Why is it that when ever I go to the Gym to do exercises in the morning I feel good

[Dr. Feel Good]

then about a? few hour hours later I end up falling to sleep for two or three hours in the late afternoon then awake in the evening which keeps me up until about 3:00 am at night & then most of the time I get back up around 8:00 am to start my day again. How can I change this so that I'm not up so late ?