Ugh....I am right there with you! I am almost 7 weeks, and I am SO exhausted every single day at the exact time that you are. Unfortunately for me, this is how every one of my pregnancies has been. My biggest pregnancy complaint has always been about how exhausted I am the entire time.
I don't have any magic answers. I do have a couple of suggestions though. First of all, make sure you are getting enough sleep at night. If you normally sleep 8 hours, try going to bed an hour earlier to get nine. Also, are you sleeping well at night? I think my problem is I suffer from a bit of insomnia during pregnancy. If that effects you, Try eliminating caffeine late in the day and in the evening.
Also, I don't know what your "lunch hour" is like, but if it is possible at all for you to sneak out to your car and take a 15 minute power nap, you should do it. It will at least help to refresh you.
Beyond that, I have no advice. Just feel comforted that you are not alone. Our bodies are working hard to create this little one.