Have you ever heard that it takes more muscles to frown than to smile?
It takes energy to be stressed or upset about something, which is most likely why you’re crying in the first place. When you cry, you release that stress and anger and sadness. It’s an emotional drainage. It takes a lot of energy to cry.
For a more scientific answer (this is from a website, not my own words) – Crying causes the body to lower the feel good hormones which are responsible for the overall general psychological health. So to recover the original blood pressure and the adequate hormone level, the brain sends chemical messages to the neurons to induce sleep so that the body can rest and recuperate.