According to a study published in the The Journal of Physiology in 2010, a twice daily 5-minute immersion in cold water (12oC) for 3 days reduced stress reactivity by about one-third.
12oC is certainly chilly, but it’s not ice bath territory. Cold tap water in the UK is around 10oC in winter.
Stress is a generic physiological response to a stresser. The amygdala senses a threat, and immediately floods the body and brain with noradrenaline and adrenaline, followed a few minutes later by cortisol. This rapidly induces a series of changes to ready the body for movement and trauma (fight or flight).
When your body is exposed to the cold, the amygdala senses danger via skin temperature receptors and kicks off the stress response. The more intense and lasting the cold, the larger the response. This is what makes you hyperventilate and want to get out immediately.
By voluntarily staying in the cold, you are exerting top down control over this involuntary response. By staying calm in the face of heightened physiological arousal, you are training your body and brain to respond less severely the next time.
The more you repeat this, the more you can dampen your stress response, not just in the cold, but in any situation (remember, the stress response is generic – it’s the same, to differing degrees, whether the threat is cold, a looming deadline, or a tamtruming child).
This study used hypoxic (reduced oxygen) exercise as the stresser. Various stress markers (including noradrenaline and adrenaline levels) were measured under hypoxic exercise before and after the 3 days of cold water immersion. In the ‘after’ hypoxic exercise session, noradrenaline and adrenaline spikes were about one-third lower than in the ‘before’ hypoxic exercise session. In other words, the repeated cold exposure had substantially reduced stress reactivity.
You can replicate this at home by immersing yourself (up to the shoulders) in a cold bath. Obviously, intensity and duration matter here. Cold showers will be less intense, so to get the same effect, a longer duration may be required. Ice baths will be more intense, so a shorter duration would work.
Then again, twice daily 5-minute cold baths may be a bit much, and not practical for most of us. A 2 to 3-minute cold shower every morning is much more doable and is likely to give you at least some of the benefits. As always, start small, embed the habit, and build up slowly.