It gets a little slower. No one books exciting, adrenaline-filled things on Sunday nights.
The upcoming week is on your mind. Monday morning. You picture yourself being a little busier. Hopefully not hectic though - that's bad.
You prepare for the upcoming week a little. Just a little. It's still a weekend.
You enjoy the slow night. Sunday night feels like a routine, but a slow one. It's a relaxing routine.
When you go to sleep, you feel very relaxed from the weekend and prepared for the upcoming week.