Sounds like you are overwhelmed. Do you feel afraid to say no to the nighttime work? Sometimes with work we have to put our foot down a bit when we're feeling overworked. However, Mondays are always the hardest days of the week. I get that way on Monday's as well. It's hard to go back into routine after having 2 days to relax and do what we want.
My advice would be if you feel overworked, know its okay to sometimes say "no" to extra tasks. Sounds like you're getting paid overtime though so thats always nice!!