Regeneration of your life, stamina and mana increases over time and is based on your hunger rate and your stats. Poison stops your regeneration. You must cure it before you can regenerate again.
Life and stamina depends on how much stamina remaining you have. Having at least half your stamina will give you the normal regeneration rate.
Mana depends on how much intelligence you have. Having 120 or more int will give you 50% more mana regeneration.
Imbuing regeneration is a separate increase from base regeneration above. Depending how many points you have in HP, mana or stamina regeneration, you may see a slight increase. Maximizing the imbued weight will give you an extra point in regeneration. Being starved will stop your regeneration.