A lot of self-help techniques don’t have as much evidence as you’d think. Cold
exposure is a great example. It has become incredibly popular. And from all the
hype, you’d think that daily cold showers or plunges have multiple randomized
controlled trials (RCTs) showing consistent benefits for all sorts of outcomes
like anxiety, depression, and energy levels.
A 2025 meta-analysis did find some benefits, including lower physiological
stress 12 hours after cold exposure, better self-reported sleep quality,
improved self-reported quality of life after 30 days of practice, and 29% fewer
sick days taken. But the same meta-analysis did not find stress improvements at
other time points (immediately, 1, 24, and 48 hours after exposure). This is a
bit concerning—why would it work at 12 hours but not 1 or 24 hours?
The most-well-known benefit of cold stress is its ability to raise dopamine and cortisol.
In a healthy person, there is diurnal variation in both of these such that for
example cortisol is 2 or 3 times higher in concentration at its peak (usually
between 6:00 AM and 8:00 AM) than 12 hours later. Moreover a temporary trigger
such as a single experience of cold stress that makes the early-morning peak
higher tends to make the concentration 12 hours later lower than it would
have been, so “lower physiological stress 12 hours after cold exposure” but not
at the other five time points is not “a bit concerning” (as you write), it is a
confirmation of one of the most often repeated explanations about
the benefits of cold stress (provided that the cold exposure occurs ideally
within the first hour, but definitely during the first 3 hours after getting
out of a bed, as is the almost-universal recommendation as to timing).
To use cold stress effectively requires a little flexibility. For example,
ketogenesis tends to cause cortisol to stay high throughout the day and night
as does “fasting” (intentionally failing to eat enough calories to survive if one kept on eating calories at that rate indefinitely), so when I’m
doing either of those things it is very rare for me to choose cold stress
because I have learned that the combination of cold stress (or any other trigger that raises cortisol as sharply as cold stress) and the state of
chronic cortisol elevation caused by either of those things seems to prove challenging (purposely avoiding “stressful” here) to my
hypothalamic-pituitary-adrenal axis (as determined by the totality of my
observations of my internal state, not as determined by objective testing) -- and the challenge seems to have deleterious effects that can persist for weeks although I have reasons to believe the my hypothalamic-pituitary-adrenal axis might be more easily affected by such challenges than most people’s.
It takes time for a person to learn how to use cold stress effectively. Not all
of this learning need be book learning. It is probably not hard for most people
(or rather it was not hard for me) to learn to tell from purely internal
non-verbal information how stressed he currently is. If a person makes
inquiries throughout the day on his current stress level and how that stress
level varies in response to various triggers, then after not too many days of
this, he will probably have learned enough to start deriving significant
benefits from cold stress. Triggers that tend to cause a spike in cortisol are cold stress, exercise, bright light and salt. Conditions that tend to cause cortisol to remain elevated all day and all night are stress, having an infection, being in ketosis and being on a low-calorie diet.
Health interventions that require flexibility in their application are
difficult for researchers (or at least our current crop of researchers) to
study, but in my experience most of the value that can be obtained from health
interventions has this property of the intervention’s requiring flexibility and
requiring the consideration of at least a handful factors that most researchers
would consider extraneous or at least too complicated to be worth tracking in their
data-gathering and analyses.
When I was 17 I moved from Massachusetts where I grew up to South Florida.
After only a few months, my performance (I was a student in Mass and continued
to be a student in South Florida) dropped drastically and the main cause of the
drop was my not applying myself (to use my father’s words) or in other words an
inability for me to find the motivation to do the things my society expected of
me. Everything related to school seemed to require great applications of willpower, even a simple interaction with the school’s administrative
apparatus whose expected duration was less than half an hour.
I have strong reasons to believe that I could have corrected this drastic drop
in my motivation and drive using what I know now about dopamine and cortisol. I
now know (or believe) for example that cold stress is the single most potent
lever I have for achieving a spike in coritsol and dopamine. (Exercise is
another lever, and although I exercised regularly in South Florida, I did so
whenever. I did not know I should be doing it in the early morning so that the
raise in cortisol always caused by exercise would do the most good for my motivation and drive.)
When I lived in Mass I walked to school (as was common in those days, the
1970s) over a mile, which more days than not provided a quite large dose of
cold stress. If when I arrived in South Florida, I had started to fill a
bathtub in the morning, then to dump some ice in the water, then to wait for
the water temperature to hit some target temperature, then get in the tub for
some target number of minutes, then get out and let the rest of the ice melt,
then repeat the next day, my college career would probably have been vastly
more successful.
Another thing I could’ve done to increase my motivation and drive
is to take great care not to expose myself to artificial light during the five-hour period starting 7 hours
before my usual waking time and ending 2 hours before my usual waking time, taping
foil to windows if necessary, then getting bright light during the first
hour of being awake, preferably outdoor light.
I tell this story to illustrate that a little learning (both book learning and
the learning that comes from simply observing one’s internal state throughout
the day and noticing patterns) about cold stress and related things can be
decisive. It can put you years ahead of where you’d otherwise be.
The most-well-known benefit of cold stress is its ability to raise dopamine and cortisol. In a healthy person, there is diurnal variation in both of these such that for example cortisol is 2 or 3 times higher in concentration at its peak (usually between 6:00 AM and 8:00 AM) than 12 hours later. Moreover a temporary trigger such as a single experience of cold stress that makes the early-morning peak higher tends to make the concentration 12 hours later lower than it would have been, so “lower physiological stress 12 hours after cold exposure” but not at the other five time points is not “a bit concerning” (as you write), it is a confirmation of one of the most often repeated explanations about the benefits of cold stress (provided that the cold exposure occurs ideally within the first hour, but definitely during the first 3 hours after getting out of a bed, as is the almost-universal recommendation as to timing).
To use cold stress effectively requires a little flexibility. For example, ketogenesis tends to cause cortisol to stay high throughout the day and night as does “fasting” (intentionally failing to eat enough calories to survive if one kept on eating calories at that rate indefinitely), so when I’m doing either of those things it is very rare for me to choose cold stress because I have learned that the combination of cold stress (or any other trigger that raises cortisol as sharply as cold stress) and the state of chronic cortisol elevation caused by either of those things seems to prove challenging (purposely avoiding “stressful” here) to my hypothalamic-pituitary-adrenal axis (as determined by the totality of my observations of my internal state, not as determined by objective testing) -- and the challenge seems to have deleterious effects that can persist for weeks although I have reasons to believe the my hypothalamic-pituitary-adrenal axis might be more easily affected by such challenges than most people’s.
It takes time for a person to learn how to use cold stress effectively. Not all of this learning need be book learning. It is probably not hard for most people (or rather it was not hard for me) to learn to tell from purely internal non-verbal information how stressed he currently is. If a person makes inquiries throughout the day on his current stress level and how that stress level varies in response to various triggers, then after not too many days of this, he will probably have learned enough to start deriving significant benefits from cold stress. Triggers that tend to cause a spike in cortisol are cold stress, exercise, bright light and salt. Conditions that tend to cause cortisol to remain elevated all day and all night are stress, having an infection, being in ketosis and being on a low-calorie diet.
Health interventions that require flexibility in their application are difficult for researchers (or at least our current crop of researchers) to study, but in my experience most of the value that can be obtained from health interventions has this property of the intervention’s requiring flexibility and requiring the consideration of at least a handful factors that most researchers would consider extraneous or at least too complicated to be worth tracking in their data-gathering and analyses.
When I was 17 I moved from Massachusetts where I grew up to South Florida. After only a few months, my performance (I was a student in Mass and continued to be a student in South Florida) dropped drastically and the main cause of the drop was my not applying myself (to use my father’s words) or in other words an inability for me to find the motivation to do the things my society expected of me. Everything related to school seemed to require great applications of willpower, even a simple interaction with the school’s administrative apparatus whose expected duration was less than half an hour.
I have strong reasons to believe that I could have corrected this drastic drop in my motivation and drive using what I know now about dopamine and cortisol. I now know (or believe) for example that cold stress is the single most potent lever I have for achieving a spike in coritsol and dopamine. (Exercise is another lever, and although I exercised regularly in South Florida, I did so whenever. I did not know I should be doing it in the early morning so that the raise in cortisol always caused by exercise would do the most good for my motivation and drive.)
When I lived in Mass I walked to school (as was common in those days, the 1970s) over a mile, which more days than not provided a quite large dose of cold stress. If when I arrived in South Florida, I had started to fill a bathtub in the morning, then to dump some ice in the water, then to wait for the water temperature to hit some target temperature, then get in the tub for some target number of minutes, then get out and let the rest of the ice melt, then repeat the next day, my college career would probably have been vastly more successful.
Another thing I could’ve done to increase my motivation and drive is to take great care not to expose myself to artificial light during the five-hour period starting 7 hours before my usual waking time and ending 2 hours before my usual waking time, taping foil to windows if necessary, then getting bright light during the first hour of being awake, preferably outdoor light.
I tell this story to illustrate that a little learning (both book learning and the learning that comes from simply observing one’s internal state throughout the day and noticing patterns) about cold stress and related things can be decisive. It can put you years ahead of where you’d otherwise be.