clow suffers from magick exhaustion.ย
later in life, after his souls been split and his magick divided between the two haves he sends out into the future; he over channels his magick because what he operates with is still on a higher level than heโs set himself at. they things he needs to do require a level of magick that is often beyond his means but events need to be done which does not favor his own comfort.
he doesnโt tend to use magick if has the option, but when he does have to cast, more often than not he over channels and it leaves him feeling winded and exhausted. he will hide it the best he can, but often times he will have to sit down and rest.ย
in some circumstances, it makes him ill and the only way he will be able to overcome it is through rest; which is why clone sakura and touya remember clow being sick towards the end. tsubasa within syaoranโs eye would recognize the symptoms for what they really were.ย