How frequently should you winter swim, and how long should you stay in?
The stages of cold water immersion go like this:
fear and dread
trying not to think as you get in
cold water shock
absolutely loving it
getting really cold
Steps 1-4 are necessary, and add to the afterglow because you feel super-human for having conquered a fear. Steps 5-7 are the stages that your body goes through, and how it becomes adapted to cold water. Steps 8-10 are to be avoided by getting out before you get there.
How long it too long?
It sounds ridiculous to say 'get out when you start to feel really cold'. But as you become an experienced winter swimming, you do get to know when you've had enough. Before you get to that stage, it's worth staying in for up to two minutes per degree of water temperature (centigrade). So, if your thermometer reads seven degrees centigrade, fourteen minutes or less is good amount of time to cap your swim.
But I feel amazing!
Getting out when you're still at step seven, so still absolutely loving it, is ideal. This is because your body will continue to cool after you get out of the water. Known as the afterdrop, as you warm up after getting out of cold water, the cooler blood in your skin and extremities starts to mix with the warm blood that your body has cleverly shunted to your core. That means that your core temperature drops, usually triggering a shiver response deep inside you.
If you stay in for too long, the afterdrop is deeper. Considering that your normal body temperature is around 37 degrees and hypothermia happens when your core temperature is 35 degrees, you don't actually have much room for error.
Of course, some incredible humans can swim an ice-mile. That's a mile in water measuring under five degrees centigrade. The fastest ice mile was swum by Rostislav Vitek from the Czech Republic in just over 20 minutes, but most of the current 408 ice-milers took 30-40 minutes. To stay in for this long, these swimmers don't just train several times a week, they slowly build tolerance over several winter seasons.
So swimming in cold water regularly does help you adapt. Adaptation simply means that those perfectly normal stress reactions including the cold water shock response and pain in your fingers and toes, gets less extreme. It also means that you cope better with stress. Just bear in mind that your body can't tell the difference between the stress you feel about doing your tax return, the stress of having a cold or auto-immune condition and the deliciously brilliant stress of immersing in cold water, getting better at responding to one stress makes you less stressed, depressed, anxious, ill etc.
How often should I swim?
So, the more frequently you winter dip, the better for you. As a minimum, dip once a week when the water's in single digits if you can. If you're dipping less often, just play it safer. Stay in for one minute per degree, but longer than two minutes; you need to let the cold water shock subside to get any benefit.
Swim once a week or more = stay in for up to two minutes per degree
Swim less than once a week = stay in for up to one minute per degree
Stay in for a minimum of two minutes or until the cold water shock response has subsided
Get out while you are still feeling happy and comfortable