Bedrest is needed any time a character is brought to zero or fewer hit points but doesn't die, and in certain other cases.
Days of bedrest do not include the day that bedrest becomes needed (e.g. the day the PC hits 0 or less HP, or the day a parasite is acquired), nor does it include the day the PC is done with bedrest. Every day of bedrest must be an entire day spent doing nothing.
Bedrest is not needed when 0 HP or less is reached due to subdual.