Sleepy while working

Hey ! I’m working on honoring my commitments to myself in terms of things I want to do in order to complete my PhD. I made my schedule for the week and started yesterday. I don’t freak out like before at all, I am calm even, but I just want to sleep soooo badly ! My eyes are closing, my head is heavy. It’s like my body is telling me, “OK, you don’t want to freak out, we will sleep instead !” As it’s so a physical reaction, I just don’t know how to manage it. Taking a nap ? (but will I just delay my work this way ?) Forcing myself to stay awaken does not help me being more productive. I will definitely pay attention to what I eat also because I know that can have an impact on my sleepy state. Can you give me some advice please ?