I can't study. Is it serious?

Respected Sir, Daydreaming as such would not be a psychiatric problem or any sort of problem for that matter, as most people do it. However if it happens excessively and is causing an impairment in your academics, then it definitely needs to be addressed. But most dreams and fantasies, during day or night, when occurring repeatedly, usually have an underlying reason that they keep occurring. For this reason to be identified, you would have to spend some time with a psychiatrist, or psychotherapist who can evaluate you in detail and help you to pinpoint the reason that you keep having these daydreams or fantasies. Even if an exact cause cannot be pinpointed, which does happen in a lot of cases, the psychiatrist can help you to better control and eventually "shake off" these daydreams/fantasies. So, in response to your query, I would say that it is daydreams are mostly harmless and not serious, but if you feel that it is severe enough to cause an impairment in your studies, get the help of a psychiatrist