An interesting quote from the Chen Tuan's volume by Hua-Ching Ni. The taoist (to be) immortal went back to his home province when he was more than 60 years old...

...to look for a certain person of spiritual achievement who could teach him how to control the breath through controlling the muscles of the nostrils. He heard that there was a master who could transform the internal essence by breathing. This master would lie down, touch the pillow, sleep for months and then wake up. Master Chen Tuan was interested in that and wanted to learn it. The master's achievement seemed to make his worn-out and old-looking skin quite similar to an insect that has left its shell; he was like a baby with fresh skin again, and his achievement made his bodily movement light like a spirit This was a true profounf attainment of internal alchemy.