6LETTERS

Because They Are Vital Signs

by Senior’s Know-How | May 14, 2026

When we get a blood test at a hospital, 

we receive a single number. 

-Testosterone 4.1 ng/ml. 

-Cortisol normal. 

-Melatonin no abnormality.

However, a thought suddenly crossed my mind: 

“Can this single number really explain who I am right now?”

Me at 7:00 AM and me at 11:00 PM are clearly different. 

The body on a day with plenty of sleep and 

on a day spent tossing and turning all night 

are completely different. 

The body on a day filled with stress and 

on a day falling in love cannot possibly be the same.

Yet for a long time, 

we have tried to understand this complex, 

dynamic system called hormones through a single blood draw.

Perhaps the problem starts right here.

Hormones Are a ‘Rhythm,’ Not a ‘Number’

Hormones in our body move continuously, 

much like a heartbeat. 

In particular, 

most hormones are strongly influenced 

by the 24-hour biological clock—the Circadian Rhythm.

A prime example is testosterone. 

Male hormones are generally highest between dawn and morning, 

decreasing as time passes. 

According to research, 

some individuals experience a decrease of 

more than 20–30% by evening compared to morning. 

That is why even Endocrine Society guidelines recommend: 

“When possible, test in the morning.”

Cortisol is similar. 

It spikes in the morning and drops at night. 

Conversely, melatonin secretion increases only when night falls.

The problem does not end there. 

This rhythm varies from person to person.

Even the Same Person Is Different Every Day

Hormones change on days after poor sleep. 

The cortisol curve collapses for people working late nights over long periods. 

As we age, the amplitude of testosterone fluctuations decreases.

Even "morning people" and "night owls" 

can yield different test results 

even if tested at the same time.

In a sense, 

we have been scooping up 

a single cup of water from a constantly flowing river and saying, 

“This is what this river is like.”

Why Measuring Just Once Is Dangerous

 

 

Traditional hormone testing mostly relies on blood draws at a specific point in time. 

Of course, these are medically very important tests.

However, there are structural limitations:

First, results can vary depending on the exact timing of the test.

Second, it fails to reflect an individual's "everyday rhythm."

Third, it loses sight of the overall flow throughout the day.

For example, 

a person might show normal levels when tested at 8:00 AM, 

but could appear to have low testosterone if tested in the afternoon.

Conversely, even if someone's hormone rhythm has already collapsed 

due to chronic stress or sleep disorders, 

they might appear completely normal based solely on a single snapshot value.

Ultimately, what matters may not be a 

"single-moment number," 

but rather the pattern in which that person is moving.

Perhaps What We Want Is an ‘ECG for Hormones’

 

 

In the past, blood sugar (both HbA1c and glucose) 

was also tested only occasionally at hospitals. 

As a note, HbA1c measures the percentage of glucose bound to hemoglobin, 

showing the average blood sugar state over 2–3 months similar to the lifespan of red blood cells, 

whereas glucose reflects immediate changes.

Later, home glucose meters became available, 

allowing people to check their glucose levels at home. 

Every morning, 

people drew blood on an empty stomach to check their numbers. 

But now, with Continuous Glucose Monitors, 

we can observe blood sugar trends continuously throughout the entire day.

Blood pressure follows a similar story. 

A 24-hour ambulatory blood pressure monitoring test 

is often more accurate than a single reading taken 

at a doctor's office. 

Recently, continuous blood pressure monitors 

have also entered the market, 

just like continuous glucose monitors.

What about hormones then? 

If hormones are also biological signals that move with time, 

won't their monitoring eventually evolve toward tracking 

a "continuous flow"?

I have increasingly come to think so.

That Is Why Continuous Monitoring Is Important

 

 

Continuous measurement is not merely 

about accumulating a mountain of data. 

It is about understanding an individual's unique baseline:

• When energy peaks

• When vulnerability to stress is highest

• How the body reacts when sleep breaks down

• How exercise and recovery actually exert their effects

These changes are invisible in a single test. 

However, they begin to reveal themselves within the flow of time.

Toward Understanding ‘Myself,’ Not Just Numbers

 

 

For a long time, we have understood human beings through averages. 

But the human body does not operate on averages.

Some people are most creative at night, 

while others are full of energy at dawn. 

For some, cortisol reacts heavily to stress; 

for others, it barely shifts.

What matters is not whether you fall within a standard 

"normal range," 

but how you fluctuate within your own unique rhythm.

The Sixth Vital Sign

 

 

Heart rate, blood pressure, 

body temperature, respiratory rate, and oxygen saturation. 

We call these vital signs.

Yet I keep thinking: 

Aren't hormones also one of the deepest biological signals reflecting the human state?

Energy. 

Sleep. 

Emotion. 

Recovery. 

Focus.

Aging.

Perhaps until now, 

we have been missing one of the body's most critical languages. 

And that language may exist not in a single isolated number, 

but within a 'rhythm' that continuously changes 24 hours a day.