I’m sorry to hear you’re struggling with insomnia. That’s really challenging. I don’t think enough people recognize how big the consequences are of sleep disorders and general sleep deprivation, sadly.
I am some sort of currently unspecified chronically sleepy, so my situation may differ from yours but I share some understanding I believe.
I think that on a hard day you might want to skip lessons. Sleep deprivation really messes with your memory, ability to learn, and perception. You may become frustrated with the results for all the effort you put in.
I think reviews should be ok to do though. Although you may have difficulty remembering, make more typos, and misperceive some items, it’s still contributing to a steady exposure to the language. High percentages correct on reviews feels nice and all, but I think that becoming comfortable with Japanese is something that is valuable and not quite the same as having perfectly memorized kanji or vocab. Having your brain feel like its a natural part of everyday life to see Japanese, I think it can change your studies for the better. And keeping up with wanikani is easier if you make it a habit anyway.
Try not to be too hard on yourself if your insomnia disrupts your studies, it’s better to be learning slower than you’d like than to give up.