This thread is a tad on the silly side - if you feel that your ability to operate the aircraft safely has been compromised then you ARE incapacitated and your only option (in the absence of a spare crew) is to land as soon as is safely possible.
There is obviously room for interpretation; the degree of incapacitation, the probability of the condition worsening and such things as familiarity with the chosen airfield are among the factors to consider along with the urgency of obtaining medical treatment.
Continueing a long haul flight is NOT an option.