To begin with, something upsetting has happened to bring you to the point of tears. This itself is stressful. Stress and tension break down the immune system and can lead to illness over time. So, when you cry, and I mean really sob, your body is releasing stress, sadness, perhaps anger, and all around tension. It isn't just the crying that is tiring, it is the emotional drainage. Did you know it takes more muscles to frown than to smile?
Well like mimiMao said, stress can make you really tired, because it's a lot of stuff going on in your head and that takes a tole on you. But it's also because your being dehydrated and that tires your brain and other organs out.. Making you tired too
You spend a lot of emotional energy and that alone can tire you out. Also most of the time when you cry it's because your upset or sad. When you cry your muscles tend to become tense. It's kind of why you van get tired from laughing.
When you cry your body starts to release a chemical (can't remember what its called) which calms you down by making you tired and sleepy. It's the brain's way of protecting your body and your sanity. Our bodies are truely amazing.
well, after you cry your eyes are heavy and sometimes itchy. that would probably be the reason because when your tired your eyes are heavy.
I think that it has to do with all the emotions and all the thoughts kinda stress you out