Are you just playing poker, or do you have a day job, too? If just poker, adjust your schedule. Learn to sleep during the day and use night and early morning as your "normal day." Lots of people have to do this for their jobs. Obviously, you should get up and walk around, take breaks, etc., just as you should if you are sitting all day at a desk.
And, of course, being fit and eating healthy is going to make it better/easier. That's a no-brainer.
If this is just for random long poker sessions and you have a day job -- suck it up