As I sit here chugging my AM Diet Coke at 5 in the morning :? , I'v found I have to set myself a caffeine curfew of about 7AM. Anything after that can mess up Natalie's entire day.
Another option you have if you need a boost mid-day is exercise. Even if you're tired, putting baby in stroller and getting out, whether outdoors or in the mall or even the grocery store, or put in an exercise video during baby's nap (yeah, I know, you'd rather nap yourself! LOL), or even get a jump rope and put on a couple minutes with that. Hard to get motivated, but no worries about messing up your sleep or baby's later.