It so happened that yesterday I went to bed 1.5 hours later than usual. As a result, I did not get enough sleep and woke up with a headache in the back of my head. But after the breathing practice, I felt significantly better, as if some burden had been lifted. My head feels much better too.
4 cycles: 30, 35, 40, 45. Retention: 4:30. Sometimes during retentions this long, consciousness "disappears." I only come back after hearing the instructor's voice. I do not use any willpower to hold my breath; everything happens naturally. Willpower is only needed to start the practice.