Wim Hof describes a special breathing technique that helps with the oxygenisation of the body, and in turn can help with the overall health of the internal systems. Video: here

Do the following 3-4 times (= 3-4 rounds):
  1. Rhythm: Inhale FULLY and then exhale normally, 30 times. Exhaling normally means simply letting the air out (like a balloon), but not actively pumping all the air out of your lungs!
  2. Retention: When you have exhaled the 30th time, just stop breathing. You can get about 2 minutes without breathing if it's your first time.
  3. Holding: When you feel the need to inhale again, inhale FULLY and hold it for 10-15 seconds.
  4. Exhale and relax.
Some advanced tips (from my experience):

First time:

I have tried out the procedure: laying on bed, relaxed, but also anxious and excited. The results were fantastic.

During the progress of round 1, I felt a slight tingling in my fingers and forearms, and I felt nice and relaxed. During the 1st retention, I was so pleasantly surprised with the experience that I laughed and had to focus not to exhale too early (it helps to open and close the jaw slowly to prevent laughing).

I started round 2 shortly after; now the tingling was present earlier (around breathe 20) and got much stronger in the progress of the round. Around breath 25 the tingling rose up to my shoulders and head and reached a higher intensity. During the 2nd retention, the tingling remained and a resonating sound started to play inside my brain. I didn't stop the time, but I relaxed like this for at least 2 minutes. Then I breathed again, and then out after holding for 15 seconds. The sound was still strong, like a small colony of nanobots was running around inside my head (weird comparison, but true). I was a bit scared because of all the sensations, so I just breathed normally for some time until the sounds faded away and I felt completely relaxed.

Then I started round 3. The tingling started earlier yet, and it got even more intense, it felt like a bubbling stream of water was flowing through my arms. The resonating sound appeared again, now even before the 3rd retention, and it was louder and had a deeper, full sound to it. During the 3rd retention, I felt like my fingers where melting into the bed where they were touching it, the sound was not scary any more (just calming and miraculous), and the tingling/bubbling was going on all over my arms, head and chest. After a few minutes, I took a deep breath and held it, which made all the effects a bit more intense, and then relaxed...