I have a mostly whole-food, plant-based diet. However, when I do occasionally eat meat or fish, I sleep poorly, get muscle cramps, and generally just feel more sluggish. Why is this? The increased protein? I don't really want to be completely vegan, but I want to feel good.

Add cloves, and an occasional teaspoon of juniper berrries. Also, eat is three hours or more before bedtime. Drinking water with a lime and 2 pinches of cayanne pepper(all organic). There are two teas we have as well.

Most American adults over the age of 25 have experienced so much stress that they under secrete both digestive enzymes (from the pancreas) and Hydrochloric acid or Hcl. Hence, meat eating is difficult for the body to handle without supplementation of these items. Further, protein is super acidifying, so it really works best in the body combined with greens, or other very low-sugar veggies.
Otherwise you will have pH issues that lead to muscle cramps and even long-term bone loss!

Foods rich in protein (animal protein) is harder for our bodies to digest. Our body has to work so much harder to break down these proteins, leaving us feeling tired and sluggish. If you look at the way our bodies are designed, we are herbivores. The stomach acid in our stomach isn't as strong as the acid in a "carnivorous" animal. Whose bodies are designed to digest meat and expel it at a much faster rate than a humans body. There are a few things you can do to help your body in the digesting process after eating animal protein. Besides cutting it out all together, you can eat foods that are enzyme rich, such as kiwi, pineapple, mango, and citrus. You can also try eating smaller portions of animal protein. Hope this helps!