Stats Mechanics 3 (Source)

Regen Stat Effects
The three regen stats (Constitution, Stamina, Willpower) determine how quickly their respective pools regenerate. All three pools appear to regenerate using exactly the same formula. All three regeneration rates are equally affected by the character stance (standing, kneeling, sitting, or prone).

Health_Sitting_Regen( per second ) = CONSTITUTION * 13 / 1200
Action_Sitting_Regen( per second ) = STAMINA * 13 / 1200
Mind_Sitting_Regen( per second ) = WILLPOWER * 13 / 1200

Kneeling_Regen( per second ) = Sitting_Regen * ( 5 / 7 )
Standing_Regen( per second ) = Sitting_Regen * ( 4 / 7 )
Prone_Regen( per second ) = Sitting_Regen * ( 4 / 7 )

It may be easier to remember that sitting has 75% faster regen than standing, and kneeling has 25% faster regen than standing.

Example: Rodan artisan (Constitution=300, Stamina=750, Willpower=550)
Sitting:
- Health_Regen = 300 * 13 / 1200 = 3.25 health pool per second.
- Action_Regen = 750 * 13 / 1200 = 8.12 action pool per second.
- Mind_Regen = 550 * 13 / 1200 = 5.96 mind pool per second.

Kneeling:
- Health_Regen = 300 * 13 / 1200 * (5/7) = 2.32 health pool per second.
- Action_Regen = 750 * 13 / 1200 * (5/7) = 5.80 action pool per second.
- Mind_Regen = 550 * 13 / 1200 * (5/7) = 4.26 mind pool per second.

Standing or Prone:
- Health_Regen = 300 * 13 / 1200 * (4/7) = 1.86 health pool per second.
- Action_Regen = 750 * 13 / 1200 * (4/7) = 4.64 action pool per second.
- Mind_Regen = 550 * 13 / 1200 * (4/7) = 3.40 mind pool per second.