync with the solar day. But if you're sleeping for 36 hours at a time or you're doing a lot of night shift work, then your circadian rhythm can be thrown off from the solar day, and if your eating schedule is on a different rhythm altogether you end up eating at times at which your body just not maximally designed to be eating and metabolizing. 

Is that connected with the longstanding health advice—which, granted, has been challenged and supported and challenged and supported—that it's very important to eat breakfast? Does eating within an hour or so of waking up have an impact on your circadian clock?
Yeah. Two things help to set your circadian clock in the morning. One is exposure to daylight, and the other is eating a meal. Your circadian clock is most—not vulnerable, but most most subject to tweaking after you've been fasting for a long period. So breakfast is a big trigger to your circadian system to start the day. Studies have shown that if you have a long fasting period between dinner and breakfast, that means breakfast the next day is the best meal trigger for your circadian system to realize, Okay it's morning now, I'm going to reset myself. Whereas if you let 16 hours pass—God forbid—between breakfast and dinner, dinner is going to gradually be perceived as the start of your day, as far as your circadian system is concerned. 

Some interesting studies have been done showing that the same meal will cause more insulin to be released at the end of the day than if the beginning of the day—the exact same meal can can make you gain more or less weight depending on when you eat it. What's the phrase? Eat breakfast like a king; eat lunch like a prince; eat dinner like a pauper. Your caloric intake should be tailing off as the day progresses, which is actually completely opposite of the actual trends in the U.S. in the U.K. There were a couple of studies in 2013 that showed that people [who were trying to lose weight] who ate their biggest meal the day before 3pm lost 25 percent more weight than people who ate the same the exact same meal with the same calories at the end of the day. 

So our bodies are better able to handle big meals earlier in the day?
Yes. [But] the problem with this idea of getting your calories in early is that your sense of hunger is also on a circadian cycle. And it's lowest first thing in the morning. That’s when you're least likely to want to eat a big breakfast. That happens in my house all the time. I have twin boys who are ten years old and one of them totally dives into breakfast. The other, he wakes up and he's just not hungry. Then like by lunchtime he's kind of starving, but he's so crazed that he doesn't realize he's starving and [Laughs.] his day just goes south. I can't get him to eat breakfast before about 8:30—and he's got to leave for school at eight o'clock. 

Oh no. 
Yeah. I still have not figured that out. 

So our body does best with more food in the morning, when it's least hungry. 
Yes. 

Well what are we supposed to do with that?
Maybe what what you do is you have a big brunch. But if you’re going to divide your day up into three meals, or if you're only going to eat two meals, then the first one wants to be the bigger one. 

And that helps reinforce your internal sense of time, so that it syncs up with your job and the sun, and that's the healthiest state for your system to be in?
Exactly. It helps with weight gain and weight loss but it also triggers your circadian clock and basically tells it what the daily schedule is going to be and when to start. So you're reinforcing a cycle not just for that day but for days to come. 

The situation in daily, or non-scientific life, in which I think about circadian rhythms most is jetlag, when our circadian rhythms are our of sync with the world. Or, as you point out in the book, when the various clocks within the body get out of sync with one another. Does breakfast become especially important in acclimating to a new time zone?
Absolutely. The advice that works for me is that when I'm going to cross several time zones, I get on a plane I immediately set my clock for the new destination. And then I eat on that schedule. You really want to really want to get your body on on the new time the new clock as fast as possible. All these peripheral, satellite clocks in your body—when they're not taking the master clock in your brain as their main signal, they’re taking their cues from your liver, from your eating times. So breakfast is a really powerful signal to all these clocks about what time it is and what time you want them to think is. 

At the beginning of the day, there's the food signal and there's the there's the daylight signal. Those are two of the most critical signals to your circadian clock. This is key to setting your clock to whatever you want to set it to. If you're traveling, even if you've only slept like two hours, getting out first thing, getting daylight in your body and eating a meal as if it were actually breakfast and you were actually hungry, will help a lot. 

How did what you learned over the course of your writing this book change your own relationship to time? In terms of breakfast or in general. I realize the answer is kind of the entire book, but to whatever extent you can answer that. 
I came out of it with a much greater respect for the time and the importance of it. This book is not about physics and it's not about spacetime or any of that. It's about time in our minds in our bodies. There's this circadian time that's really hardwired into us biologically, and then there's this other, more amorphous stuff that we perceive in our minds in all kinds of ways. 

I’ve heard the phrase that like time is a social construct, whether it's the time on watches or the time that we perceive, you know, Time is in your mind, [as if] you can kind of ignore it somehow, but you can't. I've come to think of time almost like a language, and the core language of human sociality. I used to think to think that time was something that I could just walk away from. Through this I realized that that's pretty anti-social. [Laughs.] But also, I wrote this book over ten years, and in ten years my kids grew up. They weren't born when I started this book and now they're ten years old. Just as a process of being a parent and needing to be more responsible, I've gained a much bigger appreciation for what time and planning can actually do for me. 

I wonder if that’s a bit of too many variables—having kids, writing the book, it’s hard to know what’s what.
Yeah I mean if you take ten years to do anything you're going to change a little bit. 

One hopes. My last question is: What did you have for breakfast today?
I had scrambled eggs. I eat eggs pretty much every morning. My kids are actually on break this week, so we're all around the house and—this is a total aberration, but my wife made chocolate fondue. 

For breakfast? 
For breakfast. So the kids had—well did, too—strawberries dipped in chocolate fondue. Which was awesome.