This question has been asked (and answered) on this forum before, but my question is when is the cutoff after which it is considered eating before shachris (as opposed to eating AFTER maariv). Specifically, if my work requires me to be up all night (which it has), I would wake up around 10 pm, do maariv, eat "breakfast", report to work, and around 3 am or 4 am, have "lunch". Is this permissible?
The Shulchan Aruch (OC 89:5) discusses what to do if you were eating before dawn (alot hashachar; the earliest time for Shacharit), and then dawn happened (and now you are obligated in Shacharit): must you stop eating or not. It seems from here that you are allowed to eat so long as dawn has not happened. The Mishna Berura there notes that if the eating is of a settled nature, one shouldn't start it even half an hour before dawn, lest it continue.
He also quotes the Zohar who recommends not eating before davening if one went to sleep and awoke for Shacharit any time after midnight, but he (the MB) agrees this is not the basic letter of the law.
You can find times for dawn based on your location online (eg. http://www.myzmanim.com).