Tag Archives: metabolism

How much energy is in a thought?

Sometime during the last months of grad school I was in the office late, polishing off one too many coffees, and dipping into my emergency ramen noodle stores. I was searching for that elusive (and perhaps illusory) moment of clarity that, one hopes, arrives to propel a manuscript forward. But, the long hours and coffee caused my mind to wander into distant realms of science. I had just finished teaching about neurons and action potentials and brain activity in my physiology class (100 billion neurons, forming 100 trillion neural connections—more connections than stars in our galaxy—sparking up right now allowing you to think this!) and I had a cool thought:

I am converting these cheap noodles directly into science and new insight. I am a biochemical machine that converts packs of 10 cent fake noodles into knowledge.

And then, the natural follow-up: at what rate? What is the cost of a thought? How many noodles does my brain burn to construct a statement? A paper? A dissertation?

Now that I do not have a dissertation submission deadline looming, I have some time to explore these thoughts—thankfully while burning some higher-grade fuel than emergency ramen! Warning: the calculations that follow are extremely ‘back of the envelope,’ and should be taken with a heaping helping of salt and skepticism. This is just a fun exploration.

How much energy is burned in a thought?

First, let’s gather some parameters. How much energy does the brain use? The short answer is: an incredible amount. Despite only accounting for 2% of the body’s weight, the brain uses 20% of the body’s energy (that figure is for an adult, in newborns it is 44%!!) The brain uses 2–3 times the amount of energy that the heart uses.

[Aside: the brain is extremely efficient at what it does—processing information using orders of magnitude less energy than the best supercomputers.]

So, let’s say that the brain uses 20% of the body’s basal metabolic rate, and the basal metabolic rate is 1500 kcal/day. That means the brain uses about 300 kcal/day, or 0.0035 kcal/second.

The next question is: what is a thought? How much time does one take, and what proportion of the brain’s energy is devoted to “thinking”? I don’t know! But, does anyone know? I don’t know that either. Since it is my blog, I am at liberty to define a thought. Let’s say, for the sake of argument (and feel free to argue in the comments) 100% of the brain’s energy is required for “a thought,” and all thoughts are created equal. And let’s also say that a thought is a statement, and that it takes as much time as one would take to think or read a sentence. For instance, here is a thought:

“Wow, I am thinking this thought about thinking; this is one of the things that hydrogen atoms do given 13.82 billion years of cosmic evolution, and it’s super cool.”

How long did it take to think that specific (extended) thought?  More than a couple of seconds, less than 10? Let’s say a substantial thought takes 5 seconds. At 0.0035 kcal/second, that’s about 0.02 kcal/thought!

So, how many ramen noodles are burned for a thought? At 400 kcal per block, and 150 noodles per block, we have 2.67 kcal per noodle. Assuming the average noodle is 33 cm long, we find that there are 0.08 kcal/cm of noodle—and every thought burns about 0.25 cm of ramen noodle! Your brain is incredibly efficient—no wonder that future AI are always super jealous and vindictive in sci-fi movies.

Now we can readily convert thinking-time into calories, and content creators can register their influence in energy. For instance, if 100 people read this blog post, consuming 5 minutes of calories thinking through the content, then about 100 calories would be burned on my words. 400 people and an entire block of ramen has been consumed by my words.

I wonder how much ramen has been burned by Shakespeare?


1.1 Billion

That’s the number of heartbeats in every animal’s lifetime*. Don’t believe me?

Let’s consider an extreme comparison. A mouse can live for 3 years, and has a heartbeat of about 670 beats per minute. There are 525600 minutes in a year (365 days/year * 24 hours/day * 60 minutes/hour). So, that’s 3 years/lifetime *525600 minutes per year * 670 beats/minute ≈ 1.1 billion beats/lifetime. What about an elephant? They can live up to 70 years and have a heartbeat of about 30 beats/minute. 70 years/lifetime * 525600 minutes/year * 30 beats/minute ≈ 1.1 billion beats/lifetime. Woah… what?!

Ok, ok… you probably noticed that those “equals” signs are actually squiggly “approximately” signs, and if you did the math (you should!) you would see that they are both a little off from exactly 1.1 billion. But still, they are damn close. What gives? Why would the number of heartbeats be an invariant property of the animal kingdom? Let’s dive a little deeper.

The answer lies in allometric scaling, or how different properties of life scale with the body mass of organisms. It turns out that the power (energy per time, metabolism) required to support a given unit of mass of an organism scales with the mass of that organism to the (-1/4) power- meaning that smaller organisms use energy at a faster rate per unit mass than larger organisms. Other rates, such as breathing rate and heartbeat rate, also scale with bodymass^(-1/4). Lifespan, on the other hand, has been shown to scale with bodymass^(1/4). If you want to find how the lifetime total beats scale, you can multiply those two together (beats/time * time = beats). Bodymass^(-1/4) * Bodymass^(1/4) = Bodymass^0, which is always 1, meaning that the total beats is invariant of bodymass! More on allometry and metabolism in later posts. And maybe I’ll learn how to show equations in wordpress someday.

This paper (which probably takes into account more than the 2 points I used above) cites the total number of heartbeats in an animals lifetime as 1.5 billion.

Now, given this number, can we backtrack and use the relationship to see how long humans are predicted to live? Given a certain heart-rate, how long would it take us to use up our 1.5 billion?


R code:
curve(1.5e9/(x*525600), xlim=c(40,100), lwd=5,
ylab=”Lifetime (years)”, xlab=”beats/minute”);
abline(v=60, col=”red”);abline(v=70, col=”red”)

If an animal beats its heart between 60 and 70 times a minute, it would use up its 1.5 billion beats in around 40-45 years. Is this a ballpark estimate of a human’s lifetime in the wild? (Aside: if you take the 1.1 billion heart beats derived from mice and elephants and assume a heart-rate of 70 beats per minute for humans you get 29.9 years!)

Now, don’t worry. Humans have found amazing ways to increase their lifespan, and it’s not like everyone has a set number of heart beats to get through before it’s all over. This is just an interesting result of looking at metabolism and ecology – and what’s even more interesting is looking at the animals that stray from the predictions.

*That’s about the predicted number of heartbeats in an average organism’s lifetime