It's Monday, but the coffee isn't kicking in

Fair enough, same here—on Mondays I have two cups of coffee and it does nothing lol

It’s not necessarily about going to bed early—sleep quality may be the issue. I also slept 7 hours but didn’t feel refreshed, so I tried a sleep app, and my deep sleep was less than 30 minutes. Cutting off caffeine 8 hours before bed and not drinking any after 2 p.m. definitely makes a noticeable difference.

Isn't that just caffeine tolerance? If you drink it every day, of course the stimulant effect wears off.