Lately we have quite a few alternative health doctors or scientists saying that EMF and wifi etc are quite harmful to the body
This may be possible because if you zap during too long time it may alter the composition of your blood in some way or the functionning of your cells.
Personally i highly recommend to not use a zapper while sleeping because when you are unconscious you don't know how the resistance of the body changes.
I have had my heart beat altered because of using a zapper while sleeping (several times). Bad idea !