Sleeping inside a van

Hello! This is going to be the first time me and my wife will sleep inside a van when camping. Is it safe? I'm afraid of running out of oxygen and suffocating with windows closed (I don't know if I'll be able to let windows open because the cargo area has no windows and maybe this part have no connection to the driver part. Sorry for my english, i don't know how to explain this very well). The van model will be a Toyota Proace City or some other similar van. I don't know if the cargo area is sealed or something like that. Thanks!