Awful!! Make sure that you keep hydrated and that you eat little and often. If the body lacks energy it can go into panic mode. Snack: dried fruits and nuts; bananas; porridge; plenty of protein. Every 3 hours, 24/7.
I have found deep breathing useful: beginning at the toes by stretching then relaxing all the way up the muscle groups. I've usually fallen asleep B4 I get to my waist line ;-)