Related: What parts of the human body would be negatively affected by sleeping upside down?
I have a character (a regular human in structure) who, for his own reasons, decides to act bat-like and sleep hanging upside-down.
Handwave the feet/attachment issue.
How would the body need to change for a person to sleep upside-down, safely, for approximately 8 hours every night?
As pointed out in the answers to the linked question above, the biggest issue seems to be the circulatory system and making that work without killing the character, but any changes that need to be made to the character's body are also welcome.