Why is "When I sleep then the cat sleeps." incorrect?
Semantically, however, your statement should be correct.
Eh the whole "when A B then C D" is quite awkward with the addition of the unnecessary "then". It does work fine, however, in the case of "if I sleep then he sleeps". "when I sleep the cat sleeps" (without then) sounds much more natural.
It is now accepted 2/10/2020