You know that lack of exercise makes you feel bad, and you don't need anything else making you feel bad while writing a thesis. Many people opine that a walk helps them think, or clears the head. You may find that the occasional perambulation improves your productivity. Don't forget to eat, and make an effort to eat healthy food. You shouldn't let your body fall apart, or risk illness at this critical time.
Exercise is good for keeping up the appetite.
I know that you haven't the time to cook much, but get a supply of fresh fruit, vegetables and bread. It takes less time to make a sandwich than to go to the local fast food, and you'll feel better afterwards. Thesis writers have a long tradition of using coffee as a stimulant and alcohol or marijuana as relaxants. In moderation they doesn't seem to have ill effects on the quality of thesis produced. Other people will be sympathetic, but don't take them for granted.