Also be sure to eat some food, you need energy. Remember, when you are in the studio, everything is viewed under a microscope. It may seems as though the production team is nit picking at every mistake, but they are just trying to help create the best possible product. Don't take offense, be thankful somebody cares. Whether you are recording a tape to shop to labels or clubs, they expect high quality.
Listeners are use to hearing strict timing generated from computers and sequences.
As live musicians, you have to compete with this. Again, I'm not saying the timing should be rigid, but large fluctuations in tempo won't do. If your engineer is brave, and has a steady hand, he may edit several takes together. This involves slicing the multi-track tape with a razor blade, and precisely splicing in the good take. Years ago this was common practice.