Why automatic e-mail checking is not working in my new Nextel phone?

I’ve done a quick search for this, but can’t find an answer. My new Nextel phone doesn’t seem to automatically check for e-mail in the middle of the night. That’s not the big problem, since I’m sleeping, when I grab my phone in the morning and use it, whether to make a call or whatever, it still doesn’t automatically check for e-mail. So do you know why this is happening?