During a break up I can't maintain an appetite, I barely sleep, and I hold all the pain inward (try to not let it show - last thing I want is people commenting on it - bringing it up over and over again)... all the while skimming through the memories trying to piece together how and why the break up occurred. This may last about a month or so.
I may feel tired, but I have the nervous energy that courses through me... and won't let me go or sleep.
When I finally do come to terms with the break up and find an answer to the 'how' and 'why' that satisfies... I feel much better, resume my appetite... and the next few days I sleep like the dead.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen