Thanks for the input BRITOMART!
Just to let you know, I'm in a production of Mme Butterfly. I've been at rehearsals every night (except for yesterday) this week for 3-5 hours, mostly 5 hours. I get really hot in my costume and wig. Especially the wig. Maybe that's why I'm so drained after the rehearsal is over. But I will be done after tomorrow night. And I will be able to get a good night's sleep tonight too.
Thank you for your suggestions. I'm really not in a position where I can take a nap at work, but I definitely need to bring complex carbs the next time I'm in the middle of a production week.