Welcome to Stretch—your guide to resdiscovering your body as a powerful tool (not just a wobble of flesh) for creativity, focus and resilience.

⏪ Last edition, I shared a body relaxation script to prepare your nervous system to “dive” into your day (this eye-roll analogy will make sense once you read it.) Find that here.

⏩ This edition, we’re diving into one of the most popular breathing techniques, box breathing, and how to use the CO2 tolerance test to make sure you’re doing it in a way that suits your nervous system.

Box Breathing—adjust the technique to your unique nervous system

Box breathing is one of the most popular breathing techniques, because it’s famously used in Navy Seals training to get their nervous system under control, and stay focused and precise during critical operations.

It’s a simple technique:

It involves taking an inhale, holding your breath at the top, exhaling, and holding your breath again at the bottom.

But, there’s one big caveat. Well, there are a few but I’ll start with this one:

There is a wrong and a right way to do it.

The wrong way is to blindly follow the instructions online. Most guides tell you to do inhale for 4 - hold for 4- exhale for 4 - hold for 4, or 5-5-5-5.

The problem is: the duration heavily depends on your individual nervous system. People who are highly stressed or anxious, or who are completely new to breathwork and have dysfunctional breathing patterns (without being aware of this), will really struggle with those durations and push themselves too hard too quickly. It’s like going into the gym for the first time and picking the heaviest weight.

The right way to do it…

is to first measure your CO2 discard rate, and use that to determine the duration for your box breathing.

I’ll share the instructions that were given to participants in this Stanford study, where they researched the impact of box breathing (among other techniques) on stress and moods.

Step 1: Measure your CO2 discard rate

This simple test, the CO2 tolerance test, has been developed by human performance specialist and breathwork expert Brian Mackenzie.

CO2 = carbon dioxide. It’s the gas you breathe out on every exhale. The test gives you a personalized baseline for your current breathing efficiency and CO2 tolerance, meaning how sensitive you are to the gas.

Knowing your CO2 discard rate allows you to customize the box breathing exercise to your current abilities.

Here’s how to do it:

1. Have a timer ready

2. Take 4 normal breaths in and out. Ideally these are all done via the nose.

3. Then take a maximally deep breath in through the nose.

4. Once your lungs are full, start the timer and exhale as slowly as possible through your nose.
   Don’t hold your breath, keep exhaling, but make it as slow as you possibly can.

5. Once your lungs are empty, stop the timer.
   If your breath starts to stammer or you have to swallow, also stop the timer.
   Again, don’t hold your breath at any point. This isn’t about how long you can hold your breath. This is about how long it takes you to empty your lungs on the exhale.

6. The number of seconds on your timer is your CO2 discard duration.

Step 2: Determine your Box Breathing duration

Discard rate → box breathing duration

• 0-20 seconds → 3-4 seconds

• 20-40 seconds → 5-6 seconds

• +50 seconds → 8-10 seconds

Step 3: Start Box breathing

Box breathing involves four simple, equally timed steps. So, for example:

If your discard rate was 12 seconds, you will go for a 3 seconds duration.

• Inhale to a count of 3.

• Hold this breath for a count of 3.

• Exhale for a count of 3.

• Hold empty for a count of 3.

You can download a free app to help you with the timing.

There are two ways you can use this exercise:

In the moment:

You can do a few minutes of box breathing to calm yourself down when feeling overactivated (like a Navy Seal!) By deepening, slowing and balancing your inhales and exhales, you’re bringing balance to your autonomic nervous system.

The breathholds can feel really amazing, when you allow yourself to drop into the feeling and tune into the sensations in your body. Release all tension as you’re counting. Let your shoulders drop. Relax your face.

Long-term stress tolerance:

Even better is to use this exercise to build up your stress tolerance over time. Just 5-10 minutes, 1-2 per week, can make a massive difference.

There is a very close correlation between CO2 tolerance and state anxiety; the better control you have over your breathing, the less generalized anxiety you feel. Additionally, when you improve your CO2 tolerance, over time, you slow down your natural breathing pattern at rest. Slow breathing is linked to increased parasympathetic nervous system activity and increased Heart Rate Variability.

It’s Jack Feldman’s go-to breathing technique

Jack Feldman, Stanford neuroscientist, is credited for discovering and naming the brain region that controls the inspiration of rhythmic breathing (the Pre-Botzinger Complex), apparently one of the biggest neuroscience discoveries in the last 20 years. So, a pretty big name in the breathing world!

He mentions box breathing as his go-to technique on the Huberman Lab podcast:

One more important caveat

I did say there were more caveats!

Don’t do this exercise if you’re prone to anxiety and panic disorders. This breathing technique is likely too stressful and activating for your nervous system.

A better way will be to start with a technique that is more down-regulating, like extended exhale breathing (inhale for 2, exhale for 4, no breathholds) or coherence breathing (in for 5.5, out for 5.5). The durations don’t really matter as much in the beginning, the most important thing is to slow down your breathing.

What else I’m up to…