It is pretty hard to function when you're tired, forget being in any type of flow or operating at peak creativity. Let's face it, the most trivial tasks take on a monumental feel when you are exhausted, and have a day or an afternoon or an evening ahead of you of...stuff. What to do?
I'm going to say something which may seem annoying, here: Lie down! I mean it. Grab some type of shut -eye, it doesn't even have to be actual sleep: Cop a sit on a park bench your co-worker's couch, a comfy chair in the employee lounge., anywhere that's safe and, if possible, private. And, just: Close those eyes. Breathe deeply, extending your exhales by a few seconds each time. This will calm your mind and your nervous system just long enough for you to actually sink into a feeling of relaxation, however short it may be. If you are really bone tired, and need to push forward, you must take them to time to rest, for refresh, for as long a period of time as you can manage.
When you are about to get back to your normal routine, run to the restroom (if you need the privacy) and execute eight to ten jumping-jacks, in quick succession. Rinse your face with some cool water. Brush your hair and slick on that favorite lip gloss (female or male!) and try to eat that RxBar you have stashed in your desk or bag for just such occasions.
You can also do this on the way into work, on the subway or bus, (but please not in traffic, I don't care how bad the 405 is!) If this is truly out of the question, try to get some fresh air, quick-quick, as fast as you can, and do a few turns around the building/park/parking lot, whatever you can manage. Practice this breathing a you do it, reminding yourself that this day, like every other before it, will be over sooner than you can even imagine.
Think about that Netflix you will treat yourself to this evening, or that massage or brunch you will gift to yourself. You will get through this. We promise! Just remember, the most important fact of all: You need your sleep. A ton of it. Especially if you have family/relationship/creative obligations in the mix. These things all require top-notch energy. But so do you! Don't forget about you, baby. (Now, go home, and get some rest! You deserve it. )