I have experienced this. I believe it can be contributed to stress. Our emotions get high, causing our heart race and pulse to skyrocket. Then once our heartrate subsides, it causes us to be exhausted and feel very sleepy. Similar to how we become tired after crying. Our body has to work hard to pump all the blood when we are stressed, which is why it is so bad to get stressed out. Your best option is to sleep and just let your mind drift for a while to wake up rejuvenated.