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@thinman already wrote: Wim Hof is a great source to get back into the cold shower. You will find a lot of it for free on youtube.
It also helped me to get it done.
@Fremen thanks for the tag. I have a view cold shower days on my list and know one or two things. ;-)
Well, there is no right or wrong to approach this thing, to get back into it.
And there is also nothing wrong with luke-warm to start with.
So, depending on your body - I will just write down what comes to my mind right now:
Maybe start with lukewarm, and slowly working your way to the cold side of life.
Another thing I find quite helpful is: Do not go fully into the cold shower. Start with your legs, than the other one, than the both arms, and working your way up to your heart and head, therefore your body gets accustomed to the stress of the cold. Leave the head for the last.
And another important thing: Start a cold shower either after a breathing session = for example one of Wim Hof (you find the technique on youtube), therefore your body heats up quite a lot. Or, after a nice workout when your body is already warm.
Another thing you can try is: Start your shower warm (as long as you like), than switch to cold (or work your way up . lukewarm, etc. . for a set time like 20seconds or more as you like), than go back to warm again, than switch to cold again. You can do that as often as you like, but finish your shower with „cold“. It works. Your body will get easily accustomed through it.
Time is also of the essense. Dont try to go for 5 minutes straight or something like that! Its work in progress. Start with 20-30 seconds, thats more than enough, maybe adding each view days 5-10 seconds atop. Maybe set yourself a goal of 2 minutes for the start and work yourself up to it.
And as written above: dont underestimate the lukewarm setting, slowly but steady you will automatically reach the cold side.
Last but not least and very important: continue breathing. For me personally it helped a lot to just continue breathing, I closed my eyes and breathed and tried to stay in that moment. Also: its just water, its wet, yes, but cold? no, water is only wet ;-) Mind over matter. ;-) Your body will automatically heat up.
Well, at least something that comes to my mind, not especially in a right or wrong order, but I hope I could at least help a bit. I would be interested to know more about your cold-shower journey.
In that sense: Happy shower, and remember the cold is your new warm friend, but you already know the great benefits.